[Boatanchors] Kinda off topic- Great Info

[Bob]

Thanks.  That is good information IMH engineering Opinion.  
AND...Thanks to Boatanchors for allowing this "kinda off topic" to 
continue.  Boatanchors is a great site.
I love it.
Bob Hardie-W5UQ

J. Forster wrote:
> >From an antenna expert friend:
>
> ====
>
> 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.
>
> =========
>
> Best,
> -John
>
>
>
>
>
>
>
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