Wimpie wrote in :

Never mind Maxwell, why is no-one asking "why is an electromagnet not
used as an antenna when driven by AC?" That should be a basic reality
check because I've neve heard of such a thing working, and if it did it
would be widely known.

You may know that an AC current through a loop does generate near
fields, and far (EM radiation) fields. This follows from Maxwell's
equations. The solution for radiated power from a loop carrying AC
current is:


I did know, but I didn't think it through. What I had in mind was the kind
of electromagnet usually used to simulate a magnet, one with a big
lump magnetic material to guid the flux and concentrate it, etc. I imagine
the moment you do that, you get closer to a normal magnet, but it probably
makes the coil unusable as an antenna at the same time.

The maths is beyond my easy grasp, but I do have one thought... Is this one
reason an Oudin coil is using a high frequency? (High-ish... many tens of
KHz, compared to usual mains AC frequency.) They're often used to start
erratic ion laser tubes, to avoid damage to glass and seals by arcs. I'm
wondering if the combination of extreme proximity and higher frequency than
mains-AC coils might be using EM radiation to help penetrate the tube safely
in ways an arc could not, without damage, and in ways a low frequency coild
could not at all, with or without the magnetic material as a core. If this is
the case, then an Oudin coil might be a rare practical case for EM from
something far laser to a magnet than an antenna. I'm just guessing here, but
interested enough to ask.

I like the quadrature pair of loops idea. I should have thought of that at
least. I have explored quadrature generation and circular distributions in
audio, and forgot to make the connection somehow.