A transponder antenna is only 2-3/4 inches long. That is a quarter wave
length at the transponder operating frequency. A disk as small 5-1/2 inches
in diameter will provide an adequate groundplane.

Wayne
http://www.soaridaho.com/Schreder

"Orval Fairbairn" wrote in message
news
In article , "Mads"
wrote:

Anyone with depth knowledge of groundplanes for transponders?

- their mission (any theory would be appreciated)?

They complete the electronic circuit for the antenna.


- required dimension

1.5 feet from the center of the antenna should work fine. (1/4 wave)


- required shape (is a cross OK or does it have to circular)

Eight crossing strips of aluminum tape, about 3 feet long, should do
quite nicely.


- material (is aluminium tape OK?)

Yes -- as long as you have electrical contact with the antenna base.
Remove the adhesive at the center so that the aluminum tapes all contact
each other.


- installation - follow the a/c body, or flat


Doesn't matter too much.



- how critical is the above (loss of signal strength?)


Keep it away from other antennae.