REFLECTOR: Transponder Location and Ground Plane

[Dave Black]

> Bob,

> We have a lot of carbon fiber in various places around
> our hull and are a little concerned about masking the
> transponder antenna, and are therefore thinking it may be best
> to use a protruding external blade antenna.

You definitely CANNOT put the transponder antenna inside an area where there's
carbon fiber in the skin. Carbon fiber has the same effect on radio waves as
aluminum. 

If you use the 1/4 wave (3") antenna, it MUST be mounted to a ground plane,
and the antenna MUST point down (ground plane horizontal). In your case, if
you mount the little antenna to one of the carbon areas (make certain it's
electrically connected in the center) it should do fine as a ground plane.
 
> For those using the internal antenna, are you mounting it
> oriented down with some kind of arrangement so that there's
> a ground plane above it?  

Yes, if you use the 1/4 wave antenna it MUST be mounted with a ground plane. 

If you had a normal GLASS airplane, a 1/2 wave transponder antenna would be
the logical choice. It is about 6" tall and does not use a ground plane.

> How big of a ground plane are you using?  

The ground plane radius must be at least as big as the length of the antenna.
Ideal material would be solid aluminum, but as mentioned above carbon fiber
should do fine. 

Keep antenna cable length as short as possible, as there is considerable
attenuation in signal at transponder frequencies. 


Dave Black
Shortwing RG