You know, I can't find those articles Jim Weir wrote in Sport Aviation
about ground planes. My Sport Aviation on CD set only goes up to 2001,
so those articles must have been written after that. I'll check the
EAA website, they've got later issues available online for members.

On Sun, 20 Dec 2009 13:33:55 -0800, jim ham
wrote:

rich wrote:
Here's the article: http://www.express-builder.com/docs/tip1/tip1.pdf
written by Bob Archer. It's the 5th paragraph down.
what he says is: "I do not recommend any antenna on the market that
has a little black box in the center of the antenna. This device is a
ferrite transformer which provides a very good VSWR and a very good
bandwidth but at the cost of being a very lossy (absorbs energy)
device. The very best specification that I have seen on ferrite
transformers is a loss of 2.5 dB and the worst goes up to 12 dB. As a
reference, a 3-dB loss gives an output of 50% and 10 dB gives just 10%
out. So if you have a 5 watt transmitter into an antenna like this,
you get just .5 watt out, and it works the same on receiving. Not a
bargain. An antenna you can easily make yourself would be to just
solder quarter wave elements to the inner and outer conductors of the
coaxial cable and go with it. Also if you were planning to go with Jim
Weir of RST's designs don't bother with the ferrite beads. At these
frequencies the beads don't do anything that I could detect in the RF
lab. A good balun would work better as a dipole feed because it
balances the currents on the elements and matches the impedance at the
same time and it doesn't
absorb RF energy. My antenna designs do not need a balun because I use
a modified version of a feed called a Gamma match that feeds the
antenna at the fifty-ohm point and automatically balances the currents
on the elements."

Interesting article. Note that he does say that "An antenna you can
easily make yourself would be to just solder quarter wave elements to
the inner and outer conductors of the coaxial cable and go with it.".
This is a description of the design Jim Wier promoted years ago. His
design included a couple of ferrites slipped around the coax to keep the
shield from ringing (introducing a lossy element). I'm not sure if these
count as ferrite transformers. One does have to adjust the length of the
antenna and the effective diameter to achieve the bandwidth needed.
Jim's article talks about this.

Everybody talks about transmit power, but 1W transmitted from the
antenna is plenty for aircraft. The real difference you will see with a
good antenna is receive sensitivity. A good transmit antenna is a good
receive antenna and vice-versa.

There are lots of aircraft with crummy antenna setups that seem to be
able to transmit and receive fine. So while Bob is correct, in the real
world it sure looks like one can live with _much_ less than ideal and
maybe never even notice. Strange.


I've got the Sport aviation on CD's, so I'm going to look up the
ground plane articles.

On Sun, 20 Dec 2009 11:12:40 -0800, jim
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