Logan Shaw wrote:

I'm no electronics expert, so perhaps I'm missing something, but I
thought radio waves were simply (changing) magnetic fields that
induce (changing) electric fields that in turn cause magnetic
fields, and the process repreats until the energy is absorbed
and/or an antenna picks up the signal by having one of those
generations of magnetic field turn into electricity within the
conductor.

Right, but the end result of this is a self-propagating field that
behaves _very_ differently than just a single E or B field.

The point being, I don't see how there can be a distinction between
hum caused by changing magnetic fields and hum caused by radio.

The effect is very different. An alternating B field doesn't propagate
very far. You need a lot of turns on a coil in order to pick it up because
what you are doing is picking it up through direct magnetic induction. The
field is very limited and you have to be in the path between the pole
pieces to pick it up.

But, if you have real RF, you get stuff that can be picked up just from a
short length of cable (because you are basically picking up the E field)
with no magnetic induction required.

If the two wires are really close together and the frequency is,
say, 100 Hz, then wavelength is really, really long, and in fact
the distance between the transmitting and receiving antenna is
waaay less than one wavelength. But does that mean it's not
the same thing as radio?

It's not the same thing as radio, no. There is a discussion of this in
the ARRL Handbook, I think.

And it was really a stroke of genius on the part of Maxwell, by the way,
to explain how two related but differently-behaved phenomena can combine
to make a third even more differently-behaved phenomenon.

By the way, from what I can dig up, it seems that the US Navy
uses radio at 76 Hz to communicate with submarines. Apparently
the antennas are fairly long, like way over 10 miles long.

Yup. Because they need to generate an E field at that frequency.
It's not easy to make RF at very low frequencies because of the size
of everything required... capacitive and magnetic coupling are much
more likely at low frequencies.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."