jim ham wrote:
rich wrote:
I'm using pre-made di-pole antennas for the comm radios in my Glasair.
But I need to install antennas for the transponder and marker beacon.
I've found some pre-made dilpole marker antennas, but for the
transponder, I'm using one of the blade type, and for that one, I'll
need a ground plane. 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? I've seen lots of pictures of ground planes made with strips of
copper foil tape radiating from the center. Which looks adequate, and
should work as well as a big piece of solid copper foil, which I have
no idea where to purchase.
Copper tape is available from electronics suppliers such as Digikey
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3M1181B-ND

Jim Weir of RST Engineering wrote a series of articles in Sport Aviation
many years ago on how to construct these antennas and ground planes. He
sold kits of materials at the time. You could email him to see if these
kits are still available. Otherwise you wind up buying a whole roll of
tape.
I also read an article by Bob Archer about
di-poles. He said if the dipole has a little black box in the middle
of it, not to use it, as that contains ferrite beads which greatly
reduce it's effectivness. Unfortunately I already have that type
burried inside the leading edge of my verticle stabilizer.
I'd get a second opinion about this. I'd like to see the article.
My understanding is that the ferrite beads are necessary for impedance
match from the coax to the antenna. One can check how good an antenna is
with an SWR meter. The better the SWR the better the antenna. Anything
better than 2:1 will work, but 1.2:1 makes an superior antenna. It's
always a good idea to check an antenna installation with a SWR meter in
any case. Check right at the connection to the radio. It's too easy to
introduce problems at connectors or other coax connections.
I know of several airplanes with home-made copper tape antennas that
followed the Weir design. They include the ferrite beads and they work
fine.

Rich


Actually, the SWR meter should be used right at the antenna. That said,
you won't find many people that have SWR meters that work at the
approximately 1000 MHz range of the transponder. Plus, I believe they
only transmit when they receive an "interrogation" from the radar site
transmitter, so it would be hard to "key up" the transponder to read the
SWR meter.