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The so-called ground-plane, more accurately referred
to as a counterpoise, is an integral part of the antenna element, and as such,
must be of the very highest conductivity so as not to incur tx-rx losses. Even a
steel counterpoise will degrade an antenna's performance. The counterpoise
should be either aluminum, copper, or gold.
Actually silver has the highest conductivity of all
metals, which is probably why it's used to plate the center conductor of some
coax cable; RG142B for example. Granted the plating is very thin, if
memory serves less than a mil (0.001"), but at aircraft nav/com frequencies
current only flows in the outer 0.2mils or so of the conductor due to skin
effect. Copper is not far behind silver, 5 - 10% less conductive, and
better than gold. Aluminum is about 40% less conductive than silver, but
still a pretty good conductor. Gold is used a lot as a plating
on electrical contacts, think connector pins, not because of its
conductivity but because it's more resistant to corrosion and oxidation than the
other metals.
Having said all that (sorry, I get carried away)
anecdotal data from 3 different Legacys I've had some experience with indicates
the carbon fiber skin can make a good counterpoise. I don't have any
measured data to compare signal strengths but I have heard the radios of these
planes from the ground, using a handheld, and from the air. All of
them have strong, clear transmissions and none of the owners have ever
complained about lack of sensitivity on receive. One of them, which has
been flying for 5+ years, I heard from a distance of 75 nm while we were both in
the air. His transmissions were loud and clear.
So should you add a metallic counterpoise
to your carbon fiber airframe? It's your choice. Do whatever
gives you the highest confidence level. Based on my experience so far I'm
not planning to add one to my Legacy.
Tom Gourley
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