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