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Antenna Masts: Safety and Selection
Whether you are using a mast in a permanent installation
or a temporary public service application, this valuable
advice from a tower professional can help you avoid disaster.
Don Daso, K4ZA
Nearly every amateur who contemplates a
tower will need to include a mast to support
his or her antenna — or antennas. While
some hams simply mount their beams on
the side rails (the vertical legs) and rotate
the whole tower, most of us rely on masts
installed within the tower. Masts also come
into play for temporary installations, such
as those at public service events.
Either way, choosing the proper mast isn’t
a decision to take lightly. Your personal
safety, not to mention the safety of your
antenna, depends in large part on the type
of mast you select.
Mast Material
Consider the mast material itself. I’ve encountered almost every mast material during my years as a ham tower professional,
from wood to fiberglass, conduit to fence
rail, water pipe to aluminum, and various
steel alloys. However, the fact remains
there’s neither a single nor simple solution
for every situation. In all cases, though, a
basic understanding of some physics, along
with the inherent strength of the materials,
will help you choose wisely and stay within
your budget.
Years ago, hams thought nothing of using
water pipe as mast material. The common
pipe available locally and cheaply is known
as ASTM 120. It is heavy, which led users
to think it must be strong enough for the
task. This was often true, but at other times
these masts failed spectacularly, as shown
in Figure 1.
A few minutes with the ASM International
Handbook demonstrates that for common
water pipe, no minimum yield strength is
specified at all!1 Water pipe is intended to
convey fluids from point A to point B and
is not rated for structural uses, although it
is often pressed into service for light loads.
Water pipe is also measured by its inside
1
Notes appear on page 32.
Reprinted with permission from September 2014 QST ARRL, the national association for Amateur Radio
Figure 1 — A water pipe mast failure. [Thomas Cox, KT9OM, photo]
diameter (ID), so a smaller size (1.5-inch
ID) is required in order to mate with our
common 2-inch diameter U-bolts, clamps,
and rotators. The actual outer diameter
(OD) will still be under-sized — about
1.9 inches, in fact. If you were considering water pipe for your project, proceed
with great caution. For all but the lightest
jobs, you should use mast material that is
designed for structural applications and
rated accordingly.
Calculating Stresses
and Strengths
When selecting a mast, you must ensure
it will be strong enough. That means the
mast’s strength must be greater than the
stress of the loads you will place upon it,
with an additional margin for safety.
The bending stress on your antenna mast
depends upon:
The wind load area presented by the an-
tennas specified in square feet
The antenna position on the mast above
the top bearing
The mast’s cross-sectional area
The peak wind velocity
The strength of your mast is determined by:
The yield strength of the mast material in
psi (pounds per square inch)
The cross-sectional dimensions of the
mast (wall thickness and diameter)
Obtaining the data for calculating the
bending stress is easy enough. You can
obtain the information you need from
manufacturer’s data, your own measurements, and your local county wind speed
ratings (along with additional information
that your nearest National Weather Service
office can provide).2 The data for yield
strength is obtained from the mast vendor.
Consult Chapter 26 of the ARRL Antenna
Book for the fundamental formulas to calculate stress on masts.3 You can also use
commercial mast selection software. DX
Engineering (www.dxengineering.com)
and Champion Radio (www.champion
radio.com) both sell software that allows
you to simply “plug in” your values to de-
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termine the strength of materials required
in your installation. Once you’ve done that,
choose a mast that meets the requirements.
You may find that a simple antenna system
can be supported easily. For example, if
a single small tribander is going to be
mounted only a couple of inches above a
tower-top thrust bearing, water pipe may
work because the bending stress will be
very low. But if a “Christmas tree” array
of 10 through 20 meter monobanders is
going on that mast, you must use very highstrength mast material such as 4130 steel,
which is strengthened with chromium and
molybdenum.
Mast Installation
Once you have obtained a mast, the next
challenge is to install it safely. The most
common problem encountered during
mast installation is maneuvering that long,
heavy mast up the tower and into position.
Here are some helpful suggestions.
If you are building a new tower, install the
mast as soon as it is practical. For instance,
if you are using a 24-foot length of tubing,
install the mast inside the tower as soon as
you have the first 30 feet of tower erected.
It is much easier to install the mast inside
the tower sections and pull it up, rather than
attempting to lower it into the relatively
small diameter sleeve or thrust bearing,
from above the tower.
Simply lower the mast into the tower sections and let it rest on the tower foundation
as you build the rest of the tower. (The
longer the mast, the less likely it will fit
into the tower between the rungs of regular
lattice tower, so take advantage of the opportunity!) It is then an easy task to raise
it once the tower is complete, pulling it
up and through the bearing. Use muffler
clamps as safety stops above the bearing,
and slings to lift it into place.
If you are replacing a mast and must lower
the new mast through the bearing from
above, you will likely find the following
technique helpful. Any long mast, no matter its material or how light, will exert some
serious bending forces (torque or moment)
on the person climbing the tower. It’s very
hard to hold such a thing in position, guiding it down and through a hole only slightly
larger than the mast’s own diameter. Add
some wind and some wobble, and any
climber will begin to worry. Such work is
dangerous and not trivial.
Figure 2 — A basketball hoop can be used with
a gin pole to control movement as a mast goes
into place. [Don Daso, K4ZA, photo]
Figure 3 — Two Slipp-Nott mast clamps and
hardware. [Photo courtesy of Tennadyne]
It’s critical that the mast remain as vertical
as possible, yet secure. While I’ve read
of various methods to guarantee this, the
simplest way is not allowing the mast to
move away from vertical. A loop or ring,
secured to the top end of the gin pole will
provide that stability. I use an inexpensive
basketball hoop as shown in Figure 2. It can
be easily attached and removed.
Mast Alignment and Realignment
Despite torquing things down as tightly as
you can, you may one day look up and notice your antenna(s) have turned from their
normal position. At the top of the tower,
you find one of two things: either the antenna has slipped on the mast, or the mast
has slipped in the rotator. Such slipping is
quite common on chrome-moly masts, for
example, because of the steel’s hardness.
I do not recommend pinning masts to
antennas or rotators with a bolt through
the mast. In worst-case scenarios, rotators
and antennas are then more likely to break,
which is always more troublesome and
costly compared to a simple readjustment.
Slipping arises from the typical U-bolts
used to hold the mast in place. They simply
do not have sufficient surface contact area.
Some extra “clamping power” is called for
to prevent such slippage. A commercial
product, the aptly named “Slipp-Nott,”
available from Tennadyne (www.tenna
dyne.com/slipp_nott.htm) and shown in
Figure 3, works great if you use the common 2-inch mast material. The Slipp-Nott
provides nearly 90% surface contact area,
producing greater holding force.
Another option is using U-bolts with
“flattened” clamping areas, such as those
developed by Cycle 24 (now sold by DX
Engineering). Shown in Figure 4A, those
bolts have more contact area between the
mast and the bolt, increasing the clamping
force. Photo 4B is of my homebrew version
that adds a second U-bolt with short straps
to clamp the mast above the rotator clamp.
Although the Cycle 24 clamp is shown, a
standard U-bolt can also be added and will
add to the overall holding ability.
Conditions Beyond Your Control
If you do a simple search for wind tables,
charts or wind zone diagrams, you’ll encounter confusing and sometimes conflicting data. While measuring wind velocity
is easy enough, the charts usually provide
averages of wind speed over long periods
of time. Unfortunately, it is the gusts or
peak wind speeds that wreak havoc with
antenna systems.
Because we can’t predict when gusts will
occur, it becomes necessary to gather all
the relevant data, make some educated
calculations, and proceed accordingly. But
modeling, figuring, and software-driven
conclusions will only take us so far. At
some point we have to let experience guide
our choices. If you don’t have that experience, or are unwilling to take the risk, consult a professional engineer familiar with
conditions in your area.
Besides wind, which we can design for,
there is ice to consider in some regions.
Again, we can design for ice loads, but
here is a significant point to remember:
we cannot design for what will happen to
structures when the ice comes off. Ice never
sheds in a predictable fashion. Ice tumbles
off in bits and pieces. The load changes
dramatically as the ice departs. Add some
QST® – Devoted entirely to Amateur Radio www.arrl.org Reprinted with permission from September 2014 QST
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(A)
(B)
wind and you’ve potentially set the stage
for disaster.
One of my clients recently suffered a broken boom on his C-19XR Yagi antenna
during a fairly uncommon North Carolina
ice storm. Compiling some simple figures,
I reasoned that once the beam was loaded
with ice, it likely weighed more than 200
pounds. When the wind began whipping
up, the strength of the boom was quickly
exceeded and it failed.
Despite your best efforts, it isn’t always
possible to build in a sufficient safety
margin for every scenario and still remain
within a reasonable budget. If that is the
case, consider purchasing insurance to protect you. Insurance should cover the cost of
removing damaged antennas, masts, and
tower hardware — a job best left to professionals with the proper equipment and
years of experience.
enough for the loads you expect it to support. Prepare yourself for the job of installation before you start up the tower. And be
sure to leave yourself plenty of margin both
in material selection and safety (always use
a body harness) when working aloft.
Notes
1
ASM Handbook Volume 1: Properties and Selec-
tion: Irons, Steels, and High-Performance Alloys
and ASM Handbook Volume 2: Properties and
Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International (Formerly
the American Society for Metals); products.
asminternational.org/hbk/index.jsp.
2
Structural Standards for Steel Antenna Towers
and Antenna Supporting Structures, TIA Stan-
dard TIA-222-G, Telecommunications Industry
Association, Aug 2005; www.tiaonline.org.
3
ARRL Antenna Book, 22nd Edition, ARRL, 2012;
www.arrl.org/shop
Don Daso, K4ZA, is the author of Antenna
Towers for Radio Amateurs (www.arrl.org/
shop). You can contact Don at k4za@juno.com.
.
Figure 4 — A U-bolt with a flattened clamp-
ing area (A) for greater clamping force. [Photo
courtesy of DX Engineering] A homebrew
version is shown at (B) using a pair of straps to
add a second clamp for extra clamping. [Don
Daso, K4ZA, photo]
Be Safe and Wise;
Prepare and Plan
Here is a truth that should be universally
acknowledged: an antenna system is first in
a ham’s hierarchy of hardware and if there
is a tower in your plans, the mast should be
near the top (pun intended) of the items on
which to concentrate your attention. Start
by choosing mast material that is strong
For updates to this article,
see the QST Feedback page at
www.arrl.org/feedback.
Reprinted with permission from September 2014 QST ARRL, the national association for Amateur Radio
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