[Antennas] Re: Painting Antennas and Fiberglass Spreaders for Quads

2003年5月24日 07:18:41 -0400

Barry,
Your response, albeit interesting, reminds me of a fellow whom I knew many
years ago... He was the type who, when asked, "what time is it..?", would
respond with a lengthy lesson on quantum physics and celestial mechanics...,
when all I wanted was the correct time...!
BTW, the manufacturers of these vertical antennas DO refer to the housings
as RADOMES, so that's why the article also used the term RADOME. While your
frequent responses are interesting, they tend toward being pedantic and
extreme... It would be in the interest of all readers for you to submit
simplified responses, and stick to the issue at hand, rather than submitting
a professional resume along with your comments... Since my professional
credentials are similar to yours, I find that the best response is one that
addresses the majority of readers. Also, I'm not into pissing matches,
because they bore the hell out of me... besides, I refuse to have a
'battle-of-wits' with unarmed men..!
I heard a short story about a little boy who asked his Mom and Dad, "Where
did I come from?"... Mom and Dad became flustered, and tried with much
stumbling, to explain the 'birds-and-the-bees' to their son... When they
were finished with their somewhat embarrassing answer, the little boy said,
"Heck, I knew all of that from my sex education class..., but what I want to
know is WHERE I came from... Billy is from Philadelphia, John is from
Houston and I just want to know WHERE I came from.."
and so it goes...
73, Larry - W1GOR
----- Original Message ----- 
From: "Barry L. Ornitz" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Saturday, May 24, 2003 01:46
Subject: Painting Antennas and Fiberglass Spreaders for Quads
> I should have answered this question a while back but I got
> caught up in the more interesting discussion on the complex
> characteristic impedance of coax.
>> Having worked in the research labs of Eastman Chemical, who
> makes plastics, resins, and solvents used by the paint and
> varnish industry, and for W. L. Gore & Associates who make
> true radome material, I think I am well qualified to answer
> this question.
>> Larry, W1GOT, wrote: (BTW, my call is W1GOR, not W1GOT..!_
>> > For those who asked about painting fiberglass antenna
> > radomes, please go to the following URL...
> >
> > http://www.repeater-builder.com/pdf/antennapainting.pdf
>> I read these instructions, and there is nothing wrong with
> them. But they are a little too specific for my taste on what
> brand of paint to use, and they do not discuss the effect the
> paint has on the antenna performance at all. Thus, I thought
> I would fill in some details.
>> To begin, a radome is more properly a structure used to cover
> a microwave or millimeter wave antenna. Its purpose is to
> protect the antenna from adverse environments while having an
> insignificant effect on the electrical performance of the
> enclosed antenna. Typically radomes are made of glass-
> reinforced resin plastics (fiberglass) or they may be made of
> inflated fabric materials (like the PTFE Goretex fabrics).
>> I would not call the protective housing over many commercial
> VHF antennas a radome. This is because the housing is located
> in the near-field of the antenna whereas true radomes are in
> the far field of microwave antennas.
>> A thin layer of paint usually has minimal affect on HF and VHF
> antennas if the proper paint is chosen [however, the thick
> fiberglass housing over those VHF antennas often does have an
> appreciable effect - due to its thickness.] What is happening
> it that the antenna sees a slightly different dielectric
> constant in the area surrounding the antenna. Since all real
> materials have dielectric constants great then unity, the
> result is that the antenna appears slightly "fatter" than it
> did bare. Thus the resonant frequency is lowered ever so
> slightly.
>> You can see this same result when insulated antenna wire is
> used versus bare wire. For thin coatings (at least compared
> to the operating wavelength of the antenna) the effect is very
> small. Recently someone here asked about the reason that a
> dipole is cut to approximately 95% of its free-space half
> wavelength. This is almost entirely due to end effects. The
> so-called velocity factor of an air insulated wire is
> essentially unity. Adding a thin layer of insulation on the
> wire only changes this minimally. [For example, to lower the
> velocity factor of a wire insulated with polyethylene, the
> poly coating would have to be several wavelengths thick for
> the velocity factor to approach 0.66.]
>> So assuming the paint is a "low-loss" dielectric material,
> painting an antenna really changes its performance very little.
>> But what is a "low-loss" dielectric material to paint with?
> Short answer - most common paints! Long answer - read on...
>> It is really quite difficult to get a truly conductive paint,
> i.e. one with a surface resistivity of less than a few
> thousand ohms per square [this is resistivity - not
> resistance, the measurement is made by measuring the
> resistance between the two sides of a square of painted
> material]. Even most metallic paints are pretty good
> insulators when considered this way. But still it is best to
> generally avoid metallic paints.
>> What about dielectric losses other than true conductivity?
> For example, vinyl plastics are far lossier than
> polyethylene. Again, the important issue is whether the paint
> is thin and most of the actual insulation is done by air. If
> so, the added losses of the paint will have minimal effect.
> Now if the antenna is covered by a relatively thick insulator,
> like a PVC pipe versus fiberglass shell, you do need to worry
> about dielectric losses.
>> The real issues when selecting paints for antennas and
> such are how well does the paint adhere, how does it withstand
> sunlight (ultraviolet), and how does it withstand weather.
>> Adherence is an issue with both metallic and plastic
> surfaces. Many paints, particularly water-thinned latex
> paints, do not wet and adhere to all surfaces well. For
> metallic surfaces, it is common to acid-etch or treat the
> metal surface with a primer such as zinc chromate. Usually
> with slightly oxidized metal, this is not a problem. For
> plastics with smooth, shiny surfaces, getting adhesion may be
> a real problem with some paints. Sandpaper may be often be
> used to roughen the surface to promote adhesion. Commercially,
> corona discharge surface oxidation is often used to treat
> plastic surfaces to provide for paint adhesion.
>> Solvent thinned lacquers and oil (or synthetic) based resins
> usually adhere a little better. They also tend to hold up
> somewhat better outdoors too.
>> Ultraviolet resistance is the biggest issue with outdoor
> paints. Acrylic lacquers and enamels actually do quite well
> in this regard, as do the epoxy and urethane paints. For a
> small project, ordinary Krylon spray acrylic paints will work
> quite well. Many manufacturers use carbon-black filled
> polyethylene caps to seal the ends of traps in their
> antennas. After a few years, ultraviolet light does its work
> on the polyethylene. You can see the surface of the plastic
> become porous and oxidized. At this point, a spray coating of
> Krylon or equivalent can extend the life of the insulator
> quite a few years and prevent additional degradation. Plastic
> insulators on VHF antennas are also good candidates for
> painting this way.
>> Many oil-based paints "chalk" upon exposure to the elements.
> This is where the surface of the paint oxidizes and forms a
> chalky coating. In outdoor house paints, this coating can be
> scrubbed off to present a clean surface underneath. However
> this property is not beneficial for antennas as the chalky
> surface can retain water, dirt and moisture. Modern non-
> chalking epoxy and urethane paints are much better suited for
> use here.
>> If I remember correctly, what brought up the question of paint
> was its use on fiberglass spreaders for cubical quad
> antennas. From an electrical standpoint, almost any paint
> could be used here with no measurable change in antenna
> performance. But one requirement is good flexibility of the
> cured paint as these spreaders do flex as the wind blows. It
> may be necessary to sand new fiberglass spreaders to get paint
> to adhere. Weathered fiberglass needs the paint to prevent
> moisture ingress of the spreader and to protect the spreader
> from further weathering.
>> Both epoxy and urethane paints come in air-cured and two-part
> varieties. The urethane paint recommended in the article
> Larry pointed to is a two-part type. In practice, the two
> part types are preferred, but they do have a limited pot life
> and are more difficult work with. These paints are primarily
> used in industrial applications so they may be difficult to
> find in your regular paint stores. Most larger paint
> manufacturers do make industrial lines that are suitable and
> these can usually be ordered through your local dealer.
>> As to color, consider that white reflects ultraviolet the
> best, while black absorbs it best. Either are good choices,
> although a good titanium dioxide white is my personal choice.
>> As to another personal hint, I usually spray all mounting
> hardware such as screws and nuts with clear Krylon after an
> antenna has been assembled. This can provide considerable
> extra corrosion protection. On larger hardware, I often use
> some of the "cold galvanizing" paints to spray with. This
> zinc-loaded paint provides rust protection to steel.
>> So to conclude, the best paints to use on and around HF and
> VHF antennas are those which provide good adherence, excellent
> ultraviolet and sunshine protection, and good weathering
> ability. The electrical properties of the paint are of
> secondary importance. For real radomes, the issue is much
> more complex. But then again, I doubt if many of us have real
> radomes! :-)
>> 73, Dr. Barry L. Ornitz WA4VZQ [email protected]
>