In article Tuno writes:
Saying it is' a "whip" does not tell us much.

It's a 64 inch whip on a Radiall/Larson magnetic base that contains
the frequency matching coil, from Tim@W&W.

What frequency are you planning on operating on?

If it is around 123 MHz, and a 1/4 wave, it should be about 23 inches
long. For a 5/8 wave, I calculate 59 inches in length.

If I understand you, the ground plane *radius* has to be at least 64
inches. That's one big piece of sheet metal! I'll have to make due
with 12, unless that's going to bother my radio when I xmit.

The ground plane radius should be about 1/4 wavelength plus about 5%
(these figures from radials such as wire). The wavelength at 123 MHz is
about 96 inches, so 1/4 wavelength would be 24 inches. Add 5 % and you
would build with radials of 25.2 inches. Planting the magmount somewhere
near the middle of the roof should do just fine.

Note: this is for a 1/4 wave antenna. For a 5/8 wave antenna, the
ground current should be lower due to the higher base impedance, so the
ground plane will have less impact. I suspect that the 64 inch whip was
added to a 5/8 wave base transformer, and may be just a bit long for the
aircraft band.

On top of a motor home, a 64 inch whip will hit a lot of things as you
drive.

Alan
wa6azp