GUYED TOWER TOPIC SUMMARY - Still under construction..
Version: 01-19-2000
Check http://www.qsl.net/n1lo for the latest update
The existence, accuracy, content and organization of any section may change
at any time as new discoveries, understandings, and concepts arise. I add
new sections whenever appropriate.
By Mark D. Lowell, N1LO. First posted in November 1998
This document is a series of notes that I have made concerning guyed towers
and installation that started after reading and digesting the message
archives of the TowerTalk forum sponsored by the folks at
www.contesting.com. The archive is located at:
http://www.contesting.com/_towertalk/
I have also combined ideas from other readings and personal experiences as
well. I have paraphrased some subjects after reading the general consensus
of many messages. In other cases, the originators of these messages have
already addressed the topic in the most eloquent form, and I have simply
copied their messages here.
I have concentrated mainly on subjects relating to standard, Rohn, guyed
towers, and not crankup or self-supporting types. I most whole-heartedly
agree with an opinion once expressed on the TowerTalk reflector: "There is
nothing stronger, safer, or more cost effective than a good guyed tower."
***WARNING***
Tower climbing can be hazardous to your health! You can hurt yourself and
others very easily while engaging in climbing and rigging. The information
here is provided on an as-is basis and, naturally, I can assume no
responsibility for your safety, or how you interpret or implement the
techniques I have described here. Do not perform a procedure that you are
not comfortable with. Think ahead, get familiar with all of your materials,
and teach the people assisting you about the methods and dangers. Again, in
all matters, *you* are the one who is the most in control of your own
safety. A complete understanding of both the risks you take, and the
solutions available to you, are the best tools at your disposal. I bid you
safe journeys.
***WARNING***
This is a work in progress, growing as I gather information from individual
postings by experienced professionals and amateurs in many walks of life,
from all over the globe, and from my own personal experiences. I present it
for personal use and benefit of all who read it and find something of
value. I have nothing to gain from this except the joy of learning itself,
and the satisfaction of helping others. And, of course, it will help me put
up my own tower!
If this information has helped you, I would enjoy receiving a QSL card or
email from you.
Mark, N1LO - n1lo@hotmail.com
Comments, additions, and corrections are welcome.
GUIDES 6
TOWER CONSTRUCTION GUIDES 6
TOWER ENGINEERING GUIDES 6
LIGHTNING PROTECTION AND GROUNDING GUIDES 6
CLIMBING GUIDES 7
TOWER TYPE SELECTION 7
BRACKET SUPPORTS 8
BASES 8
SELECTING BASE TYPE 8
DIGGING THE HOLE 9
THE RE-BAR CAGE 10
THE CONCRETE 11
CONCRETE DO'S AND DON'TS 12
CONCRETE STRENGTH 13
MAST AND BOOM MATERIAL SELECTION 13
PROPERTIES OF MATERIALS 13
STRENGTH 13
STIFFNESS AND ELASTICITY 16
THRUST BEARINGS 19
ANCHORS 19
SOIL MECHANICS PRIMER 19
ANCHOR TYPES 20
SCREW ANCHORS AND STRENGTHS 20
INSTALLATION OF SCREW ANCHORS 21
GUY CABLES 22
GUY CABLES ACT LIKE SPRINGS 22
HOW GUY CABLES STABILIZE YOUR TOWER 22
ORIGIN OF THE 10% PRELOAD RULE 22
EFFECT OF GUY SIZE 23
TYPES 23
USING PREFORMED GUY GRIPS AND THIMBLES 26
TENSIONING 28
GUY TENSION MEASUREMENT 30
USING LOOS BRAND TENSION METERS FOR GUYS 30
CHECKING GUY TENSION BY COUNTING OSCILLATIONS 32
THERMAL EFFECTS ON GUY TENSION 32
MEASURING TOWER PLUMB 33
TEMPORARY GUYING 33
TEMPORARY GUYS FROM EHS 34
TEMPORARY GUYS USING ROPES 34
LIGHTNING ABATEMENT 35
LIGHTNING PROTECTION THEORY 35
HOMEBREW STATIC DISSIPATORS 36
GROUNDING FOR LIGHTNING PROTECTION 37
EXAMPLE TOWER GROUNDING METHOD FOR LIGHTNING PROTECTION 39
GROUNDING GUY CABLES 41
GROUNDING FEEDLINES 41
COMMERCIAL FEEDLINE GROUNDING CLAMPS 43
HOMEBREW FEEDLINE GROUNDING CLAMPS 43
ASSISTING FEEDLINE AND CABLE GROUNDING WITH CHOKE COILS 44
HOMEBREW GROUNDED ENTRANCE PANEL 44
GROUND ROD METAL SELECTION 46
SINKING GROUND RODS 47
IMPROVING GROUND ROD EFFECTIVENESS 47
CLIMBING GEAR 48
CLIMBING BELT 48
SOME THOUGHTS ABOUT FALL ARREST 50
CLIMBING LANYARDS 50
CARABINERS 51
CLIMBING SAFELY WITH HARNESS AND LANYARDS 51
CARRYING TOOLS 52
HOMEBREW TOOL AND PART POUCHES 53
ROPES & KNOTS 53
MAKING YOUR OWN LANYARDS 54
COWTAILS LANYARD 54
POSITIONING LANYARD 55
CORROSION PREVENTION 55
DISSIMILAR METALS AND GALVANIC ACTION 55
CATHODIC PROTECTION 60
PROTECTING ANTENNAS FROM CORROSION 60
PROTECTING THREADED FASTENERS 61
ANTI-SEIZE FOR FASTENERS 61
THREAD LOCKING 62
WATERPROOFING CONNECTIONS 63
ACCESSORY MATERIALS AND SERVICES 64
INSULATING MATERIAL 64
ELECTRICAL TAPE 65
ACCESSORY STEP 65
ROTOR REPAIR 65
FIBERGLASS SPREADER RODS 65
CRIMP-ON PL-259 CONNECTORS 65
PULLEYS 65
TOWER BOLTS 66
TOOL AND PART POUCHES 66
COLD GALVANIZING PAINT 66
ROTATORS 66
SELECTION 66
ROTOR WIRING 67
ROTOR WEIGHT DISTRIBUTION 68
PHYSICAL INSTALLATION TIGHTENING SEQUENCE 68
TROUBLESHOOTING ROTATION PROBLEMS 69
HYGAIN ROTOR PRIMER 69
HYGAIN ROTOR IDENTIFICATION 70
HYGAIN ROTOR LUBRICATION 70
HYGAIN ROTOR INTERNAL WIRING 70
HYGAIN ROTOR TYPICAL ELECTRICAL MEASUREMENTS 71
YAESU ROTOR PROBLEMS 71
ATTACHING COAX AND CONTROL WIRES 73
ROUTING CABLES 73
ATTACHING CABLES TO TOWER 73
FORMING ROTATION LOOPS IN THE COAX 74
MAINTAINING ANTENNA SWITCHBOX RELAYS 74
THRUST BEARINGS 75
ROHN TB3 THRUST BEARING 75
INSPECTING YOUR TOWER 77
INSPECTING NEW TOWER SECTIONS 77
INSPECTING USED TOWER SECTIONS 77
ASSESSING BENDS IN TOWER LEGS 78
CORRECTING MINOR BENDS 78
INSPECTION CHECKLIST FOR GUYED TOWER INSTALLATIONS 79
ASSEMBLING TOWER SECTIONS 82
PRE-ASSEMBLY ON THE GROUND 82
GUY ATTACHMENT POINTS. 82
GUY CABLES 83
TOP SECTION 83
GIN POLES 83
GIN POLE TYPES 83
HOMEBREW GIN POLE MAST 84
GIN POLE ROPE 85
RIGGING THE GIN POLE AND TURNING BLOCK 86
GIN POLE KEEPER LOOP 86
RAISING MASTS 87
CLIMBING MASTS 87
RAISING ANTENNAS 88
CHECKING ANTENNA TUNING BEFORE RAISING 88
TRAMMING 88
REMOVABLE TAG LINES 89
ALIGNING BEAMS 89
MINIMIZING ANTENNA INTERACTION 90
CHILD PROOFING A TOWER/ANTI-CLIMB GUARDS 90
ANTI CLIMB DOCUMENTATION 90
METHODS 90
TOWER STRENGTH INFORMATION 91
CALCULATING WIND LOAD AREA AND WIND LOAD 91
WIND SPEED ZONE 91
CALCULATION METHOD 92
NEWER CALCULATION METHODS 94
ANOTHER TAKE ON EFFECTIVE PROJECTED AREA 97
REFURBISHING USED TOWER 97
TOUCHING UP RUST SPOTS 97
SEPARATING OLD TOWER SECTIONS 98
ADAPTING CATV HARDLINE FOR AMATEUR USE 98
CHOOSING LENGTH 98
HARDLINE CONNECTORS FOR AMATEUR USE 99
WASPS 102
BUILDING YOUR OWN BALUN 102
ATTACHING ELECTRICAL ENCLOSURES TO YOUR TOWER 105
GUIDES
TOWER CONSTRUCTION GUIDES
See the Frequently Asked Questions (FAQ) page for the TowerTalk reflector
at http://www.contesting.com/towertalkfaq.html. This resource is quickly becoming the
internet's Bible on tower construction techniques, and you will also find a
link to the latest version of this document.
Steve, K7LXC, of Champion Radio, sells copies of this catalog and other
publications concerning tower erection, such as "The 10 Most Common Tower
Building Mistakes"
De Steve K7LXC:
On a commercial note, my company - Champion Radio Products,
888-833-3104,(http://www.championradio.com) was formed to provide tools,
equipment and resources for amateur tower and antenna building projects. If
you're interested in a catalog, an SASE to Box 572, Woodinville, WA, 98072,
will get you one. Topics of reprints (they're almost free!) available from
Champion Radio include grounding for amateurs, building a one tower
station, etc.
TOWER ENGINEERING GUIDES
Rohn's consumer catalog is regarded as the engineering Bible to define the
right way to erect a tower, with outlines of height and guying plans to
meet windload criteria. Copies of the latest Rohn color Amateur catalog are
available from Rohn dealers or Champion Radio (888-833-3104,
http://www.championradio.com)
The commercial tower erection standard is EIA/TIA-222-E. This document
contains all of the currently accepted engineering standards, reference
materials, and equations for the safe design of towers. The -222- spec may
be obtained from Global Engineering Documents, 1-800-854-7179 (for those
who need it).
LIGHTNING PROTECTION AND GROUNDING GUIDES
Anyone interested in understanding lightning protection and grounding
systems should get themselves a copy of MIL-HDBK-419A. Some consider this
to be the best single reference source on this subject. This manual is also
available online as an Adobe PDF file.
However, they still permit the download of the document in .PDF format.
This distribution mechanism is still free of charge. And it does not
require any registration number to obtain. Please note that if your browser
does not have the Adobe Acrobat .PDF viewer plug-in installed, you will not
be able to download the information. Here is what I did just a few minutes
ago.
1. Point browser to: This is the Department of
Defense Single Stock Point for Military Specifications, Standards, and
Related Publications web site.
2. Look at the left side of the page. Click on the link titled "ASSIST
Quick Search!" This link requires no registered account or password.
3. In the entry panel titled "Title", type (or paste) the following:
GROUNDING, BONDING. Do not fill in any other panels or add anything to this
one other than what is shown here.
4. Click on the "submit" button. This will return a panel showing several
records found, one of which will be the 419 handbook.
5. Click on the 419 link to go to the profile page for this document and
once there, click the pages icon in the top line where it says "click here
to access document images. Finally, you arrive to the page where the
document is located.
6. Under the "Media" heading in the leftmost position in the revision
history panel, click on the "document image" icon (Acrobat .PDF file icon).
This action will fire up your Acrobat plug-in and begin the handshaking. If
you use IE, right click on the PDF icon and use save as.
7. In the browser pulldown menu area select "file" and then select "save
as". Then configure the save as dialog window to put the file where you
want it and name it like you want it. Click "OK".
8. Go get a cup of coffee. Step 7 is where the file is acutally downloaded
to your box in its complete form.
BTW, the NFPA document is now called NFPA 780. Here is the URL to the spot
on the NFPA site where this reference can be obtained:
http://www.nfpa.org/home.html You will have to follow the links in this
order: 1) On Line catalog, 2) Codes and Standards, 3) NFPA 780: Standard
for the Installation of Lightning Protection Systems, 1997 Edition.
CLIMBING GUIDES
"On Rope", by Padgett & Smith is a truly excellent source of information on
climbing: ropes, knots, gear, techniques. The equipment and techniques
described here adapt very well to tower climbing. The new, second edition
has 380 pages and 700, (repeat: 700!!) drawings. Check it out from your
local library or buy one of your own.
TOWER TYPE SELECTION
For towers up to 40 feet, you can eliminate the expense and work or guying
by using Rohn 45 and bracket mounting it next to a house or garage roof
eave. Rohn 45 has more foot room for standing on rungs. However, it is
heavier and requires more care to assemble. Rohn 25 is much lighter. First
decide what antennas you want to mount and collect all of their weight and
windload information. Then, refer to the Rohn catalog for the allowable
total windload areas of various configurations of tower.
Definitely select Rohn 45 or 55 if you plan large beam antennas in the
future such as 40m beams or multiple stacked beams.
BRACKET SUPPORTS
If you don't have the Rohn catalog, my recommendation is to get it. It has
house bracketed and freestanding specs in it. For example, 50 feet of 25G
bracketed at 36 and 18 feet will take 14.6 square feet of load at 70 MPH;
50 feet of 45G (same brackets) will take 34.8 square feet. Either choice
will give you a respectable load capability and reliability.
BASES
SELECTING BASE TYPE
Most guyed towers are built on top of a concrete base that has a pier pin
or bolt embedded in it, rather than embedding a tower section in the base.
If you use a pier pin, not only do you not have to worry about having the
bottom section plumb, you also achieve the following benefits:
1) You don't have to worry about water in the tower legs, as it will
naturally pour down the legs and out the weep holes in the base plate.
2) You are in essence putting a bit of a shock absorber on the base of the
tower, the tower can turn a bit from side to side to absorb torque in high
winds, resulting in less stress on the bottom section(s) of tower.
3) You don't have to worry about how the tower "short base section" will
interfere with the steel re-bar in the tower base.
The purpose of the base on a guyed tower is two-fold: to keep the tower
from sinking under the dead weight of not only the tower but also the
pressure of the guy wires, and to keep the base from kicking out. A pier
pin/base plate somehow seems easier to deal with than worrying about making
a base section plumb. The only drawback is the requirement of having to
put temporary guys on the first tower section(s) when the tower is being
erected or dismantled.
It is much easier to construct this way, and use a base plate adapter.
This method actually allows the tower to rotate a little on its base which
helps dissipate torsional stresses in a high wind situation rather than
wrenching the bottom of the tower. The weep holes in the base adapter
completely eliminate the potential problem of water collection and
corrosion of the bottom of the legs. Alignment of the tower is much
simplified, although it is more difficult to erect the first few sections,
which will require temporary guying. In the case of Rohn 25G, however, you
may be able to assemble the first three sections with guys on the ground
and then stand it up.
DIGGING THE HOLE
Bases for smaller towers aren't too bad to dig by hand, but this quickly
changes for larger ones, particularly the self-supporting, un-guyed types.
Hiring someone who owns a backhoe and is experienced with it makes all the
difference. An experienced operator can make short work of digging your
hole. It is important that the base be surrounded by undisturbed earth to
help keep it from shifting. Take care not to dig away more dirt than is
necessary to form the sides. If necessary, have the backhoe operator dig to
rough dimensions and touch up the walls by hand.
If you use or contract for a backhoe to dig the holes for your bas and
anchors, plan on their final volume being larger than you expected. Since
the backhoe isn't always precise, you may get larger dimensions. Also, if
dirt falls off of the sides (sloughs) into the hole, which is common in
larger holes, your hole will become bell shaped after you remove the loose
dirt. The end result is that you may end up needing at least 25% more
concrete (or greater) than you originally thought.
Try to avoid using wood forms below grade. Over time, the wood will rot,
and a mushy gap will form around the concrete, reducing its stability. If
your soil is poor enough that you must use forms, remove them after curing
and take care to thoroughly tamp the soil against the concrete to try and
restore some of the stability of the undisturbed soil condition.
THE RE-BAR CAGE
A re-bar cage is required to give your concrete the tensile strength it
needs to support the load of your tower. Re-bar is sized in reference to
1/8" steps in diameter. For instance, #4 re-bar is 1/2" diameter (4/8) and
#6 re-bar is 3/4" (6/8) diameter, etc. Re-bar of any grade should be
adequate for your tower base as long as it is the right size. Here are some
do's and don'ts for re-bar:
1. Welding weakens re-bar. Tie the pieces together with wire ties cut from
steel or copper wire.
2. Keep re-bar away from any outside concrete surface. The purpose of the
cover concrete cover for re-bar is to keep it from corroding. If re-bar
starts to corrode inside of the concrete it will expand and cause spalling
of the concrete. Where concrete is cast against and permanently exposed to
earth (bottom and sides), the cover should be 3" minimum. Where the
concrete is exposed to weather (portion above grade), the cover for bars
larger than No. 6 should be 2" minimum and for No. 5 and smaller it should
be 1.5" minimum.
3. Use a minimum size of #5 re-bar (5/8" diameter).
4. Cut re-bar easily with an abrasive cutoff blade in a grinder or circular
saw.
Three ways of addressing # 2 above are as follows:
a) Support the cage on bricks, broken-up pieces of concrete step stones,
or home-made concrete pedestals to keep re-bar "within" concrete. You can
use the small cardboard tube forms for testing concrete to pour little
pedestals. Alternately, you may be able to buy some 3" 'dobie' blocks with
wires at your local building materials yard. These are 3" square blocks
with tie wires embedded in them made for just this purpose.
b) Pour 3" of concrete first and let it cure a little before placing your
cage, and;
c) Pour most or all of the concrete with the re-bar cage resting on the
bottom and then use hooks to pull the cage up about 3 inches. It will
SLOWLY move with some force. Decent concrete should pass a slump test (and
not be so watery) so that the re-bar won't sink. Just make sure that you
tie the cage together with the appropriate twist tie wires and it will hold
together, allowing the whole thing to be moved upwards.
THE CONCRETE
Mixing concrete your self is a lot of work. One 80 pound bag of Quickrete
will make 2/3 cubic feet of concrete. It takes about 10 minutes to mix one
bag of this in a wheelbarrow and dump it into the base. A tower base
3'x3'x3' is one cubic yard, or 27 cubic feet, requiring 40 bags! That would
be 400 minutes or over 6 hours of mixing by hand! For smaller bases, such
as a 2'x2' diameter base with a 2x2 square top extending 4" above grade,
the volume is only about 8 cubic feet and can easily be mixed up by hand.
For the larger bases, though, it is much easier to order 1 cubic yard of
concrete and use wheelbarrows to shuttle the concrete from the truck to the
pour site. Another option is to rent a concrete mixer on site.
If you have concrete delivered, and the concrete truck cannot get to your
anchor and base excavations, you can set up a brigade with several friends
using multiple wheelbarrows to carry the concrete to the holes. You can
also rent a motorized wheelbarrow, which takes a lot of the strain out of
the job. Alternately, contract for a concrete pumper truck, some of which
can deliver their loads up to 400 feet away from the truck.
You can also purchase dye to color the concrete that will show above grade
to match the landscaping. A company named Colorcrete makes a range of dyes
that you can purchase from your local concrete plant. Colorcrete #CC50,
used at the rate of at least 2 pounds per yard, or 2-4 heaping tablespoons
per 80 lb bag of Quickrete, makes a pleasing, light rusty brown concrete,
similar to the color of exposed aggregate concrete. Quickrete also sells a
limited number of shades at home centers.
For hand mixing, a large wheelbarrow and a hoe will be required. Use about
1-1/2 gallons of water per 80 pound bag of Quickrete. In any case, a
tamping/vibrating tool is a must for flowing the concrete into your form
and around your re-bar cage. For large pours of several yards, you can rent
a flexible concrete vibrating tube to make the concrete flow. For smaller
pours, you can make a manual tool by screwing a 3" diameter disc cut from
1/2" thick scrap plywood to the end of a 6 foot long piece of 1x2 board
stock. The plane of the disk should be at right angles to the length of the
board. When you submerge the disk and shake the stick, the vibrations will
make the concrete flow and level itself nicely. Take care not to use the
vibrator too much or the gravel will begin to sink to the bottom, weakening
the concrete. Use only enough vibration to make the concrete flow and
level.
Setting bolts into existing concrete bases can be done. For minor stuff,
expanding anchors will suffice. Since they'll never come out unless you
chisel out the concrete around them, the stainless steel version is
preferred. Waterproofing hint: put some Araldite (an epoxy glue) in the
hole before setting the anchor. As the anchor expands, it pushes the
Araldite into all the voids that would otherwise retain water & eventually
wick through the concrete. For any serious stuff - the pier pin for Rohn
45 would probably count - use Hilti brand anchors. This company makes a
literally dozens of different anchors for concrete. They have a whole
series of chemical anchors, which would be perfect for the application.
They come with either a plated or stainless steel threaded rod & a glass
epoxy-filled cartridge. Drill the hole, blow out the dust, drop in the
cartridge, drive the rod in with a hammer to break the cartridge, attach
drill chuck to end of rod & drive it home (which mixes the epoxy at the
same time). About 20 minutes later, you have an anchor that is stronger
than the concrete it's set in.
CONCRETE DO'S AND DON'TS
Concrete continues to gain strength as long as it stays moist. Concrete
does not "dry," it undergoes a reaction called hydration, which requires
water. The longer you can keep the concrete moist, the longer it hydrates,
and the stronger it gets. If it dries, then the reaction stops and it stops
gaining compressive strength.
Concrete gains strength with decreasing momentum, i.e. most of it's
strength is gained early on in the curing period. If you have a proper mix
of "Cement", sand and gravel and not too much water (this is a strength
killer) the majority of the strength will occur in the first ten days or
so. The consensus seems to be to wait at least 7-10 days before putting
stress on the concrete.
Here is a guide for concrete strength versus curing time taken from a civil
engineering handbook. The percentages are of the concrete's normal rated
strength, and apply only as long as the concrete is still moist and
hydrating!:
Days Strength
3 25%
7 60%
28 100%
90 120%
180 125%
Strength Killers:
1. Sun beating on freshly poured concrete. Keep it covered with wet straw
(or old wet rug) plus plastic or tar paper.
2. Excessive heat. Don't pour concrete when the temperature is high.
3. Pouring concrete into a hole that is dry. Wet the bottom and sides of
the hole prior to pouring concrete otherwise the dry soil will suck the
water out of the concrete and you will surely have a weak mix when it
cures.
4. Stressing the fresh concrete by rocking the tower base or premature
assembly and climbing.
5. Letting the surface get dry while it is curing. Give it a spray with
water as often as possible to keep it wet because it WON'T CURE IF IT GETS
DRY, IT WILL ONLY GET DRY.
CONCRETE STRENGTH
ULTIMATE STENGTH (these materials just break without yielding - brittle)
Bricks, common light red - 40 (tension), 1,000 (compression)
Portland Cement, 1 month old - 400 (tension), 2,000 (compression)
Portland Cement, 1 year old - 500 (tension), 3,000 (compression)
Portland Concrete, 1 month old - 200 (tension), 1,000 (compression)
Portland Concrete, 1 year old - 400 (tension), 2,000 (compression)
Granite - 700 (tension), 19,000 (compression)
Note the difference in tension and compression for the rock-types. This is
why re-bar is used in concrete, to add tensile strength for a better
composite building material. Fiberglass is another example of this. The
resin has compressive strength and the cloth has the tensile strength.
The tensile and compressive strengths of metals are much more evenly
matched, but can still vary.
Take care to level the surface of the base before the concrete cures. The
plumb of the base section of your tower will partly depend on how well you
do this. Also, the compressive loading of the tower legs will be better
equalized with a flatter, level base top. Your re-bar cage should not touch
the pier pin and should not come within 3 inches of any surface of the
concrete.
For the embedded tower section method only, your foundation will have a
hump that causes water to run away from the legs. No cavities in the
foundation near the legs for water (or mud!) to accumulate. Foundations
should rise several inches above the surrounding soil so that mud cannot
wash onto the foundation and accumulate. After the concrete has fully cured
for at least 30 days, seal the exposed concrete with "wet-or-dry" asphalt
roof cement - it's asphalt with encapsulated asbestos, and appears to drive
out or absorb water at the adhesion surface with tin roofs, concrete, you
name it. Cost is good too - only about $4 per gallon!
MAST AND BOOM MATERIAL SELECTION
DO NOT use common water pipe unless the mast will only extend a few feet
above the last tower section.
Use "structural tubing" instead of "pipe" for strength and known strength
properties, and buy it new for assurance of its properties.
PROPERTIES OF MATERIALS
STRENGTH
Rohn offers a 2" x 10' High strength galvanized steel mast, Part Number is
M200H I believe ... when I checked with them years ago, the spec was that
this is a 50,000 psi mast. It is VERY heavy, I think the wall thickness is
0.125"
The strength of a mast, or any metal part, for that matter, is highly
dependent on the composition of the metal and its treatment, resulting in a
specific yield stress value. The yield strength of a material is the
stress, expressed in pounds per square inch (psi), at which a material
begins to deform permanently, resulting in some sort of lasting change of
shape after the stress is removed.
The ultimate strength, usually somewhat higher, is that where the material
has already yielded, and stretched or bent, and finally breaks. You
generally want to design things to stay below 50-67% of the yield strength
of the material (safety factor between 1.5 to 2). The translation of the
stress level to the actual allowable loads on the part in question, and
vice-versa, is the tricky part that requires an analysis of the geometry
and math. Those calculations can get hairy!
The strength of metals varies greatly with the method of their manufacture
and composition.
1) Quenching, cooling very rapidly from a glowing hot temperature, can
dramatically increase the hardness, but introduces brittleness.
2) Tempering, re-heating to a lower temperature followed by a slow cooling,
'draws' the hardness back down, reducing the brittleness and adding some
toughness.
3) Annealing, heating to a high (glowing) temperature and allowing to cool
slowly, softens a metal, reducing hardness and adding considerable
ductility (ability to be bent and formed).
4) Cold working, when parts are bent, mashed, drawn, hammered (wrought),
flattened, etc, by machine or by hand, causes the hardness and strength to
go up somewhat.
5) Repetitive bending causes fatigue and drastic strength loss. This, in
turn, can further cause your wallet and credibility to vaporize if you have
not accounted for it!
Different metals and alloys of the same metal respond very differently to
these treatments. It gets complicated!
It depends on the alloy *and* the treatment.
The point to remember is that identifying the type of metal is *far*
different from knowing its actual strength. The advice of not using a pipe
or tube of unknown origin for a mast is good because even though you may
know that it is steel or aluminum, you still don't know it's properties
unless you bought it from a manufacturer or reseller or have it tested.
Of course, you can always count on minimum strength values for types of
metals, with the knowledge that it may still be much stronger.
Hardness has an excellent correlation with the strength of the metal. The
harder it is to prick with a center punch, the higher its yield strength.
Again, It depends on the alloy *and* the treatment.
Here is some data from Machinery's Handbook, 23rd edition:
SOME REPRESENTATIVE YIELD STRENGTHS
Aluminum, 6061-O (fully annealed) - 8,000 (surprise!)
Copper, annealed (soft) - 10,000 psi
Brass, cast - 12-15,000
Aluminum, 6061-T4 - 24,000 (surprise!)
Wrought iron - 23,000 to 32,000 psi
Wrought Steel (water pipe) - 23,000-32,000
Steel, stainless, 304L & 316L, annealed, 30,000 psi (at its softest)
Steel, common structural (I-beams, etc) - at least 33,000
Steel, stainless, 304 & 316, annealed, 35,000 psi (at its softest)
Aluminum, 6061-T6 - 40,000 (surprise! - it's the treatment)
Copper, wrought, up to 53,000 (what the book says!)
Brass, wrought - up to 62,000 (what the book says)
Steel, 1025 low carbon (cheap fasteners) - 50,000
Steel, 1050, quenched and tempered "typical" - 95,000
Tool steel, 4140, quenched, tempered to 1200F - 95,000 (tough)
Steel, Stainless, 316, tempered and work hardened, up to 100,000
Tool steel, 4140, quenched, tempered to 400F - 238,000 (!) (brittle)
(4140 is commonly referred to as ?chrome-moly steel?)
Ok, you see that the alloy and the treatment affect the properties.
Be very careful to know what the alloy is and what the heat treatment is.
The little "T6" behind the 6061 aluminum is easy to overlook but is SOOOO
important.
By the way, 6061-T6 is one of the most common structural aluminums.
4140 is but one of many, many tool steel alloys.
SIZE DATA FOR WATER PIPE (INCHES) (count on about 20,000 psi yield)
SIZE, SCHEDULE, ID, OD, WALL THK
1.25, 40, 1.380, 1.660, .140
1.25, 80, 1.278, 1.660, .191
1.50, 40, 1.610, 1.900, .145
1.50, 80, 1.500, 1.900, .200
2.00, 40, 2.067, 2.375, .154
2.00, 80, 1.939, 2.375, .218
I don't know if the MARC program accepts input of yield strength
information for materials and independent sizes in the calculation of mast
strengths, but this data along with the size data will tell you
*approximately* what a mast will take, provided you *know the alloy and the
treatment*
If in doubt, go and buy something of known properties.
STIFFNESS AND ELASTICITY
SPRINGINESS OF MATERIALS
Greetings from Virginia's Middle Peninsula,
At the risk of boring some, I will make an attempt to describe and quantify
'stretching' for those who are interested. Forgive me if the majority have
no interest in this level of detail or consider the topic already beaten to
death. Reviewing it sure helps me, anyway.
The phenomenon that is being described, "stretch", is elastic deformation
(also deflection), a temporary change of shape that makes a material act
like a spring. Materials can stretch elastically (temporary), plastically
(permanent), or a combination of both, in any direction.
Just about any part will act like a spring under certain conditions. When a
load is applied to a part, it moves a little (deformation). Strain is
actually defined as the amount of movement per unit length of the part. If
its yield stress was not exceeded, it moves back to its original shape, and
that is called elastic deformation (deflection). In this manner, a part
(such as a boom, mast, or guy wire) acts like a spring. If it deflects too
far and its yield stress was exceeded, however, it may move back toward its
original shape, but it will retain some amount of permanent change of shape
(elastic deformation + plastic deformation) and the material suffers damage
in the form of a permanent bend.
We want to avoid the permanent, plastic deformation! We design parts to be
strong enough so that they don't break (yield stress is not exceeded).
However, and this is the point: Just because a part won't break does *not*
mean that it will not bend elastically and be quite springy! And sometimes
more than you intended! Parts have to be designed to control their
deflection (related to springiness) as well as their ultimate strength.
Who else has tried to straighten some wire from a spool or your whip
antenna? (who else has had to use their 2m whip to unlock their car door?
) You have to bend it way back in the opposite direction (elastic),
and then carefully a little more (exceeding the part's yield stress) to get
the right amount of permanent bend (plastic) so that when you let go it has
the shape you want.
The relationship between the size of the load and the amount of deflection
(elastic movement) is controlled by the size and shape of the part and the
"Modulus of Elasticity" (modulus for short) of the material that the part
is made from. Just as the yield stresses can vary for different materials,
the modulus is also dependent on the type of material. The higher the
modulus, the *less* a part will change shape elastically. The modulus of
steels is well known, and varies very little for different steels. I don't
have data for aramid fiber. Perhaps Kurt can find this or someone will
contact Phillystran's manufacturer for this data.
Fiberglass is a composite material, and has a wildly different modulus
depending on the direction in which the load is applied compared to how the
glass strands are oriented. Quad spreaders are quite elastic in bending,
but much stiffer in tension. I have no data for these materials. Now we're
really getting complicated!
We tend to think of wire cables as fixed in length, but they will deform
with a load, and we hope they will always be elastic deformations!
A straight, solid rod is easy to analyze for strain. As you can imagine, a
lot of force is required to make it change length (high spring constant).
Plain, straight rod makes a crummy extension spring, but a spring
nonetheless! However, if you coil it, the stress is applied in a different
way, and there's much, much more length of wire per unit of length. When
you pull on a coil spring, you are actually causing the wire to twist in
torsion rather than just extend in length. You can get a lot more elastic
movement from this shape without exceeding the yield stress (lower spring
constant).
Think about the shape of a piece of EHS guy wire. Its strands are twisted
into a gentle spiral. Nothing like a coil extension spring, but some small
amount of torsional loading will occur, slightly increasing the overall
deflection/change in length. Also, there is less cross-sectional area of
steel as compared to a solid rod of the same diameter, also increasing the
deformation.
Now, for those of you who haven't hit delete yet, and without dragging you
through too much more mumbo-jumbo, here are a few numbers to give you a
feel for the amount of movement we're talking about:
EXAMPLE: 100 feet of *solid* steel cable, with a 400 pound tension:
DIA, TENSILE STRESS, TOTAL CHANGE IN LENGTH
1/8, 32,600 psi, 1.3 inches
3/16, 14,600 psi, 0.58 inches
1/4, 8148 psi, 0.33 inches
You see how using a part that is way oversized for stress alone helps
control deflection/deformation/'springiness'
I don't have data in my handbooks for the modulus of wire rope in tension,
but the above numbers should be a good starting point. I would venture a
guess as 10% more for EHS.
This means that when you pre-load your 3/16 guys, for example, they will
stretch elastically somewhere around 3/4 inch, I'd say, just due to the
change in length. Something else happens, too. Guy wires have droop, or
sag, which, due to gravity, requires more length of cable between two
points because it's not in a perfectly straight line. This introduces yet
another potential for elastic change in length. Let's now guess about 1
inch of total change in length for our 3/16 cable. Once the sag is pulled
out of your guy, not too many turns of your turnbuckle are needed to raise
the tension!
Reducing sag and the spring effect it introduces is another reason for
proper pre-loading of guys. Bigger guys are heavier, will sag more, and
will require more pre-load. It seems to me now, after thinking about all
that I have learned about towers here on the reflector, that the 10% of
breaking strength rule of thumb helps out here.
OK, long again as usual... for those of you who are still reading... What
happens to the tension in guys and to the movement of the tower when the
wind blows on it?
As the wind forces build, the tower moves a little. This movement stretches
the upwind guy(s) elastically, adding to the pre-load tension on the upwind
guys and resisting the movement. However, the downwind guy(s) will
*release* their pre-load and lose tension, also resisting the movement. In
this system, the guy forces react synergistically to hold the tower closer
to, but not exactly in its original position. The more the guys act like
springs the more the tower will move in the wind. The more the tower moves,
the more fatal bending moment will be applied to the tower section.
Therefore (I must be getting toward the end), larger guys made from
materials with a greater modulus will control your tower better and keep
the bending forces lower.
Thank you for the bandwidth. This post turned out longer than I wanted, but
I hope it helps someone understand some of the engineering and materials a
little better. And if so, then they will build their towers more safely.
Hi Mark, good explanation! I found some info on the aramid cable. A rigging
supplier in Portsmouth, RI provided the data. They specialize in marine
rigging
For the sake of others who haven't been exposed to modulus of elasticity
values, here is a list: Note: Msi stands for millions of Psi:
Fiberglass 3.5- 4.0 Msi, (epoxy/e-glass Mil Spec G-10 material)
Aluminum 10 Msi 9(common 6061 & 6063 alloys)
Aramid fiber 18 Msi (Kevlar 49 used for most aramid guying cable -
Phillystran)
Aramid fiber 25 Msi (Kevlar 149. I only found listings for 8600Lb - 32500
Lb. cable)
Steel 29 Msi (commonly used steels mild, chrome-moly, and stainless)
I think the bulk of aramid cable sold to amateurs is Kevlar 49, the 149 is
more expensive, but it can be had.
THRUST BEARINGS
Use two separate thrust bearings, one on the top plate, and one below the
top plate, only when you have a long mast that you need to keep steady.
This way a rotor can be made removable without making the mast unsteady or
unsupported. Leave the lower bearing loose while the rotor is in place.
This is important because it is extremely difficult to line up both
bearings AND the rotator without having the mast bind up somewhere. Rotors
are designed to hold the vertical weight of a mast, and that weight helps
the races in the rotor wear evenly. Raise the mast and tighten the lower
bearing only when you need to remove the rotor for servicing. However, if
you have a long, very heavy mast, you could tighten both bearings to
support the entire mast and use a short connecting section with a flexible
coupling between the rotor and bottom of the mast. All rotors will
eventually need service and this scheme makes maintenance easy.
The Rohn TB-3 has aluminum races. It does not have to be packed with grease
to extend its life.
The most secure way to support a heavy mast with antennas is to place a
muffler clamp around the mast just above the bearing and use the bolts of
the bearing mostly as guides to center and snug the mast. Over tightening
of these bolts can flare their tips, making them impossible to remove from
the bearing.
PROTECTING THRUST BEARINGS FROM SNOW AND ICE
Are you concerned about accumulation of ice on your thrust bearing or
pointy top? This is not a problem with all types of installations, but some
almost encourage water and ice to build up. One way to add protection is to
mount a rubber sewer fitting on the mast with a hose clamp. The fitting in
question adapts a small size pipe (your mast in this case) to a much larger
size pipe (the area you are trying to protect in umbrella fashion, in this
case). Water runs away from the thrust bearing or pointy top with this
arrangement. Works slick. Looks neater if you think about it before you
install the mast, but it can even be installed after everything is in the
air.... just cut the adapter with a pocket knife and seal it back up with
RTV once it's fitted over the mast. One manufacturer of these rubber
fittings is "Furnco", and some people refer to these fittings as
"Furnco's."
ANCHORS
SOIL MECHANICS PRIMER
The 400 psf/ft of depth figure (pounds per square foot of anchor area per
foot of depth buried below the surface), mentioned in the Rohn drawings for
"normal soil," is for lateral bearing of guy anchors. The value does not
contain the required factor of safety of 2. If you work in allowable
stresses you end up with 200 psf/ft of depth with a maximum of 2000 psf/ft.
Now you need to know the depth and thickness of your anchor.
For towers having anchors less that 10 feet deep, start by finding the
depth to the center of the anchor block and call it D. Next, multiply D by
200 psf/ft and you will get the allowable lateral bearing pressure for the
foundation, called Q. The lateral side bearing area of your anchor,
multiplied by Q, must be greater than the horizontal load for the guy
anchor to prevent anchor pullout.
Now for your question of "What is normal soil?" It is a cohesive soil with
no water (water table below foundation depth). What is cohesive? Soils are
classified by their grain size. In layman's terms -- there are boulders,
gravels, sands, silts, and clays (large to small). Solid rock has its own
system with RQD's and other properties (another subject). Soils come in
various mixtures and have two major properties -- angle of internal
friction (phi) and cohesion (C). To simplify - pure sands have phi and pure
clays have C. This is definitely an oversimplification! Soils are hardly
ever just one type, so most soils are classified according to charts rating
their grain size. This test can be done in the laboratory with a grain size
analysis (a series of various size screens) or it can be done by hand and
"feeling" the soil.
A sand will feel gritty and a clay will feel smooth. A silt is in between
and can fall either way. Silts are the most difficult to classify. There
are some beach sands that are classified as silts and have phi angles and
some silts are hard as clays. Hard silts will lose strength when wetted and
clays don't. Now what is Normal Soil? Normal soil is a cohesive soil -
normally a clay but could be a silt. To make a comparison - take Q from
above and if it greater than C (cohesion) it meets or exceed the normal
soil parameters. C can be measured by various means. The laboratory test -
unconfied/2, The standard penetration test (N /8), The pocket penetrometer
test /3. There is even a system of estimating C using your thumb nail. The
answer to which test to use depends on your available equipment and
experience.
ANCHOR TYPES
There are many types of earth anchors and their strength depends on the
type of soil they are installed in. You must determine the type of soil you
have to determine the pullout rating of an anchor. The anchors are
critical. They are truly the only thing that keeps a tower in the air. When
you lose a guy, you lose your tower.
The concrete type anchors specified by Rohn in their catalog have greater
holding power than the screw type anchors, but they require more effort and
cost to construct.
SCREW ANCHORS AND STRENGTHS
Some screw-type earth anchor information available from one of the largest
manufacturers, AB Chance Co. @ http://www.hubbell.com/abchance. The
Virginia distributor is JA Walder, P.O. Box 1272, Ashland, VA 23005. Their
website is http://www.walder.com, email is email@walder.com, and their
telephone is 1-800-335-3605.
The pullout strength of anchors is highly dependent on the properties of
the soil. Here are some pullout strengths for AB Chance screw-anchor models
for 'NORMAL SOIL'. I don't know how to adjust the ratings exactly
for other types.....(* indicates galvanized). Soils with clay will provide
more pullout strength. Softer soils that have more sand and loam, or that
become saturated with water during season rainfall will have much less
holding power.
MODEL # SCREW DIA SHAFT LENGTH PULLOUT PRICE
315SA 3 IN 15 IN 200 LB $5.25
330SA 3 IN 30 IN 1400 LB $6.00
430SA 4 IN 30 IN 2500 LB $7.50
404 4 IN 40 IN 3000 LB $12.90
604 6 IN 48 IN 4000 LB $15.24
*4345 4 IN 54 IN 3000 LB $26.76
*6346 6 IN 66 IN 4500 LB $34.08
*816 8 IN 66 IN 10000 LB $52.86
Clearly, there is a relationship between the screw diameter, depth, and the
pullout strength. For a 100' or taller tower, screw anchors should be down
about 6 feet, and have a minimum 6" diameter screw. You can get 6 ft.
anchors from a cable tv supply company. An 8 inch model has nearly enough
reserve strength for the full breaking strength of 3 3/16" guys (12000 lb).
It may also be possible to buy ground anchors from the local electrical
power company.
INSTALLATION OF SCREW ANCHORS
My first set I screwed into the ground (clay!)... enough of that
nonsense...since then I use a post hole digger (power where possible, by
hand elsewhere)... I drop a half bag of Quikcrete down the hole, then screw
the auger down the hole until it bottoms, and backfill... pull tests with
my backhoe have convinced me that the guy wires (4000#) will part long
before the anchor fails...
Water softening the ground will help if you insist upon doing them the hard
way. Don't waste too much water, the ground can only soak up so much. A 2-3
day rain tends to soften up the ground considerably. After this you may
have to try the anchor at different angles until it grabs, then slowly
start tilting the anchor back toward the tower at the average angle of the
guy cables. I would wait a week after installing to allow the ground to dry
before putting any load on the anchor in that case. It may be easier to dig
part way down with post hole diggers to help set the screw. Then back fill
the hole, tamping firmly every 6 inches. Another trick to help start these
is to have someone pound the end with a sledgehammer while two others turn
it using a 5 foot section of 1" pipe for a torque bar.
GUY CABLES
GUY CABLES ACT LIKE SPRINGS
Refer also to the above section on springiness of materials. Guy cables
that are not perfectly vertical act like extension springs in two ways:
Mode 1) They change length relatively easily without significant elastic
stretching as the droop in them is pulled tight, resulting in a very low
spring rate until all the slack is pulled out, as you approach the
proverbial "straight line between two points."
Mode 2) Once they are tight, they can still change length mostly by
stretching elastically, although only with much larger changes in tension
(much larger spring rate)
HOW GUY CABLES STABILIZE YOUR TOWER
So....before a guy wire can really do its thing, which is to keep the tower
legs from moving, (ideally), it must pull tight for upwind guys or already
be tight for downwind guys. Upwind guys will increase their tension, and
downwind guys will release their tension to balance the forces (of wind,
let's say) that are trying to move the tower. But because of the elasticity
effect, the tower *must move* first to reach a new equilibrium. It flexes.
ORIGIN OF THE 10% PRELOAD RULE
How do you decide when you have pulled all the slack out? Thanks to
gravity, it is very difficult to get the guy wire to be a perfectly
straight line unless it is vertical. There will always be a "catenary"
curve in it that includes excess slack, even when the cable is pulled well
beyond 10% of its breaking strength. Well, at some point, you have to pull
it so tight that the tension starts to make the guy wire stretch
elastically (going from mode 1 to mode 2 above.) And the cable still isn't
perfectly straight. I believe that the 10% of breaking strength rule has
been worked out to where, for the weight of a typical cable, practically
all the slack has been pulled out, putting the cable into mode 2 as
described above. If you preload your wire with much more tension, you are
simply reducing its ability to absorb additional load from wind before you
reach its breaking strength. However, (don't we always run into these), if
you reduce your guy anchor spacing from the base below the 80% of tower
height, then an increase of guy preload to 15% of breaking strength (600 lb
for 3/16 EHS) helps compensate and control tower flex without cutting too
far into your reserve cable strength.
EFFECT OF GUY SIZE
Another factor: the larger the diameter of the guy, for the same material,
the higher will be its spring rate, and the better it can resist a change
in length (and movement of your tower) for the same loading force. Since it
is heavier, it requires more preload tension to pull out the slack. Thus,
the 10% rule keeps up with things. Using a thicker guy gives you more
control over the flexing of your tower since it has a much higher spring
rate, and much larger forces are required to make it change length. If you
play a stringed instrument, you can see this effect when you change, say,
from extra-light gauge strings to medium gauge. It's a lot tougher on your
fingers to fret them!
TYPES
3/16" type EHS galvanized wire works well. Preforms, or "dead ends" are the
most reliable and easiest way to terminate the guys. Old type cable clamps
are cheaper but tend to loosen with age as the joint forms itself to the
cable after tightening. Use thimbles at ALL terminations. Although the
tower leg gives you a nice convenient radius for the preforms, this
technique does nothing for the wind-induced torque that will try to twist
your tower down. This is the function of the guy assemblies; to add torque
resistance. There is a specified size of thimble for each part of the guy
wire system; i.e. a 3/16" preform grip takes a 7/16 to 3/8 inch thimble.
BTW, the 'seat diameter' (which is the distance/radius required) for a
3/16" preform is one inch minimum. Since 25G is 1.25 inch OD, it does give
an acceptable seat diameter for installing the preform grip directly on the
leg. A 1/4" EHS guy cable takes a 1/2" thimble to properly match the bend
radius. Have you ever seen a 1/4" thimble floating inside the loop of a
1/4" guy grip? The mismatch makes the thimble essentially useless.
Moving along, you'll need to use thimbles when using the guy assemblies but
they are smaller diameter than the legs and you shouldn't have as much
trouble getting the thimbles over them. There are different kinds of
thimbles. Many are teardrop-shaped; these are the ones that you'll have to
open up when installing them. Check with your local suppliers; there are
also thimbles that are U-shaped with enough clearance in the mouth that you
should have a minimum of fuss installing them. Yes, the whole process is
tedious but just think how well you'll sleep nights knowing you did
everything correctly. You can easily bend the thimbles open using two
adjustable wrenches. Place one on each free end of the thimble and bend
them away from each other, perpendicular to the plane of the thimble's
curve. What you end up with is sort of a spiral, creating a large opening
in the thimble end.
3/8" EHS guy cable (7 strands, 0.138" dia each) is rated at 15,400#
breaking strength, 5/16" EHS guy cable (7 x 0.104") is rated at 11,200#.
1/4" EHS (7 x 0.080) is rated at 6600#. 3/16 EHS (7 x 0.063") is rated at
3990# strength. This material is called galvanized guy strand, and should
be made to ASTM spec # A475-78, grade EHS. Back calculating from these
numbers, the steel in this case, has an ultimate tensile strength of about
185,000 psi, except for the 3/8", which works out to about 145,000 psi. The
yield strengths would be about 3/4 of these values.
Phillystran is a non-conducting guy wire material made out of aramid fiber
and is like Kevlar - strong and lightweight. It comes in different
diameters and strengths. It appears Texas Towers may be the only
Phillystran supplier in the ham market.
HPTG1200I - 1200 pound strength (545 kg), .19 inch diameter (4.8 mm)
HPTG2100I - 2100 pound (953 kg), .24 inch dia., (6.1 mm)
HPTG4000I - 4000 pound (1816 kg), .30 inch dia., (7.6 mm)
HPTG6700I - 6700 pound (3,042 kg), .37 inch dia., (9.4 mm)
Phillystran consists of a Kevlar (aramid fiber) fiber core and a PVC
jacket. The purposes of the jacket are: 1) to protect the cable from
abrasion during installation, 2) to prevent moisture from wicking into the
core and 3) most importantly, to protect the core from UV damage.
Phillystran has only been manufactured since 1974 so thus far the longest
service life of the product has been 23 years. No one knows how long it'll
last past that because it hasn't been around long enough. 40-50 years?
Maybe. The service life may also be related to the region and environment;
the more UV, heat, wind, etc. may have an impact on how long it retains its
characteristics. Some of the oldest Phillystran (with the old jacket
material) is still being used in southern Florida. In places the jacket has
disappeared and the core is out in the Florida sun. After 23 years in this
worst case scenario, it still retains 75% of its rated tensile strength.
Bottom line? It'll last a long time and is a worthwhile investment
particularly if you're planning on having a populated tower/antenna system
and want to minimize any potential interaction problems. You need to use
factory preformed guy grips for the bigger sizes but for boom trusses using
the smallest size, cable clamps are okay. It is a commercial product and
its PVC jacket gives abrasion and UV resistance so that its service life is
probably 20 years or longer. Phillystran can be terminated with a special
preformed grip made by Preformed Products.
Pultruded fiberglass rod has been proposed by some for use as guy material.
The elastic modulus is 4.83 times less than steel, however, meaning that it
is much more elastic. In order to have the same spring rate as steel guys,
and therefore the same ability to stabilize your tower in the wind, the
cross-sectional area of the fiberglass must be 4.83 times that of the
steel. This works out pretty close to double the EHS diameter when you
account for the stranding. So, the equivalent solid fiberglass rod diameter
would be twice the EHS size you want to replace. With this rule of thumb,
3/8" solid, pultruded fiberglass rod makes a good substitute for 3/16" EHS
guy cable.
EHS guy wire has a different twist than wire rope, and requires preformed
grips made for that specific type of wire.
Using Rohn's guy assemblies (not torque arms) allows a secure attachment
for the guys (as opposed, for example, having them looped around the legs
where you have the forces being held by the diagonal welds). Imagine a
force big enough to pull the leg out, bending it and possibly breaking a
weld or two. This would not happen with a guy assembly. The guy assemblies
allow the forces to be spread across the faces of the tower instead of just
a leg. (See 'big force' above.) The guy assembly allows Rohn towers to
comply with the TIA-222-E structural tower standards. Lack of guy assembly
makes them non-compliant and probably has an impact on their rated wind
load figures as well. Rohn came out with the current product to upgrade to
one of the recent TIA-222 revisions. When they did their calculations, they
found the "old" torque arms really didn't contribute anything to the
torsional resistance of the tower. What they did do was keep the twist down
as it was being climbed. They discontinued the old product but hams put up
such a fuss that they re-introduced them. If you use the current guy
attachments and tension the guy wires properly, there's not much need to
have the old torque arms installed.
Although it entails two more guy ends and hence ups the cost per guy it
seems like the EHS pigtail at both ends of a Phillystran guy is a nice
inexpensive comfort margin. I am thinking in terms of tool buckets,
climbing belts and the like bouncing off Philly as a potential source of
problem, if you bump into EHS who cares!
As far as down below and sizes that fit, etc.....what we use at the
equalizer plate IS the turnbuckle. The lower end eye of the turnbuckle is
sandwiched between the equalizer plates.
This is nice in that it "freezes" the lower end of the turnbuckle from
rotation. When you are done tensioning everything you just need to "freeze"
the top half of the turnbuckle and the center of the turnbuckle which can
be accomplished with a single loop of cable through those two. Watch the
combination of bolt size that the equalizer plate uses with eye size on the
turnbuckle, make sure the eye is big enough to handle that diameter bolt.
This will present the top end of the turnbuckle as what you need to
actually connect the guy to. A simple thimble through the eye of the
turnbuckle (I prefer the eye type over the jaw type as there is fewer
things to go wrong [K.I.S.S.]) and a preform and you are good to go!
Cutting EHS cable can be difficult without bolt cutters. One method is to
use a hand grinder. Tape the place where you want to make the cut and it'll
zing through it in less than 10 seconds. The tape keeps the strands from
unraveling after its cut. If you don't have a hand grinder, you can use a
steel-cutting aggregate blade in your skill saw. They only run four or five
bucks. BE REAL CAREFUL - use eye protection when doing this because you'll
be throwing sparks all over the place. This method also works for cutting
concrete rebar. Another method is to use a steel "cold chisel." Place the
guy wire across any metal surface (metal that is softer than the chisel!),
put the chisel on the wire and strike with a 2 lb. or bigger hammer. Wear
safety glasses!
USING PREFORMED GUY GRIPS AND THIMBLES
Please be aware that Rohn recommends only the "Big Grip" series of guy
grips for tower installation. The regular grips sold by most supply houses
are for utilty pole guying applications and much shorter then the Big
Grips. There are even some brands of utility type grips with only three
strands of steel instead of the four in a Big Grip. Grips are also called
`preforms', since the are literally pre-formed for a thimble and spiraled
ends for wrapping.
Preforms are color-coded for EHS wire sizes. The coding is:
1/8" Blue
3/16" Red
1/4" Yellow
9/32" Blue
5/16" Black
3/8" Orange
The colored marks on the grip has two functions: 1) they indicates size
and 2) they mark the cross-over spot which is where you start wrapping the
legs around the cable. There are two crossover paint marks - the first is
for normal cable and the second (farthest from the termination loop) is for
use with insulators. The end of the cable of interest should extend at
least to the crossover marks. If they are longer and extend past the marks,
it is of no consequence. Wrap the first leg (either one) at least 2 wraps
around the cable. Then wrap the second one insuring that the wrap starts at
the crossover mark. Continue wrapping one leg until it is about 3/4 done,
then wrap the other leg to that point. Finish the short leg first, then the
longer leg. The grips are designed to be installed by hand so you may need
to bend the cable in order to seat the ends. Do not completely apply the
wrap - leave the last inch un-snapped into place until you are sure and all
your lengths have been adjusted. Trying to unwrap them (as you probably
have found) is a LOT easier is you don't have to use the pliers to get at
an end....oh, and one other thing...watch out for the loose flesh of your
finger tips and palms as - they can be pinched as the wrap snaps into place
creating some massive blood blisters! Attach a black tie wrap or end sleeve
to the end of the grip to prevent it from unraveling. Repeat as many times
as necessary.
The preformed grips can be removed and reapplied twice if necessary to
readjust guy wires. If removal is necessary after a guy grip has been
installed for a period longer than three months, it must be replaced. No
thimbles are needed when using an insulator. But be sure to use the SECOND
crossover paint mark when installing a preformed grip through an insulator.
If you use the first mark on insulators, the grip has too acute an angle
and puts more strain on it.
When you are ready to set the grip completely, use a flat sided that has a
square shank. Installation is much easier if you install the short leg
first, followed by the longer leg. The legs are different lengths to ease
installation. When you use the screwdriver to wrap that last turn onto the
EHS you should have the tip of the screwdriver bearing on the cable, NOT
the free end of the grip. You can do it either way, but when the tip bears
on the central EHS you cause the grip to follow the guy wire, and voila -
success. By putting the screwdrivers tip on the end of the guy grip you
will get that sloppy result where you have to push on the grip to get it to
seat properly.
Putting Preformed grips on Phillystran isn't as easy as installing them on
stiff EHS. I've found the best thing to do is to do only 3-4 inches or two
complete wraps at a time on both legs. It keeps the Phillystran from
bunching up which necessitates un-doing and re-doing it. The first leg goes
on with no problem. The thing that works the best on the second leg is to
push/bend the leg that you're winding on slightly towards the loop in the
grip. That is, pull the two legs apart as you're winding the second one
while you're turning it on and it should work better. If possible, put some
tension on it after the first few wraps.
The local tower guru also strongly recommends installing the metal "ice
sleeves" on the upper end of any grip that's installed so that water can
run down into the end of the wrap. Apparently the freezing of the water can
exert enough force to begin a sequential unwrapping (gets worse with each
freeze). They only cost a few cents apiece and drove readily on the end.
Tape wraps about the end of downward-pointing guy grips may not be the best
way to do this. On the upper end of the guys the tape creates a place for
water running down to accumulate, and stand, causing the guy wire to rust.
Preformed has what they call an end sleeve but is commonly referred to as
an ice cap. It is a small tapered sleeve where the little end is on the guy
wire and the bigger end goes over the end of the Preform grip. They're easy
to apply and their purpose is just what you described - that is, to keep
ice from getting under the grip strand ends and lifting (and unwinding)
them. There are other products (the Ice Knocker comes to mind) that are
bigger cones/devices that go over the guy wire and are supposed to shed any
ice coming down the guywire. You can also install cable clamps over the
ends of the grips to keep them from unwinding.
The purpose of the thimble is to keep a constant radius on the
termination. An insulator already has adequate 'seats' built-in and doesn?t
need a thimble. You won't find thimbles big enough to fit through the
insulators anyway so don't worry about it.
To clarify some confusion on thimble sizes:
1)If cable clamps are used to terminate guy wire, the thimble size is 1/16"
larger than the wire.
2) If preformed grips are used, the thimble size is 1/8" to 1/4" larger
than the wire. Pick the thimble that best matches the curve in the grip.
3) If 4000lb Phillystran is used with Preform Grips, the thimble is 7/16".
4) If 6700lb Phillystran is used with Preformed Grips, the thimble is 1/2".
5) If Phillystran is used with cable clamps (highly not recommended) use
one size smaller than above.
In all cases the heavy duty version of the thimble is used.
TENSIONING
Rohn specifies that guys should be tensioned to 10% of the breaking
strength of the guy size that is recommended for a particular tower. One
rule of thumb is 8% if the guy is out at 100% of tower height, 10% if at
80% of tower height (standard Rohn drawings) and up to 15% if the anchor
point is at 65% of tower height. You lose a lot of wind load in this last
type of installation.
For Rohn 25, 3/16 EHS is recommended, having a breaking strength of 4,000
lb. Therefore, 400 lb. of tension is appropriate for Rohn 25 tower. The
primary failure mode for Rohn 25 is in compression of the legs, so it is
important not to over tension the guys, resulting in greater compression of
the tower legs.
1/4 inch EHS is has a breaking strength of 6650 lb. and the preload tension
should be 665 lb. for towers where Rohn specifies 1/4.
For Phillystran, there is some new information from the factory and it
looks like it doesn't stretch as much as it 'relaxes'. What they recommend
is that the Phillystran be initially tensioned to 15% of its ultimate
breaking strength and then over time, it will 'relax' to the 10% desired
tension. According to their chart, It goes from 15% down to 12% within
about 10 hours and then finally reaching 10% within 30 days (a guess since
their graph doesn't extend out that far).
The TIA-222 tower spec allows a tolerance of 1 part in 400 for tower
alignment; that's 3 inches per 100 feet so your tower doesn't have to be
perfectly plumb. Start with the bottom set of guys and an intermediate
tension around 100 pounds, verify the plumb (or pull into plumb) using a
long level (4-6 feet) and then adjust to the final tension. If all the guy
anchors are at the same level, you only have to measure one guy; they
should all be the same. Once you've got your intermediate tension and
plumb, it doesn't take much travel in the turnbuckle to get to the final
tension - maybe as little as 1 turn. Actually, using this method you don't
need much turnbuckle to adjust. Going from 100# to 400# tension might be
less than 6 turns of the TB, so there's not much problem with pulling the
tower out of plumb. Move up to the next set and repeat until finished.
Use your arithmetic measurement for how long the guy should be and then
make the piece of guy wire closest to the ground on that first one 10 feet
too long. Since you are splicing the guys by insulating them this first one
will give you a good feel for how close your arithmetic guesstimate is.
i.e. if you have ten feet too much your math is one hell of a lot better
than mine! I assume you are using a bolt cutters for cutting your
EHS...they can be had cheap at flea markets...you have seen them they have
the big long red handles and menacing black jaws. If you are using an AB
Chance or similar anchor into the ground/concrete you have a closed eye
that is your attachment point. You need to pass something through that eye
which will act as a place for you to attach a come-along. Depending on the
installation you use this will vary as you will need to try and avoid the
actual guy wire's path as best you can. If you have an equalizer plate you
can use an adjacent hole on the plate as an attachment point.
With the come-along and a Chicago grip (or another, second, guy grip
applied several feet up the guy wire) moderately tension the guy wire. I
say moderately so you don't pull the tower over or throw it out of plumb
from the start. Once the guys are moderately taught check the tower for
plumb, adjust the guy that needs to be tighter first and, if necessary,
later on you can let out the far side guy(s). If you can tighten that first
guy and bring the tower into plumb there is a good chance you will have
also tightened the other guys in the process. If increased tension does not
plumb the tower, then you should consider letting out on the other guys.
You will have a loose end pointing at your guy anchor with the come-along
doing the work. I recommend you have a turnbuckle there as it will allow
you to fine tune your adjustments later on. Start with the turnbuckle 3/4
out. With the force on the come-along, and the bottom side of the
turnbuckle attached to your anchor you know how long the wild end of the
cable needs to be. Cut it so that it corresponds to where it should end at
the high side of the turnbuckle. Trim it, and marry it to the turnbuckle's
upper end with a preformed guy grip. It should only take a couple of twists
of the turnbuckle at this point to transfer the load off the come-along and
onto the turnbuckle. It will take a couple of hits/misses for you to find
how far up the guy wire to attach your come-along/Chicago grip so that you
will not interfere with the turnbuckle, still be able to take up, and - be
able to reach that upper point! Don't make it too high.
We have had great luck with using the Loos gauge as a method for
equalizing the force on the guy wires. While it may not give an exact
number it does give you a repeatable number, strive to have all your guys
have equal tension (this assumes the end points are all the same distance
from the base of the tower, of course). If you are going into an equalizer
plate, remember that as the other upper guys attach to the plate it should
want to change its angle with respect to the ground as the later guys
attach to it. This creates a situation where the bottom set will be drawn
tighter than when initially installed. The best way to handle this is to
compensate for it by having several inches of extra take up on the lower
turnbuckle when it is originally installed so that they can be backed out
as the upper guys tighten, allowing the eq plate to rotate. I encourage you
to purchase a Chicago-Grip (the Florida Rednecks call it a Pork Chop...when
you see it you will know why) - this device when used with the come-along
makes the job of tightening the guy wires no big deal. Having a second
person is a big plus on this job as you can really zip from one to the
other with one guy in charge of attaching the hardware and the other in
charge of tightening, etc.....I recommend a Dad!
After you have done the first level (assuming you took my advice and got
that pork chop - don't leave home without it) you will zip through the
subsequent guy levels. If you are a member of a club you might want to
encourage the club to buy a pork chop for all the members to use. We have
used these techniques successfully, repeatedly. Oh yeah, one other thing -
the pork chop is a great way to grab the lower end of your tram line when
you are putting up your antennas....but, we will wait for you to ask
about that in a month or so :-)
Using preforms, you do not cut the turnbuckle end of the guy wire at all.
Just let it lay on the ground or coil it up if you like. Only when you are
sure your tower has grown tall enough would you cut the excess length with
bolt cutters.
Make sure you put a cable, or one of the long ends through all the
turnbuckles to prevent them from loosening. Also, loop a cable through all
of the thimbles (in a circle) in case one of the turnbuckles breaks. If you
are afraid of vandalism, you should put the loop through the centers of the
turnbuckles as well, rather than the loose end serving this purpose. The
advantage of using the loose end, is that tightening of the turnbuckles
requires less time, since no cable clamps need be removed.
To tighten the guys, I use a preform about 6' up each guy wire and a come-
along attached to the preform (lever end of come-along). The cable end of
the come-along hooks at a convenient place on the anchor or equalizer
plate. Make sure the tower is vertical to the first set of guys via 4'
level (what I use) or plumb bob (never tried this). Then, as long as the
first part of the tower is vertical, you can site up the legs to see which
way you need to go with the rest of the guys. There will be interaction
between adjustments of sets of guys.
GUY TENSION MEASUREMENT
USING LOOS BRAND TENSION METERS FOR GUYS
The best, easiest and cheapest way to measure guy wire tension is with a
LOOS guy wire tension gauge. It can measure 3/16 to 9/32 sizes, which is
perfect for hams. The LOOS guy tension gauge works on Phillystran about as
well as EHS wire.
Loos & Co., Inc.
Cableware Technology Division
900 Industrial Blvd.
P.O. Box 7515
Naples, FL 33914-7515
1-800-321-LOOS (5667)
1-813-643-LOOS (5667)
One friendly distributor, well known in some tower circles, is:
Champion Radio Products
http://www.championradio.com
LOOS model B calibration chart
SCALE 3/16 7/32 1/4 9/32
10 240
15 320
18 380
20 420 300
22 480 360
24 540 410 250
26 620 480 280
28 740 560 340
30 880 660 400
32 1080 780 480
34 1400 960 580
36 1210 680
38 1600 840 400
39 1850 940 460
40 1060 530
41 1210 600
42 1460 670
43 1800 750
44 850
45 980
46 1120
47 1330
49 2000
The Model B also works well with 5/16" guy wires if you file away a small
amount of the rivet that blocks 5/16" cable from entering the mouth of the
gauge. Currently, no data is available to create a calibration chart for
5/16" EHS.
Here is the calibration chart for the PT-2
------------------------------------
SCALE | 3/16" | 7/32" | 1/4" |
-----------------------------------|
13 | 300/6% | | 315 |
18 | 500/11%| | 385 |
21 | 640/14%| | 438 |
24 | 840/18%|500/8% | 500 |
28 |1240/26%|740/12% | 615 |
32 |1060/17%| 780/9% | 780/9% |
34 |1300/21%| 900/11%| 900/11%|
36 |1680/27%|1100/13"|1100/13%|
38 | | |1300/16%|
40 | | |2000/24%|
------------------------------------
CHECKING GUY TENSION BY COUNTING OSCILLATIONS
Here is an interesting technique that one person has described. He has 100
feet of Rohn 45 up and guyed like Rohn. What he did was install an "in
line" tension measuring device (dynamometer) and crank the tension to the
prescribed amount. Note that a Loos tension meter could also be used here.
He then set the guy wire into oscillation by gently pushing it sideways
with his fingers. It takes a dozen or so pushes to get it moving. He
carefully observed that it was not oscillating in more than one section
meaning that it was oscillating at its fundamental frequency rather than a
harmonic. The oscillations are slow, like one per second or so. He counted
them for one minute, timed with a stop watch, and recorded the number of
oscillations. The frequency will be different for each level of guy since
it depends on guy length, wire size, and tension so you have to install the
dynamometer at each level to determine the correct frequency for that set.
He then removed the dynamometer, reattached the guy to the tower and
tensioned it until its oscillation frequency was the same as before (this
is much simplified with the Loos-type clip-on tension meter). To check guy
tension now, all he has to do is check the oscillation frequency with his
stop watch against the numbers recorded originally. No computer. No
software. The key, of course, is to own (or borrow) a tension measuring
device. BTW, you can check the accuracy of the device you use by hanging
known weights from it.
THERMAL EFFECTS ON GUY TENSION
Guy tension can change from natural temperature induced
expansion/contraction. Here is the temperature change information relating
to ambient temp and guy wire tension at 10% of breaking strength (these are
the values for desired tension at different temperatures):
3/16 inch EHS
120 degrees F 300# tension
90 350#
60 400#
30 450#
0 500#
1/4 inch EHS
120 degrees F 500# tension
90 585#
60 665#
30 750#
0 830#
You can see that these are basically linear relationships. These figures
include the effect of tower height change. This information is taken from
the Rohn Tower Inspection Manual. So you can see that you need to
compensate for temperature. If the initial tension was done in the winter,
then they will 'loosen' up due to higher temperature expansion. But if you
use the above factors, the tension should be within spec for the whole year
no matter when the reading was taken.
When Phillystran introduced the new jacket material a couple of years
ago, they didn't have any Phillystran grips so they initially allowed using
cable clamps. Unfortunately there was deformation of the strands and cold
flow problems caused by the cable clamps. Now Phillystran Big Grips are
available and they are the ONLY termination devices approved by the
factory. Always do what the manufacturer specifies.
MEASURING TOWER PLUMB
Now that your guys are in place and your ready to fine-tune the guy
tension, you need to have a way of measuring how plumb your tower is. One
of the least sophisticated methods, yet highly effective and often
recommended is to use a plumb bob. Suspend one from the center of the tower
at the first guy point and adjust the guys to plumb this lower section of
tower. If your first tower section is embedded in concrete, use a long
spirit level to make the first section as plumb as possible before the
concrete cures.
The wind will tend to push the plumb bob around, depending on how heavy
the weight is. Place a bucket of water at the base of the tower (or
whatever will fit through the rungs) to dampen the swinging of the bob.
Once you get the first section (or lowest guyed sections(s)) plumb, you can
simply sight up the tower legs. Even slight bowing is readily apparent with
this method. A low-tech but very effective technique to check the plumb of
the higher sections is an extension of this same idea, using a portable
plumb bob. This can be improvised with some string and any heavy object.
Hang it from a tripod or ladder. Sit on the ground with the bob line
between you and the tower. Line up the edge of the string and the tower
with your eye to check plumb. Perform this from two positions, 90 degrees
apart from each other. For taller towers (over 120 feet), use a transit
from two positions, 90 degrees apart to sight the tower legs for plumb.
TEMPORARY GUYING
While assembling your tower, you will inevitably encounter stages of
construction where the tower extends above the last guy point. The first
few sections above the base won't have any guys at all, for that matter.
Temporary guys do not have to withstand the full loads that permanent guys
will take, such as wind. Assuming that you only assemble your tower when
the weather is fair, and that you will finish with permanent guys before
your construction session for each day is over, temporary guys can be much
lighter. The important thing is to choose a material that has very low
stretch, such as lightweight steel cable, or static-type rappelling rope
accessory cord, having kernmantle construction. Twisted polyester also has
very low stretch, compared to nylon. The typical nylon you get from the
hardware store will stretch. In any case, even ropes that stretch somewhat
are much better than having no temporary guys.
Most people seem to feel comfortable climbing two sections above the last
guy point. The more adventurous go three before adding temporary or
permanent guys. Some professionals feel comfortable climbing 4 sections
before attaching the next set of guys, but the sway becomes so great that
it would be extremely unnerving and disorienting for all but the most
seasoned climber. I think a good rule of thumb would be to use temporary
guys any time you are 20 feet (2 sections) or more above the last permanent
guy set.
For towers that start with an embedded base section, things are simpler.
Once the concrete has cured, you can climb the first section and add the
next, then attach temporary guys until you get to the next section and the
first set of permanent guys.
For pier-pin based towers, things are a little different. DO NOT CLIMB a
tower having a pier-pin base without having temporary guys in place! The
single bolt on the base plate is not designed to resist bending at all! The
way this is usually accomplished is by pre-assembling the first 2 or 3
sections of tower, complete with permanent or temporary guys, then standing
it up, setting it in place on the base, and attaching the guys to their
anchors. Only then can you start climbing.
TEMPORARY GUYS FROM EHS
With this method, you use the precut top set of permanent guys or some of
the same material from which your permanent guys are made. In either case,
you need spare grips that are matched to the size EHS cable that you will
use. If you pre-cut your permanent guys, use the top set of guys as
temporary guys. To accomplish this, partially attach the grips (3-4 turns)
on the tower ends of your top guys. Use carabiners or quick links (single
chain loops with a threaded fitting that opens on one side of the link) to
attach the grip to a tower brace. Use a partially attached grip through
your anchors to hold the other end of the temporary guy. As long as you do
not fully wrap the grips, you will be able to remove them. If you are using
guy bracket assemblies (stronger) to attach your guys to the tower, you can
go ahead and fully seat the upper grip ahead of time, since it will be
retained by a bolt on the assembly and will not have to pass around a tower
leg.
TEMPORARY GUYS USING ROPES
This method is easier and faster, since ropes are easier to work with than
the steel. The best rope to use would be kernmantle construction, low
stretch static rope, such as Blue Water II. 6 or 7mm climber's accessory
cord, such as PMI 6mm would also be acceptable. Obtain three lengths, each
long enough to reach from the anchors up to the highest guy point required.
Stuff (not coil) each rope into its own nylon ditty bag, and place the
bags adjacent to each tower leg. As an alternate, pile (again, do not coil)
each rope, hand-over-hand, on the ground next to each leg. Tie the upper,
free end of each rope to the corresponding anchor using a figure-eight
knot. Note that since the anchor eye is closed you will have to tie these
in a special way. Tie a single figure-eight first, with an extra long loose
end. Pass the loose end through the anchor, and `chase' it back through the
knot to complete it (see the knots references for more information on this
must-know technique).
Pick up each rope at the base and take them up with you as you climb. It
helps to clip a carabiner over them so that the rope can pay out straight
up from the ground, through the `biner, and out to the anchors as you go
up. You can also tie a tag line to the `biner, climb up, then pull the
anchor ropes up.
Once you arrive at the temporary guy position, separate and tie each rope
to the tower. To accomplish this, pull up some extra slack in each rope and
form a closed loop in the rope (this is called a bight). Keep the bight
closed and use it as if it were a free end. Pass the bight around a tower
leg, and tie a taught line hitch around the portion connected to the
anchor. Now, Boy Scouts, adjust your taught line hitch to apply tension to
your temporary guys. Alternately, if you have three ascenders (a one-way
sliding climbing device), you can simply attach the ascenders to the tower
with carabiners and pull the ropes tight through the ascenders to hold
tension. Once you have installed the next set of permanent guys, untie the
ropes and repeat as necessary. No one on the ground has to get involved,
and you control the tensioning process so you will not get jerked around
accidentally.
LIGHTNING ABATEMENT
LIGHTNING PROTECTION THEORY
This is a two-step process. First, you can prevent or reduce the static
buildup which allows the strike. Second, (preferably starting from a point
above your antennas) you provide a very low impedance path to a good ground
to handle the energy from strikes that you can?t prevent. There are several
companies that sell the equipment and monitors/alarms. Basically they sell
or install a good ground system, and a static dissipation system. A static
dissipator is typically a spike ball, really a bundle of sharp
spikes/wires, bent so the tips are at different angles, looks like a ball
at a distance. When the static charge between cloud and ground starts to
build the corona from the spikes prevents the charge for rising to a level
which would support a strike. The charged cloud passes over the protected
area without a strike.
A cone of protection is provided by providing a high strike point to
create a desired spot for a strike, but preventing a strike is still more
desirable. This can be accomplished by letting your mast extend above any
antennas. There will be some noise from the static discharge corona. There
is a company in Boulder, CO (I've don't remember the name) that
manufacturers the prevention equipment. That have sufficient proof that it
works.
Coiling your feedlines and control lines will add extra impedance to them
that will make lightning energy look elsewhere for a better path to ground,
such as your shield ground, MOV?s and your coax lightning arrestors at the
base of your tower.
HOMEBREW STATIC DISSIPATORS
One method is to use a piece of solid 1 1/4" aluminum bar stock, drill it
full of holes and cut up a bunch of old CB whips into about 200 6" pieces.
Spin one end on the fine grindstone to a sharp point, stick all of the
blunt ends into the holes (Snug fit) and, with a dull punch(like a nail
set) lock each one into its hole by a sharp rap on the punch at the top
edge of each hole. Then bolt the "Porky" ( it kind of looks like one!~)
to the top of the tower at the base of a mast or on an outrigger.
Another method, if you?re using steel EHS guys, is to simply leaving the
guy wire ends untrimmed at the top. Unfurl them and sharpen the points, and
aim them up and away from the tower.
Another very simple one can be home-brewed with a bundle of sharp wires
sharp on one end and stuffed, blunt end first, into a heavy electrical
terminal of good size(#4 or even as large as 1/0. A properly tightened set
screw and shaping the the wire into a ball like a "Porky," and it's ready
to attach to mast or tower top.
Frayed stainless steel wire rope (SA) would work, of course, if one
separated the wires completely and secured the ball with a low resistance
means on the way to a good ground. Any number of sharp points, from 1 up to
1000 is good.
Here?s another one. Go to Lowes and get some of that galvanized fence
wire, cut it into about 2 ft. lengths. Make sure that you cut the wire at
an angle, so that its pointed on the end. Then take a piece of 1 inch x 4
or 5 ft. electric conduit, cut some vertical slices into one end and spread
them apart slightly. Then jam as many peices of the fence wire in it as you
can. You only need to put the fence wire in a couple of inches. Clamp the
wire in the conduit with a stainless steel hose clamp. Then take your
handy-dandy torch and some good flux and solder the whole thing.
Now if you spread the wires out they will form a ball. It ends up looking
kind of strange. Make sure you bend the conduit at the bottom (where you
would attach it to the tower). The bend should be about 35 degrees.. so
that when attached it will stick out from the tower. Connect the "spline
balls" to the conductor mentioned above and thats it.
For static discharge problems across insulators, such as when an
electrical storm is in the area, drain resistors may help. Put some 100
Kilo ohm, 2 watts resistors across the points where it's arcing. The
resistors will keep the arcing from creating carbon tracks across the
insulators, and will help keep your (localized static generated) noise
levels lower.
GROUNDING FOR LIGHTNING PROTECTION
Nothing in a ground path should be attached by soldering alone -
everything should be clamped only. The reason is that a soldered connection
might heat to the point of melting and separating the connection if it is
required to carry a large amount of current. This then causes that joint to
have a relatively high impedance, causing the current to seek a different
path - one you might want it to take. Joints should be clamped first, then
soldered to preserve the best electrical properties.
I would put out a spread of 2 or more ground rods, spaced 16 feet apart,
and running heavy, bare wire from rod to rod. Even 1 extra rod will help
tremendously... if you don't have enough of a yard for a spread of rods,
then run them around the perimeter of the house. Studies show that a ground
rod will drain excess charge (in average ground) for a radius of (roughly)
8 foot. Putting the rods too much closer than 16 foot apart overlaps their
influence area. Conversely, much more than 16 foot will allow a significant
charge to build between rods. In dry, sandy soil you might close them in
about 25%, and in wet, highly organic soil, spread them out 25%.
In general a large diameter conductor is better than a smaller diameter.
a flat ribbon, 2" or 3" wide, cut from aluminum roofing flashing (copper
would be great, but expensive) will have lower inductance and be even
better. a parallel run of smaller conductors will also lower the inductance
per foot, compared to a single larger conductor. #6 gauge solid copper
should be considered a minimum size.
The ground wire should run over to and up the leg so that it makes a very
gentle angle; no right angles are allowed as they present a high impedance
point to the charge that's trying to get to ground.
The whole point of a ground system is to have all of the conductors at
the same voltage potential so that the voltages all rise and fall at the
same rate. If they have different potentials, then you'll get arcing and
damage. BTW, if your guywires are not insulated and not grounded, a
lightning strike will probably weld the turnbuckle threads together.
The consensus seems to be to connect ground straps to each of the three
tower legs and run them down like radials to at *least* three ground rods
adjacent to the tower base. The radials can be up to 50 feet long for
maximum effect, with appropriately spaced ground rods. Lightning energy
will not travel much beyond this distance. The radials and straps should be
buried below your local frost line, and at least 6 inches.
Again, the idea is to collect and/or dissipate charge from/to the earth.
a single ground rod alone cannot do this effectively. One rod saturates an
area around it radially out to a distance about equal to its depth.
Therefore, adjacent, 8 foot rods should be spaced 16 feet apart. Three rods
spaced to form an equilateral triangle, 16 feet on a side, around your base
would be a minimum start. The rods should also be connected to each other
in a ring with straps that would trace the circumference of this triangle.
This makes an effective connection to a *much* larger volume of earth. For
best performance, add 3 more rods for a total of 6 rods. Number 8 gauge is
a little small. One approach is to use 3/4 common copper water pipe for
ground rods and straps (HQ to the rescue!) to make most efficient use of
the copper, accounting for skin effect. Connect the ends by flattening
them, drilling holes, and joining with a copper split bolt, to keep all
similar metals. a joint compound with copper particles would be ideal
between mating surfaces (Penetrox E). This way there are no dissimilar
metals, promoting greatest longetivity for the joints. I'm not sure that
just soldering them together is good enough for underground service.
Here is some ASCII ART (view or print this with a monospaced font) to
illustrate:
o
|
|
|
o
/|\
/ | \
/ T \
/ / \ \
/ / \ \
|/ \|
s-------o-----------o
/ \
/ \
/ \
o o
This is not quite to scale, of course. "T" is your tower base. "o" is a
ground rod. The radials, tied to the tower legs, spread outward, with 8
foot deep ground rods connected every 16 feet. The first set of rods is
connected together to form a triangle. There should also be a ground strap
from this system to the ground system for your home (service entrance,
"s").
a separate wire from the lightning rod on top down to the ground system is
probably not required if there is good contact all the way through the
tower legs. For a crankup/foldover, you may need a jumper across pivot
joints or rollers, as these might not be reliable conductive paths like
bolted leg joints. Some companies insist on a separate copper conductor
running up the tower, which is the best way to absolutely assure a good
path, but I don't think this will be necessary in fixed towers.
All grounds are tied together, as you can see from the "art". Multiple
rods are what you are counting on to dissipate the strike current without
making it want to 'search' elsewhere for a ground path.
EXAMPLE TOWER GROUNDING METHOD FOR LIGHTNING PROTECTION
After perusing the load of messages in the archives about tower grounding
methods and materials, I used the following scheme to build a low-impedance
grounding system for my planned tower. In true Ham fashion, I improvised
with very commonly available materials wherever possible. Let me share this
method as one example of 'how-to' that I chose.
1) Use 1 ground radial per tower leg, 2 ground rods per radial, 6 rods
total.
2) On each radial, the 1st ground rod is spaced 8 feet from base, 2nd rod
24 feet from base (16 foot rod spacing). For better or worse, I did not
encircle the base with a ring.
3) Use 3/4" x 10' type L (heavy wall) common copper water pipe (about 80
cents/foot) for both radials and ground rods. This pipe provides lots of
smooth copper surface area for low impedance, yet enough total copper cross
section for current-carrying capacity.
4) Provide a flexible connection between the radials (rigid) and tower legs
using 2, 5/8" diameter by 3' long sections of flexible copper refrigeration
tubing in parallel. These come up out of the ground from the radial end,
and arc up, parallel to the tower leg, and are easy to bend, yet provide
lots of smooth copper surface area. Since the wall thickness is lower than
the 3/4" pipe, two in parallel are required for current capacity.
5) Make connections in the 3/4" pipe (radials & rods) by flattening the
ends, polishing with Scotchbrite abrasive pad (no metal wool!), and joining
with #6 size copper split bolts. A 1/2" through hole is just right for
these bolts. Since these bolts don't have much of a shoulder on them, they
must be modified. Modify the split bolts by flattening (in a vise) the 2
little staked 'wings' that capture the anvil (the little jaw that slides in
the slot) in the nut so that you can take the nut off and leave the anvil.
Put the bolt and anvil through your drilled hole from one side so that the
'T'-shaped anvil head helps provide a shoulder to keep the small bold head
from pulling through, and apply the nut, of course, to the other side. This
little exercise earns you a copper bolt to avoid dissimilar metals.
6) Use copper-filled antioxidant liberally on all the copper-to-copper
joints. An excellent source is Versachem anti-seize paste #13, available
from Advanced Auto parts. It comes in a container with a brush attached to
the lid and is a heavy grease LOADED with copper particles. Hey, you can
also use this stuff on fasteners in your engine! After bolting the
connections, seal the antioxidant in the joint with electrical tape such as
Scotch Super 88.
7) Drill weep holes vertically through radial pipes every 4 feet or so
before burying. You don't want steam-flashover during a strike to explode
your work (wouldn't it be neat to watch, though?)! I used shallow V-
trenches, 4 inches deep, for the radials.
8) Sink the ground rod pipes by making them into water drills. Flatten one
end to form a nozzle with about a 1/8" elongated opening. Solder a garden
hose adaptor assembly on the other end. I used 2 90 degree street els, a
pipe thread adaptor, a 1/4" lever valve, and a garden hose adaptor in my
assembly. Using the 30-40 psi water from my well pump, and twisting the
pipe like a drill bit, back-and-forth, while pushing down, I was able to
sink each 10 foot pipe in less that 5 minutes. Don't sway the pipe side-to-
side, or you will make the hole too wide. Use the minimum valve opening to
make it work, so you don't wash out too much dirt and compromise the
contact between the pipe and ground. Take it all the way below grade in
your trench, then raise it up 6" or so (tap it back down in the trench
later after connecting to radial). Then, cut off the water, cut off the
pipe with a tubing cutter just below the adaptor assembly, and carefully
desolder the stub so you can reuse the assembly on the next rod. Flatten
the end with 2 hammers for electrical connections per above.
9) Connect the flexible jumpers to the tower legs by first gently hammering
the ends flat against the leg to form a large, curved contact patch over 2"
long. Apply a piece of stainless steel shim stock over the tower leg, and
clamp the tubing ends over the shim with all stainless hose clamps. Use
Noalox on all surfaces as an antioxidant. Don't use the copper-filled stuff
between the stainless and the galvanized tower leg (bad metal combination).
I found a perfect source of stainless steel sheet metal by using the
tubular shaped element that comes in the flexible compression pipe
couplings used to join sections of abs pipe. These little jewels, also in
the plumbing aisle, have a tubular rubber boot, a stainless cuff,
(essentialy a rectangle of thin sheet formed into a tube), and 2 hose
clamps. Sorry, I don't know what they are called.
10) Extend one radial up to your house and tie it in with the service
ground. Bring the pipe above ground to an entrance panel for your cables.
Solder the pipe together above ground using tees. Since this section is
primarily for dissipating surges and not direct strike current, soldered
joints should hold up fine, and are easier to make. Leave the branch of the
tee open so the pipe cannot collect water. Stuff the branch opening with a
copper potscrubber to keep bugs out.
Ok, it was long, but I hope you enjoyed reading about how I went about the
grounding. Thanks to all who have contributed to the reflector and spawned
my ideas. I hope this post helps someone else build a good ground system. I
feel that this will give me much protection from lightning when a hit
occurs, and I will sleep better at night when those 2am boomers roll
through. Well worth the $200 or so I spent in copper.
Now, I am shopping for tower sections......
Mark, N1LO, Gloucester, VA
GROUNDING GUY CABLES
What I did was to use regular guy cable clamps to fasten short straight
lengths of guy wire to each guy near the anchor. The short pieces (4 in my
case because there are four levels of guys) are then clamped together in a
clamp designed for grounding a copper ground wire to galvanized water pipe.
These clamps are relatively cheap and designed to segregate the copper wire
from the galvanized pipe (wire). The bundle of guy wire stubs is put where
the water pipe would normally go. If you need to increase the effective
diameter of the guy wire bundle, you can insert short segments of extra guy
wire in the middle of the bundle as "shims". I put the pipe clamp about one
foot above ground and use a heavy copper ground wire from the small hole in
the clamp down to the ground rod.
While there are clamps made specifically for this application (they're a
little expensive), most people use regular cable clamps. Since you're
connecting a round object to another round object, you don't get a whole
lot of contact area; hence the aforementioned clamp blocks. Make sure you
don't have any sharp bends in the ground wire and have it drop straight
down to your ground stake and that should do it.
One word of caution. At some point you'll have dissimilar metals in contact
(steel and copper). Since attaching the copper wire to your guy wire will
cause long-term corrosion to occur (interestingly enough, usually ONLY the
strand that is in contact with the copper as it washes off the galvanized
coating), use steel wire attached to the guy and move the steel to copper
contact point to the ground stake. As always, use an appropriate
anti-oxidant material on all joints.
If the purpose of guy wire grounding is for lightning protection,
soldered connections are good for ONE hit. After that, the solder has been
melted and you don't have a connection anymore. The only acceptable ground
wire connections allowed by the NEC are either compression (like a clamp)
or
exothermic (like a cadweld). Using solder introduces yet another dissimilar
metal to your guywires. This
is another situation that is not recommended.
GROUNDING FEEDLINES
Yes, all coax lines should be grounded to the tower at the antenna and At
the base of the tower before the cable goes into the conduit or Before it
goes into a junction box. Yes, use feedthroughs (or better, a PolyPhaser
lightning arrestor) and use pl-259s on the cable. Then water proof the
connections.
You must establish a single point ground, preferably just outside your
shack outside wall. PolyPhaser goes into detail on this in their "grounds
for lightning protection" book.
Simply burying the cables help reject the pickup of additional rf energey
from nearby strikes in the section that is buried, but by itself, burying
is inadequate.
If you really want your cables to be protected, they should be grounded at
the top of tower where the cable run begins, grounded at the point at which
they leave the tower and turn towards the building and then grounded at the
building entry where your Single Point Ground System bus resides. THEN you
can consider your cables protected. Note: I'm skipping the part about the
rest of the ground system.
Merely disconnecting your cables is a false hope. An average direct
lightning strike won't think twice about jumping from the end of your
disconnected cables to another convenient place that offers a lower
resistance path to ground. It could be the adjacent radio, telephone,
computer, the cable on your TV, etc. The main thing that you're trying to
do is to keep the lightning transient OUT OF THE HOUSE in the first place.
If you do a good job of that, it really doesn't matter what you do IN the
house. By having your cables in the house unprotected but disconnected,
you're still inviting that massive voltage and current into the house.
Eight-pin connectors allow you to disconnect the cables from the boxes
within seconds (easy slip in - slip out connectors) anytime you are leaving
the operating position or whenever you hear distant rumbles of thunder
heralding an approaching storm.
Each wire going into/out of your shack must be protected, in some way, from
lightning. That means a dc-blocked unit in series with coax cables, shields
shorted to ground at the antennas and at the base of the tower and at the
single point ground, and other lines (control lines) parallel connected to
MOV?s to ground at the base of the tower. PolyPhaser sells the latter too.
I have every control line connected to a PolyPhaser antenna control
lightning protector (MOV unit) inside the box at the base of each tower.
The PolyPhaser unit has eight (8) wire capability per unit. You must verify
that your operational voltages (rotor control, relay control voltages) do
not exceed the threshold voltage of the MOV?s. The MOV voltage can be
specified to PolyPhaser for unique applications.
As an additional measure of protection, recommended by Polyphaser, you can
add MOV?s to tower ground at the top of the tower for those control lines
that are the upper most lines on the tower. That is, the highest
antennas/rotors/relays on the tower have another set of MOV?s to ground at
their mounting point on the tower.
COMMERCIAL FEEDLINE GROUNDING CLAMPS
Take a look at the grounding blocks offered by Industrial Communications
Engineers (ICE) in Indianapolis. They are heavy machined aluminum blocks
that clamp around up to 4 coax shields with a mating ground bar to attach
to the tower. First, you remove about 1 inch of the coax jacket of each
feedline. Then you apply the supplied anti-corrosion compound to smear
between the braid and the clamp. Last, you close the clamp and attach the
assembly to a tower leg. Each block can handle up to 4 coax's and there are
2 models - one for RG8x type and one for the larger stuff (9913 etc).
ICE also has other goodies-preamps; "lightning arrestors" for coax, open
wire, phone lines, and control cables; and transformers to use surplus
75-ohm TV hardline.
ICE - Industrial Communication Engineering
1-800-ICE-COMM/ (800) 423-2666
HOMEBREW FEEDLINE GROUNDING CLAMPS
One method for coaxial cable starts with a piece of 3/4 inch aluminum plate
about 2 inches square. Drill the appropriate size hole through the plate
for the outer shield (you have to strip a short section of the outer
insulating jacket). Then flip the piece 90 degrees and drill two 11/32
holes on each side of the center hole and perpendicular to it through the
edge of the plate for the 5/16 inch clamp/mounting bolts. Then saw the
whole thing in half and the saw cut provides about the right crunch.
Depending on where it is going, you can make the clamp/mounting spacing fit
a U bolt and then grind/file radius to fit tower leg. When clamping to a
tower leg, use a stainless steel bolt, and a thin piece of stainless sheet
(machinist?s shim stock) between the aluminum block and the galvanized
tower leg with lots of anti-ox compound. Also apply anti-ox to the socket
where the clamp grips the coax. Then seal the clamp, leg and cable like it
was a coax joint to keep the water out. No moisture means no electrolysis.
It is cheap and takes about 15 minutes to make with a drill press, band saw
and grinder.
Another method is simply to cut the coax at the grounding point and install
PL-259's on each end. Join them with a barrel connector. Us the silver
plated types, please. This provides an exposed electrical terminal for the
shield. Make a strap from a thin piece of stainless steel sheet and clamp
the exposed silver to the tower leg. Apply anti-oxidant compound first, of
course, and then weatherproof the joint.
Here's another method. Strip off the jacket for an inch, and lay a 1/2 inch
wide copper foil along the braid with two flying (free) ends.
Tightly wrap the shield to the foil with a single flat layer of thin CLEAN
solid buss wire, about number 22, and flash solder the wire ends in
place.
Then weather-proof the whole joint. The copper foil leads hanging out can
be grounded to the tower leg or entrance panel. When connecting the copper
to a dissimilar metal, apply anti-ox xompount and separate the two metals
with a piece of stainless steel sheet or shim stock.
A method that does not break the coax should be more reliable than adding
two connectors and a barrel, especially if one is careful in the
weatherproofing stage of the process.
ASSISTING FEEDLINE AND CABLE GROUNDING WITH CHOKE COILS
Form a coil of several turns in each cable just before the cable passes
through the entry panel of the shack. This is just like an air-core balun
that you would make at the feedpoint of a dipole. This coil, commonly
referred to as a `solenoid coil' or `choke coil', forms a high impedance to
lightning-induced energy (mostly RF) and helps force this energy into the
grounding points up stream at the tower. The coil is no longer the `path of
least resistance', or in this case, the `path of least reactance.' This
also will choke off other sources of coaxial feedline radiation and EMI
interference.
HOMEBREW GROUNDED ENTRANCE PANEL
Here is one approach to protecting your gear from lightning. You can make a
'grounding switchboard' that fits in a window. The approach is simplistic,
inexpensive, and effective, based on the following goals:
1) Disconnect of all conductors between tower and shack when not operating.
2) Shunt all conductors from the tower to a low impedance ground when not
operating.
3) Allow varied rig/antenna connections.
4) Isolate gear from ground when not operating.
5) Do not operate during threatening weather.
6) Develop a habit of always disconnecting after operating.
7) Don't spend too much money!
8) Don't drill holes in your walls and aggravate the XYL!
9) Have fun building something that really works!
Here is the scheme, in summary:
1) Install a conductive panel in a window.
2) Bond the panel to the electrical service and tower ground system (single
point) with a low impedance conductor, such as copper pipe or wide strap.
3) Connectorize and label all cables coming into the shack.
4) Make shorting plugs that mate with the connectors and shunt all
conductors to the panel. They should be quick "push-on" types so it will be
easy enough that you will actually *use* them! :~)
5) Make choke coils in the conductors between the tower and shack.
In practice, the switchboard is real easy to use - which is a key element
in human habits! Just reach over to the panel, pull the shorting plugs out
of the jacks you want to use, and plug in your jumpers. At this point, the
rigs also become grounded through the shields of the jumpers. It's
essentially a switchboard, too, so you can connect any rig to any antenna.
When done, pull off the jumpers and push on the shorting plugs. Rigs become
ungrounded and present no path to stray lightning energy.
You can build an insert for one of your shack windows. It consists of a
rectangular, 1/8" thick aluminum plate (try 5052-H32 soft temper, easy to
drill, from an *old* road sign ), framed with
pressure treated lumber. Rabbet the edges of the frame with a router to fit
the contours of the sill and the sliding operator such that the insert is
captured and secured when the window is closed on it. Paint the frame with
a suitable primer and paint to match your window. Add some self-stick
weather-stripping "V" tape to complete the seal.
Install Amphenol UHF female bulkhead connectors in the plate for each
coaxial cable desired, plus spares.
Make a rain shield from a piece of 1/16" aluminum or stainless sheet. Mount
it with screws to the upper frame piece on the outdoor side. Make it long
enough and bend it out such that even rain coming down at an angle can't
hit the connectors.
For each wire antenna fed with open wire line, install two bulkhead
connectors and use shielded parallel lines from two conductors of RG-58
(see ARRL antenna handbook). If you like remote baluns, you can get by with
a single connector and one coax.
For rotors and switchbox control cables, use 6 or 8-pin, molded trailer
connectors. You can make a clip that retains the tower side connector in a
cutout in the panel.
Make insulation panels from pieces of celotex sheathing. Use the kind that
has aluminum foil bonded to the outsides for a nice look. Cut holes in the
sheathing for each connector and glue one to each side of the aluminum
panel.
Acquire some PL-259 quick 'push-on' adapters. These have an SO-239 female
connector on one end and a PL-259 male connector on the other. However, the
threaded female ferrule on the male side has been replaced by a springy,
slotted sleeve that allows the fitting to be pushed on to another SO-239.
To make these into shorting plugs, solder L-shaped pieces of 12 gauge
copper wire into the center sockets of the female UHF side of the adapter,
with the short legs soldered to the edge adjacent to the threaded part.
Don't let the short legs jut out past the threads. Then, using a vise,
press 2" long pieces of 1/2" PVC over the threads on the female sides,
making short, insulating handles. Then, fill the PVC cavities with hot melt
glue. Paint them, if you like, for a neat look.
You can make a shorting plug for the rotor or switchbox cables using mating
connectors. Solder all the wires from the connector (wires are already
molded into it) into one large copper cable lug and bolt the lug to the
panel. When you mate the connectors, all of the conductors are shunted to
the panel.
Next, make jumpers of RG-8X for every single antenna connection for all of
your rigs and put push-on adapters on the panel ends. Make a jumper for the
rotor or switchbox using a short length of control cable and the mating
connector for the one in the panel. Label and color-code all ends.
Put a little silicone grease on the quick disconnects and connector pins.
One cheap source is dielectric boot grease at an auto parts store.
You can ground your panel to ground using 3/4" common copper water pipe for
its large, smooth surface area. If it is convenient to run your cables
alongside the ground pipe, as in the case of a second story shack window,
install 8-10" stub legs in it using "T"'s every few feet to cable tie the
hardlines and coaxes to it for support. To attach the pipe to the panel,
hammer the pipe flat and bend it over at a right angle. Drill holes in it
and the panel for 1/4" stainless bolts. Don't forget to use pieces of
stainless such as washers or foil, plus antioxidant paste, wherever the
copper and aluminum mate.
GROUND ROD METAL SELECTION
The corrosivity of the soil is another important issue. One should measure
the pH of the soil, preferably at the depth at which the grounding system
is to be installed. The pH testers sold at spa and swimming pool places
will suffice for ascertaining whether your soil is acidic or alkaline.
Other kits, designed specifically for soil testing, may be found in lawn-
and-garden shops (check Home Depot) and feed-and-seed type businesses such
as Southern States. Dig down to the area where your grounding system will
be (generally 6-18" or more depending on how far the soil freezes during
the winter) and collect a small sample of dirt in a container that you have
thoroughly cleaned and rinsed with distilled water. If you are using a
pool/spa type kit, take some of your sample dirt and put it in the test
tube with some distilled water, shake it up, and test with the strip. If
you bought a kit designed for soil, follow the instructions that came with
your kit.
If your soil is acidic (most of the eastern US is), you want to go with
galvanized, tin, or aluminum-clad products because the acid will attack the
copper. If your soil is alkaline, however, you want to avoid galvanized,
tin, and aluminum grounding components because those metals are quickly
attacked in that environment.
If you are installing galvanized towers and galvanized guy anchors side by
side with copper ground rods, then it looks like you're creating an
electrochemical corrosion cell, and accelerating the already corrosive
effects on the galvanized steel. So unless you're using copper for towers
and copper for guy anchors, I'd use the SAME material as the guy
anchors/towers for ground rods driven within several feet of the anchors.
Yes, they'll corrode. But won't they corrode anyway? And wouldn't it
corrode faster if dissimilar metals are used as you pointed out seeing in
pipes that were side by side? And would it not be better for the ground rod
to corrode than the guy anchor holding up your towers? It's a hellava lot
easier to replace ground rods than anchors!
It's the copper rod that causes the galvanized steel anchor to corrode at a
faster than normal rate. It is clear there is a BIG problem in some areas
with corrosion. It's probably OK to use copper rods (with tinned copper
wire) in most places in the antenna system like for the interconnecting
grounding between ham shack, AC, telco, tower radials, and so on; however,
you should definitely consider using galvanized steel rods for the
grounding of guy wires near galvanized steel guy anchors and maybe the
tower base, at least in certain parts of the country.
SINKING GROUND RODS
To drive ground rods the traditional way, try a steel ?T? fence post
driver, available from a farm-supply store or fencing distributor.
a faster way is to rent an electric jackhammer that has a chuck that will
fit around the top of your ground rod. The ?water method? is an easier way
to sink ground rods than just wailing on them (or your knuckles) with a
maul. a bucket full should do the job. Dig a small hole where the rod is
going and pour a quart of water in the hole. That way you can keep the hole
full and it will self- feed the water down along the ground rod. Don't
get in too big of a hurry doing this job. Let the soil have time to soak up
the moisture. Ram the rod up and down by hand. Somewhere around the four
or five foot depth, pull the ground rod all of the way out and fill the
hole again. Go get a cup or coffee or a soda and let it soak for 10 or 15
minutes. From that point on, you will not need anymore water. You may
need to take a hammer and drive in the last two feet. Wear some gloves
because blisters will appear quickly and be cautious when pushing the rod
in on top of the water else it will squirt back at you with vigor.
IMPROVING GROUND ROD EFFECTIVENESS
There is a mix of chemicals to be used for grounding electrode
backfill that promotes excellent contact with ground, possibly even
available as premix in bags. It is possible to have a drilling
company buy the separate components for you and mix them on site.
The formula is:
75% Gypsum
20% Bentonite Clay
5% Sodium Sulfate
This is available from galvanic protection distributors and is known as
"standard galvanic anode backfill". I'll know for sure in a day or two.
These items are all dry powders, and mixing them on a windy site is
difficult. Anyhow, it works this way... The gypsum absorbs and retains
water. So the moisture level seen by the bentonite in the mix does not
vary enough to cause the shrinkage problems. The Gypsum has very little
expansion or shrinkage due to external moisture level variations and it
improves the conductivity of the mix. There is enough bentonite so that the
expansion of the bentonite still operates to pressurize the electrode and
hole walls for good contact. For this to work, the mix must be put in dry
and then have moisture added. The sodium sulfite is probably not strictly
necessary in a system with no continuous source of DC current for the earth
terminal. But it helps prevent polarizing the earth terminal if there is
some DC flow.