ground planes (radiation surface)really can never be too big but often there
are times when the "ideal" ground plane just simply cannot be used...as a
simple rule of thumb the ground plane radius can be equal to the radius of
the length of the conducting mast...with this as a ROT you'll find the
typical ground plane for a 5/8 wave antenna tuned to 118-136 MHz is going to
be quite large. A car, van or SUV roof top though is getting pretty close to
the ideal even for a long mast like this...also keep in mind the ground
plane may not always be the center of the mast, in these cases the signal
sent may be better from the area or direction with the most ground plane...
Even when it isn't possible to have a single large metal ground plane you
can use simple foil tapes (Home depot) "metal" duct tape or even copper or
metal wires in a radial "X" Pattern (like the whiskers (ground plane)you see
on many base antennas...
also ideal...not always available...in the case of your handheld radio the
radio itself is the ground plane.........certainly not anywhere near the
"ideal" but it works....with some necessary compromise (BTW, your handheld
antenna isn't really 6" long but is a coiled 1/4 Wave mast equal to about
22"-24" for airband... even many glider manufacturers have installed simple
rubber duck antennas in gliders for ELT antennas using little more then the
coax cable running back to the ELT.....certainly not ideal...but a
compromise that will still work....with limitations.
Best to try the antenna/radio combination with a SWR meter..try adding and
removing mast and ground plane where possible and get the best signals
output you can for your situation...it may be a compromise but all things
considered may be the best you can possibly get..and likely, quite good
enough
tim

Please visit the Wings & Wheels website at www.wingsandwheels.com


"Tuno" wrote in message
...
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.

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.

2NO