In article , 'Doc wrote:

Jeff,
Try using the AC as a mounting point for a horizontal
bar running out far enough to mount an antenna on, such
as some type of 'base'(?) antenna.

I've tried to figure out a way to do exactly that. I'd love to get a
"stand off" far enough away so I could put on a simple quarter wave
ground plane, but it'd take a more mechanically clever person than I to
engineer such a thing off this AC box. Not only to do that, but to do
it so the maintainance guys here don't get upset with a modification to
the apartment's property.

All antennas have to be tuned and/or impedance matched.
Any antenna near obstructions will have undesirable
characteristics compared to the same antenna in the 'clear'.
I think what you're seeing is fairly 'normal' for your
situation. Good luck...


But the difference between the smaller and larger magmounts is what
puzzles me. Shouldn't both mounts give similar SWR readings?
--
--
J. Rogers KB1HOL
KB1HOLatARRLdotNET

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
"Others place some sort of thin, conductive metal foil across the bottom
of the magnet."

Skin effect confines r-f to conductor surfaces. It is difficult to get
common-mode current through spaces as tiny as glue and paint thicknesses
on either side of the metal foil at the bottom of a mag-mount.. Polarity
of propagation at any instant is the same on all surfaces involved. The
little electrons avoid each other as much as possible on all surfaces.
Also, any current generates counter-emf which in a combination of such
surfaces tends to strongly oppose current, just as flowing into the
depths of aluminum or copper is opposed at r-f.

Dave also wrote:
"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."

An end-fed antenna, regardless of its length, requires a ground or
something else to provide the second connection for the antenna.

A balanced antenna such as a dipole or loop is the only system
independent of the ground plane.

Best regards, Richard Harrison, KB5WZI

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

A thought - mag mounts are designed to work on a painted surface
through capacitive coupling. If your metal box is unpainted metal,
you might be getting a DC path through one mag mount and not the
other.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil,
Paint the airconditioner??
'Doc