Dr. Anton T. Squeegee wrote:
. . .
An antenna is a coupling device. Its purpose is to efficiently
"couple" or radiate your signal into another transmission medium (open
space, including our atmosphere).

Not really. The power going into the antenna isn't a "signal", but the
power leaving it is. An antenna is a transducer -- it changes power in
the form of electrical voltages and currents into electromagnetic E and
H fields.

The antenna (effectively an inductor) needs to be resonant at your
operating frequency of choice.

That's completely untrue. There's nothing magical about resonance, and a
resonant antenna radiates no better or worse than a non-resonant one.

As for being an inductor, the impedance at the feedpoint of a small loop
antenna is inductive, but an antenna in general certainly aren't
"effectively an inductor".

It seems to me that the density and
composition of the transmission medium you're coupling to has an effect
on any antenna's resonant frequency.

The conductivity, permittivity, and permeability of the medium in the
immediate field of the antenna have a profound effect on the antenna's
resonant frequency. Density in and of itself doesn't.

This, to my understanding, means that an antenna designed for use
in open air will not work underwater, nor will, say, a VLF antenna
designed for submarine usage work in open air.

It certainly won't work the same as it does in the very different medium.

If you believe I'm misunderstanding, then please point me to some
documentation on the subject, and I will cheerfully STFU.

Any basic antenna text will do. Kraus's _Antennas_ is a good starting point.

Roy Lewallen, W7EL