Transponder antenna ground plane - Help!

Bastoune wrote:
While finalizing the installation of a TRIG TT21 transponder, I am
running into less that consistent information regarding the ground plane
spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod
antenna inside the fiberglass fuselage.

The TRIG TT21 Manual has this to say about the dimension of the ground plane:
"[...] As the ground plane is made smaller, the actual dimensions of the
ground plane become more critical, and small multiples of the wavelength
should be avoided, as should circles. Rectangles or squares are much less
likely to create a critical dimension that resonates with the
transmissions. The smallest practical ground plane is a SQUARE around
120mm per side; as the size increases the performance may actually get
worse, but will be better by the time the ground plane is 700mm on each side."

From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly
states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."

Now, when I look at all of the Schleicher TNs, I see that they call out
in most case a 130mm ROUND aluminum disk to be used in conjunction with
the RAMI antenna mentioned above.

So what gives? Round or Square? Short of a very large plane, what
dimension to use for the ground plane? The fact that the information
available does not seem to converge on a given dimension or shape for a
ground plane tells me that this is probably more than one "OK" answer.

Thanks,
B

Does your glider manufacturer have a TN or similar for a transponder
install in this or a similar glider ? if so just follow those directions.
Ground planes are usually better off on the larger side and round. If
making this up a fuselage internal antenna install yourself you *might* be
better off with a dipole antenna that does not need a ground plane.

The Schleicher TN ground plane diameters are often limited by internal
clearance, however again you should follow the glider manufacturers
directions if they exist. There are reasons that an external antenna
install may be preferred to an internal one.

It is fantastic that you are installing a transponder, but I get nervous
with questions like this, if you are having to ask these questions are
there other things as well you are overlooking or misunderstanding? it is a
great idea if you can to have an experienced glider A&P or knowledgable
person check on your plans well before you start the install.

Darryl