[Boatanchors] Kinda off topic

[J. Forster]

>From an antenna expert friend:

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I have used pulleys and weights to support wire antennas between trees
for nearly ten years, with great success despite ice and wind storms
that have broken many other antennas, overhead utility lines, etc. in
my area.  IMO, the most important rules are:

(1) Use double-braided, black, Dacron line.  No other combination of
material(s) and construction -- at least nothing that a ham can afford
-- stands up as well to extended exposure to weather, including
especially sunlight.  So-called "double-braided" line has a braided
outer jacket and, inside, a bundle of straight, not braided and not
twisted (at least, not twisted much), fibers.  The jacket does not
bear tension but protects the inside fibers both from sunlight and
from abrasion.  Dacron stretches and creeps less than any other
affordable fiber.  Also, because the tension-bearing fibers in "double-
braided" line are not twisted, there is no twist to unwind and
lengthen the rope.  Kevlar may sound better than Dacron but it's not
better in this application.  It doesn't last, and it's brittle.
Dacron withstands shock better.  Shock happens when a tree limb falls
on your antenna or its halyard.

(2) The tension in a Dacron line, and in any antenna wire, should be
set (by pulleys and weights) equal to ten percent (10%) of the rated
breaking strength of the line or wire.  Ten percent may seem too
conservative, but it's not.  It allows for shock, for gusty wind
blowing on an ice-covered line or antenna, for the weight of ice, for
the gradual weakening of a line that occurs due to fatigue, internal
abrasion, sunlight, acid rain, etc.  If you take suitable
countermeasures against fatigue and abrasion (see below), a Dacron
line will last five years.  If you don't, it may fail in one year even
if you loaded it to only 10% of its breaking strength.

(3) Before considering fatigue and internal abrasion, I'll state the
obvious, that a line must be protected from external abrasion.  A line
should never contact or wrap around a tree branch, a structure, or
another line.  If it does, then it will wear through at the contact
point as wind causes the line to rub one way and then another.

(4) Fatigue and internal abrasion damage may be imperceptible
externally, and may occur faster than you imagine.  Wind causes the
tension in a line to increase and decrease in potentially thousands of
cycles per day, a hundred thousand cycles per month, or a million
cycles per year.  Like any solid material, Dacron is damaged by stress
cycling.  Even in the absence of abrasion, the tensioned fibers inside
a line are slowly but surely weakened by the accumulation of fatigue
damage.  Remember that the "elastic limit" of a line is far short of
the breaking point; and that a line is not perfectly elastic even at
10% of its breaking stress.  Each stress-strain cycle leaves behind
some broken and slipped molecular bonds.

     Internal damage to a line occurs also by abrasion, when the line
alternately rolls around a pulley wheel or "sheave" and straightens
out, as wind causes supporting trees to sway, antennas and feedlines
to bow out, and so on.  When a line curves and partially flattens
itself against a pulley wheel, there is differential stretching and
rubbing of one fiber against another; and, when the line rolls off the
wheel and straightens out, the fibers rub the other way.

     An antenna halyard is most likely to fail where it rolls over a
pulley wheel.  The smaller the diameter of the wheel, the faster the
halyard fails.  When I have looked at a double-braided Dacron line
that has failed at a pulley wheel, I have been struck by how the
outside braid looks pretty good; but all the tension-carrying fibers
inside have broken.

    There are two simple and effective ways to reduce the rate of
damage that occurs at a pulley wheel: (1) increase the diameter of the
wheel; and (2) replace the wheel with a wider one or (better) with two
or more parallel wheels, and replace the original line with two or
more parallel lines, dividing the load equally between the multiple
lines.  Once upon a time I used single pulleys with wheel diameters of
1.5 to 2 inches.  Then I went to 3-inch pulleys.  Now I use double-
sheave blocks with five-inch diameter wheels, and I double up the
line.  Also, in the neighborhood of a pulley, I use thicker, stronger,
line.  If you have a 60-ft halyard, you don't have to double it or
make it thicker for its entire length.  Just double and/or make it
thicker for the six or ten feet (or whatever) that will roll back and
forth over the pulley wheel(s) when the wind blows.

(5) Prevent abrasion of a line where it's attached to an insulator,
counterweight, or whatever by using a thimble or at least wrapping it
with a couple layers of Scotch 33+ or 88 electrical tape.

(6) It's good practice to inspect your lines annually, and to replace
them every two years.  If I observed this rule, then none of my lines
would ever have broken.  Because I have been lax, I have had lines
break -- in every case where the line rolled over a pulley wheel.
(See above re pulleys.)  I have _never_ had an antenna break.  In
heavy ice storms, I have seen my wire antennas sag to the ground (and
the counterweights on the ends of the halyards lift way up).  In heavy
wind storms, I have watched my counterweights rise and fall more than
I ever expected.  As long as the counterweights have enough room to
move (important!), nothing breaks.

(7) In a wind storm, a freely hanging counterweight not only rises and
falls; it swings like a pendulum.  Be sure to hang it where it won't
swing into something and get caught or fouled up, either while it is
rising or while it is falling.

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Best,
-John