Anyone know what differentiates a Tesla coil from any other old coil?
Is it simply that they all seem to be EHT types or is there some
particular quirk concerning the way they're wound? IOW, what sets 'em
apart?
============================

Paul, I've never seen a Tesla coil or even recollect seeing a photo of one.
But I can imagine.

They are a means of generating very high voltages at relatively low radio
frequencies.

Low radio frequencies to minimise RF radiation losses.

The coil is quite an ordinary, single-layer, solenoid, close-wound or
relatively close wound in the form of a helix. Grounded at the lower end.
Open circuit at the top, high-voltage end.

It is coupled to the RF generator via a few turns link winding at the bottom
end.

Or the generator may be tapped into the bottom few turns.

The working frequency is that at which the coil is self-resonant with its
own distributed self-capacitance.

The generator, by virtue of the number of turns on the coil and the number
of the turns on the link coupling, is impedance-matched to the Q of the coil
such that all the generator power is dissipated in the coil's RF resistance.
This, of course, corresponds to maximum voltage at the top of the coil.

The situation is identical to a short HF helical antenna without a whip on
the top, operating say in the 160 or 80 metre band - but without the
radiation loss.

The radiation loss in the Tesla is low because its operating frequency is
low. Most of the power being dissipated in the wire resistance.

The theory is very simple. The coil inductance with its self-capacitance
must resonate at the working frequency. The coil Q must be very high and
the wire resistance must be low to generate a very high voltage.

Many turns of wire are needed to resonate at LF but the wire diameter must
be large enough to have a low resistance. Hence the larger the physical
size of the coil the higher is the attainable voltage.

The few number of turns on the link (or tap) coupling is calculable but is
better determined by experiment. One will always end up by pruning to match
to a particular generator impedance for maximum Tesla voltage.

The coil's inductance is easily calculated from well known formlae. The
coil's self-capacitance is more complicated. But what is really required is
the resonant frequency. Or it can be found by experiment. (Which is what
Tesla must have done.)

It so happens that my program SOLNOID3 will calculate the isolated
self-resonant frequency of a solenoidal coil from its dimensions. For Tesla
coils dimensions will be in metres and inductances of 10's of milli-henrys.

The practical design problem is insulation between adjacent coil turns. And
of course the driving power. Having never seen one my instinct guides me
towards a coil length to diameter ratio of 15-to-1.

Have you any intentions of making one?
----
Reg, G4FGQ