In article ,
Jeff wrote:

But the difference between the smaller and larger magmounts is what
puzzles me. Shouldn't both mounts give similar SWR readings?

It would be worth checking to see whether the same antenna, on the
larger mag-mount, on a larger ground plane gives better results.
It's possible that it won't.

Most mag-mount antennas that I have seen, are designed to establish a
capacitive coupling with the metal ground plane, so that it'll
actually serve as an RF ground. The shield of the coax cable is
connected to a portion of the mag-mount base, and this is then
capacitively coupled to the car body (or whatever) in one way or
another. Some mag-mounts seem to depend on the magnet itself being a
good enough conductor to provide the coupling. Others place some sort
of thin, conductive metal foil across the bottom of the magnet
(sometimes with a protective plastic covering) and then connect this
foil to the coax braid somewhere inside the base.

It's possible that the large mag-mount base you're using doesn't have
enough coupling capacitance. Possibly, it was designed for use on an
HF antenna base, and isn't _intended_ to provide capacitive ground
coupling. At HF frequencies, capacitive coupling doesn't work very
well for mag-mounts, and the bases are usually designed to be grounded
to the car body via a wire or braid.

If your big mag-mount doesn't couple the ground to the air conditioner
base well enough, you'd probably end up with an ineffective ground and
a high SWR, and possibly one which changes as you move the coax
around. The outer portion of the coax braid would be working as a
counterpoise... and how well it works would depend on its position,
whether it was coiled up, how close it came to other metal objects,
etc.

If you want to be independent of the ground plane provided by your air
conditioner, you might want to investigate an end-fed half-wave
antenna. You can get mag-mountable commercial half-wave antennas
which use a matching coil in the base, and you can make (or perhaps
buy) a J-pole made out of something like thin brass or aluminum rod.
If you have an outdoor balcony with one of those large sliding glass
doors, I'll bet you could fabricate an on-the-glass J-pole, or a
center-fed vertical, out of adhesive-backed copper or aluminum tape
stuck to the glass.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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