[Antennas] Re: antenna support rope question

2002年10月14日 21:26:47 -0400

David Ashworth, NC6P, asked about antenna support rope, 
specifically Dacron covered Kevlar rope.
The reason for the Dacron covering is to provide abrasion 
resistance to the Kevlar. Kevlar can cut the metal on 
pulleys, or even cut itself, as it rubs. The covering 
eliminates this. It should have excellent strength and 
should hold up quite well outdoors.
I wrote a fairly extensive post on rope for this group 
about a year ago. I am reposting it below for those 
interested.
 73, Dr. Barry L. Ornitz WA4VZQ [email protected]
From: "Barry L. Ornitz" <[email protected]>
To: <[email protected]>
Subject: Re: Rope
Date: Monday, October 01, 2001 08:10 PM
A few days ago, Tim Makins, EI8IC, mentioned guying an 
antenna mast with rope. He specifically mentioned 
polypropylene rope. In a private note to Tim, I suggested 
that he avoid this material for long-term use noting that it 
has poor ultraviolet resistance. I made several suggestions 
to Tim which he found useful enough include on his website. 
He asked for my permission (thank you), and I decided to 
write up my comments in more detail and forward them to the 
list in addition to Tim. I hope everyone finds them useful.
Since I subscribe to the Digests, it may take a few days 
before I see any follow-up questions.
 Barry L. Ornitz WA4VZQ [email protected]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hi Tim,
Yes, I guess your country is known for the rain and mist and 
beautiful green landscapes! Here in the sunny South, 
ultraviolet resistance is important.
Feel free to write up something on this, or you may include 
my notes as you please. All I ask is to credit me as the 
source! :-)
I originally did not post to the Antennas list as I did not 
have time to write a full reply. I am generally more active 
on the various "Boatanchor" lists where I am sometimes called 
the "resident chemist." Actually I am a PhD chemical 
engineer with a minor in electrical engineering. Before 
retirement, my work specialty was online instrumentation. 
Having worked for several manufacturers of plastics and 
chemicals, I have quite a bit of experience with the physical 
and chemical properties of plastics. One of my former 
technicians had even previously worked for a rope 
manufacturer. He knew all the important things about twist, 
stranding, lubrication, etc. that I know nothing about. But 
I am familiar with the chemical and ultraviolet resistance of 
polymers.
Polypropylene rope is most commonly used here in the States 
as "ski rope." It is inexpensive, light weight, strong and 
it floats. But the polymer structure of polypropylene (and 
most other polyolefins like polyethylene, etc.) is not 
resistant to ultraviolet. When exposed to sunlight, it 
quickly degrades from the ultraviolet radiation. The result 
is considerable weakening of the rope along with surface 
oxidation. The loss of tensile strength is naturally a bad 
thing for a guying application. The surface oxidation is of 
not much concern here, but when used as an insulator (such as 
on the ends of a wire dipole), the oxidized surface becomes 
wettable allowing surface contamination to adhere. This can 
increase the RF losses of the rope slightly. If you observe 
polypropylene rope after a season or two outdoors, you can 
visibly see the surface degradation and you can often seen 
numerous broken strands.
To slow the degradation of polypropylene rope, or any plastic 
for that matter, anti-oxidants and ultraviolet inhibitors may 
be added. These increase the life of the rope somewhat, 
but they do not offer ever-lasting protection. Certain 
colors offer better ultraviolet protection too. I am sure 
that most people are familiar with the fact that organic dyes 
and pigments fade. Red is probably the worst offender, 
followed by yellow. Bright white and black generally hold up 
the best. In polypropylene rope, the black will generally do 
better than the white. Since the polypropylene is naturally 
translucent, it takes little white pigment (usually titanium 
dioxide) to make it white. Black is cheaply obtained by 
adding carbon black to the polymer melt. The white pigment 
tends to reflect the ultraviolet while the carbon black tends 
to absorb it and convert it to heat. With its low pigment 
loading, black polypropylene is probably better than the 
white.
A much better choice is nylon rope. It is strong, readily 
available, and has a fairly high ultraviolet resistance. It 
does stretch considerably, and this is an important 
consideration in antenna work. Chemically, the nylons are in 
a family known as polyamides. Their chemical resistance is 
normally good except in areas where acid conditions exist. 
These tend to occur in industrial areas with high smog or 
where acid rain is prevalent. Again bright white or solid 
black is to be preferred in outdoor use.
Probably the best rope materials for outdoor use are the 
polyesters. Typically these are polyethylene terephthalate 
(PET) or polybutylene terephthalate (PBT), although I suppose 
polyethylene naphthalate (PEN) ropes exist today. The most 
common material is PET [more properly called poly(ethylene 
terephthalate)], known under such trade names as Dacron 
(DuPont), Fortrel (Wellman), Kodel (Eastman), A.C.E. 
(Honeywell). etc. [For some reason, rope vendors at many USA 
hamfests tend to charge a premium for Dacron, probably 
because of the name recognition.]
Polyester ropes have very low stretch making them excellent 
for guying applications. They are excellent in both chemical 
and ultraviolet resistance. The polyester materials have an 
interesting property that makes them especially ultraviolet 
resistant. These materials fluoresce upon exposure to 
ultraviolet light; they absorb ultraviolet radiation and re-
emit it as light at a longer wavelength. Generally the 
emission is in the near-infrared region but some materials 
fluoresce in the visible spectrum too. This is an effective 
way of getting rid of the energy absorbed from the 
ultraviolet and it provides much of the ultraviolet 
resistance seen in these materials. Their chemical 
resistance is even better than the nylons. Black or white 
colors are still to be preferred, although I have seen 
surplus military rope in the traditional olive-drab color 
that should perform well. Since the polyesters are generally 
clear, they take more white pigment to color them than to the 
polypropylene materials. Thus the white rope is almost as 
good as black for long-term use.
There are other specialty ropes available such as those made 
from polyaramides (Kevlar and Nomex by DuPont) and 
polyimides. The Kevlar ropes are extremely strong, but they 
require special stranding and covering to avoid abrasion 
problems; their abrasion resistance is quite poor. There are 
also composite ropes made with a core of one material which 
is clad with a woven covering. These tend to be rather 
specialized and are probably not worth much discussion. 
However there are polyester covered polypropylene ropes on 
the market today. These rely on the good strength of the 
inexpensive polypropylene core, while the jacket provides 
considerable protection of the core from sunlight. Wire 
ropes are also commonly available, and entire books have been 
written on their design and application. It is usually best 
to consult the manufacturers directly about applications of 
these specialty ropes.
I hope this gives a better explanation of why you might not 
want to use polypropylene in many applications. However, I 
do like to use polypropylene ropes for Field Day antennas. 
These are used only a few days each year, and the bright 
colors can provide contrast to prevent people from walking 
into them! They are inexpensive enough to discard too.
 73, Barry L. Ornitz WA4VZQ [email protected]