[Antennas] Painting Antennas and Fiberglass Spreaders for Quads
Barry L. Ornitz
[email protected]
2003年5月24日 01:46:16 -0400
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:
> 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]