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Re: [canard-aviators] Re: Conductive fiberglass


> >Why not use a dipole antenna for your transponder? No ground-plane, and
> >it'll fit inside your nose. Sportcraft Antennas sells a nice one.
> Hmmm.  Don't we want the transponder antenna to be a bit more
> omni-directional...?  ...I believe a dipole antenna is, in general, a
> bi-directional device (actually more like a doughnut; the hole in coverage
> also applies).

I am hardly the final word on RF matters, but from my study of antenna theory a
few years back I seem to recall a 1/4 wave with ground-plane produces exactly
the same radiation pattern as a half-wave dipole. As I understand, the
ground-plane acts as a mirror, creating an image of the 1/4 wave antenna. Thus,
to look at it, you'd see a dipole. In any case, both produce the same
doughnut-shaped omni-directional pattern when vertically polarized.

If you turned the dipole (or the 1/4, for that matter) to horizontal
polarization, not only would it become bi-directional, but it would be
cross-polarized with the ground antennas. In that orientation it would work very
poorly, if at all.

Quarter-wave antennas are used in spam-cans because:
1) The ground-plane is already installed.
2) No antenna can be mounted internally due to the RF shielding effects of the
aluminum skin.

Half-wave dipoles are better in glass planes because they:
1) Do not need a ground plane, saving weight.
2) May be mounted internally, reducing drag.

I know the drag from a transponder antenna is almost negligible, but it all adds
up. And why dirty up your nice, clean glass plane?

Dave Black
Velocity RG