On Wed, 23 Dec 2009 14:02:11 -0500, rich
wrote:

Boy, those are a lot smaller than I thought they had to be. The pie
pans sound like a good idea since they aren't so thin like foil is
and easier to keep from damaging. But solder doesn't stick to
aluminum, so how to make a good contact that won't come loose to the
pie pan? I may just get that antenna kit you sell at RST with that
copper foil.

The transponder and GPS antenna bolt directly to the ground plane.
That makes both the mechanical and electrical connections. Copper
tape is not being used; the transponder can be the little whip
monopole with the BB on the end. The GPS can be any one of a number
of commercial units that bolt directly to the ground plane. Copper
tape dipoles are used where you need isotropic (whole orange)
radiation pattern. Ground plane antennas are where you want
hemispherical (half an orange) radiation. Transponder radars are
always down. GPS satellites are always up. Unless you fly inverted
{;-)


I had started to think that you didn't write any ground plane articles
in SA, since nothing comes up about that online or in the CD
collection.

That's true.


That belly panel on a Glasair is fiber glass with two layers, and a
half inch foam core between them. Do the ground planes need to stand
off from the inner skin if they are seperated from the lower/outer
skin by a half inch? The space between the upper skin of the belly
panel and the lower wing skin it covers is less than a quarter inch,
so there's no room for the ground plane to stand off. I could mount
them back in the tail cone if that is needed.

I do antennas. I don't do mechanicals. That is left to the
homebuilder.



I had thought about installing one of those pre-made di-pole
transponder antennas. But to mount it vertically, about the only place
I could put it would be on the back side of the spar in the wheel
well. And that would put the microwave energy only a half inch away
from the fuel cell. Not sure that would be kosher. On a Glasair 3, the
entire leading edge of the wing from the spar forward and from tip to
tip is all fuel.

Transponder antennas should NOT be dipoles. They should be monopoles
on ground planes for the half-orange radiation as described above.


It makes sense to me that the ferrite beads are needed, as without
those, I can't see how the antenna would know where to start and stop.
It seems without those, the antenna and the coax would all become the
antenna, and be totally out of tune for just about any frequency.

That's not true. The antenna stops where the pigtails converge into
coaxial cable. The beads are only there to keep unwanted reflected
power from coming back down the outside of the coax cable.

Jim


Rich


On Tue, 22 Dec 2009 10:28:31 -0800, RST Engineering
wrote:

On Sun, 20 Dec 2009 11:13:46 -0500, rich
wrote:

I'm installing it on the belly panel. What I'm
wondering is how big to make the diameter of the ground plane, and how
to make contact with it to the blade type transponder antenna. I would
assume it somehow needs to connect to the outer portion of the BNC
connector? And what about the GPS antenna, does it need a ground
plane?

Make both of them out of aluminum pie pans and stand them off from the
fiberglass surface so that you don't have any agitated porcupine
quills sticking out from the airframe.

Transponder diameter about 5.5" diameter; GPS about 3.75" or 11" if
the 3.75 is too small to work with.

Jim