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Counterpoise (ground-plane) antenna
performance drops off in the direction where the ground-plane is shortest. They
perform especially poorly when the ground-plane is below the stub, as in
a top-mounted antenna, since most of the radiation is directed
upward. I made all half-wave dipoles for my radios, since they have
exceptional radiation in the horizontal direction, unmatched by 1/4 wave stubs
over a ground-plane with abbreviated length. The glide-slope and
VOR are bow-ties made from 0.010 aluminum flashing with bazooka baluns. The
VOR is in the left wing stub, and the GS is in the right wing stub. My comm is
brass sheet imbedded in the back of the rudder spar, with a ferrite balun. My
transponder antenna is a slot-fed dipole as shown in Fig. 8-5a, p 246, of the
Rad Lab series vol. 12, Microwave Antenna Theory and Design. I had a little
trouble with it initially since it pulled the Xponder frequency off beyond spec.
I added short BNC components in the line, such as barrels, right-angles, etc
until I got the reactive component of the reflected wave to pass through zero at
the transponder. The KT76 output stage appears to be the oscillator, so it is
sensitve to VSWR. I used only Andrews 1/4" FSJ1-50 coax throughout for minimum
loss and no shield reception/radiation to put nulls in the pattern. It's
expensive, but why scrimp with thou$and$ spent on avionics. Perhaps someone
could figure out how to cut slots in the wings and fuselage of carbon planes,
backed by fiberglass, and feed the hoizontal slots to get vertically-polarized
radiation, and vertical slots to get horizontal radiation. I proposed to some
biplane builders who didn't want the comm antenna to show that they conceal it
in a wing strut! Where there's a will, there's a
way!
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