REFLECTOR: Transponder antennas

[Dave Black]

Al,

> I concluded I would use the small “monopole” antenna in the
> nose for a short lead.  Aluminum ground plane was mentioned –
> would light gauge SS be just as good?

Sure. Aluminum is normally chosen because it's lighter. But at the size of a
transponder ground plane, even lead would not be too heavy. If I recall
correctly, a ground plane should have a radius equal to the antenna's length.
So for a transponder antenna, the ground plane is pretty small. 

> I find the small monopole antennas priced over a broad range,
> all look and sound the same.  Aircraft Spruce: TED - $19.95,
> AV-22 - $48.95, Comant CI-101 - $63.95; and Aeroelectric -
> $34.50.  Oh, yeah; and Velocity store $37.95. What’s the
> difference?

Quality of construction? Markup? There should be little or no difference in
performance. All it is is a 1/4 wavelength piece of wire with a connector on
one end and a knob on the other end so you don't poke yourself in the eye. 

> AS also has the Advanced Aircraft Electronics L2 that
> advertises high gain, and no ground plane required – mount
> anywhere inside composite aircraft - for $85.95.

That's a 1/2 wavelength antenna, hence does NOT require a ground plane. In
theory, it should be a bit more efficient and provide better signal strength
than the 1/4 wave antenna. It's designed to be mounted INSIDE your glass
plane, so is not suitable for use in a metal aircraft. But as was noted here
recently, inside mounting can result in objects like batteries and people
'shadowing' part of the radiation pattern, and producing a dead spot in
certain directions. 

All transponder antennas must be vertically polarized. Since Velocity is a
glass plane, even the 1/4 wave antenna can be mounted inside the fuselage or
nose, if you choose. There's nothing magic about having that little antenna
stick out in the breeze. But if you do use the 1/4 wave antenna, you MUST have
a ground plane, the ground plane must be horizontal, and the antenna should
stick down from it. Aside from that, you want to mount it at least a foot from
any object which will 'shadow' its transmitted signal. 

For an explanation of shadowing, consider the antenna as a light bulb. It
happens that the 'light' (or in this case radio waves) travels through
fiberglass, but not through metal or people. So you need to mount your antenna
where its 'light' can be seen 360° around your airplane. 

Due to the great amount of signal loss within the transponder cable, it's
always best to keep your cable length as short as possible. This pretty much
says your antenna should be mounted in the nose or keel.


Dave Black
Shortwing RG