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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. It should have its longest dimension in the
direction to which you want the greatest sensitivity. A curved counterpoise will
affect the radiation direction. An antenna on the bottom of a curved fuselage
can benefit by the counterpoise running up the sides of the fuselage. In this
respect, it will take on more of the length of a 1/4 wave section, basically
forming a 1/2 wave dipole. A short, flat counterpoise will have very poor
radiation characteristics. Since carbon fiber is more at one with a carbon
resistor, it makes a poor counterpoise, absorbing much power.
As to GS antennas, I just installed one in a carbon
fiber wingtip extension. I cut off about an inch along the
18" trailing edge, attached a foam wedge to form the new trailing edge,
then folded two 1" wide by 8 1/2" long pieces of .01" sheet brass
over the foam, then covered it with fiberglass. The two brass pieces were
separated by 1/4" in the center, with coax attached to the separated ends.
Several ferrite beads were placed over the coax near the feed to serve as a
balun. In the opposite tip I installed a similar antenna, but this one had 1/16"
wires coming straight back 6" from the dipole ends, with 1" diameter loops
formed on the wire ends to act as capacitance to increase the radiation
resistance to 25 ohms. This serves for VOR and Localizer, and with the wire
ends, does not have a perceptible null in the direction of the
array.
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