spectral analysis of Tesla coils

[Physikfan]

Hello Tesla friends

I have two questions:
1. Has anyone done an electromagnetic spectral analysis of Tesla coils in operation?
2. Has anyone determined the self-resonances of Tesla coils in the off state, perhaps with shorted spark gaps?

Just before Christmas, two atmospheric Tesla discharges:

[Berni]

The spectrum depends a lot on what kind of tesla coil it is. A big rotary spark gap tesla coil run at high power to produce lots of corona discharge will cause noise all over the spectrum, not just at its resonant point(Tho still be by far the most powerful there). If you have a solid state or vacuum tube based tesla coil run at low power and no modulation you will get only a single sharp peak of RF power at its resonant frequency, acting as a powerful CW radio transmitter.

Checking the resonance while its off is easy. You just put a loop of wire around the tesla coil and connect it to a network analyzer or any other piece of equipment that's capable of measuring the return loss on an antenna. When you feed the resonant frequency into that loop its energy should be sucked up by the secondary coil and radiates out so you get very little reflected signal back out. When its not at the correct frequency the RF power you put into the loop of wire will simply return back to your equipment showing a lot of reflected power.

[Physikfan]

Any nonlinear behaviour of the Tesla coil circuit will cause higher frequencies, of course high power will create a lot of nonlinearities.
More than 10 years ago I did some spectral analysis on a rather very big Tesla coil
and found frequencies up to 1.5 GHz on a logarithmic scale for the y-axis.

"Checking the resonance while its off is easy. You just put a loop of wire around the tesla coil and connect it to a network analyzer or any other piece of equipment that's capable of measuring the return loss on an antenna."

Is it really that easy?

In this case what are you doing with the spark gap, shortcut?

And the dynamic behaviour of a working spark gap:
Is is possible tp measure the impedance of such a device?

Is it difficult to compare working and non working Tesla coils?

[Berni]

Yes as soon as you have any sharp transitions you get plenty of harmonics all over the place, well into the GHz range even.

As for the resonance point, yes you can find other resonance points trough various circuits found in the tesla coil. But the LC tank circuit formed by the secondary inductance and the top load capacitance is the largest one. Since that is what creates the output means that it will want to generate an output at the resonance frequency of it. Due to the high Q factor and the loose coupling to the primary coil this resonance is pretty sharp and when all of this is put together it sort of forces the whole tesla coil to operate at that.

But that's not to say the primary side of the coil is not important. Having a resonance frequency in that circuit be close to the resonance of the secondary will be a big help in getting more energy across to the secondary and so give you more output power. This is why the primary coil often has a adjustable tap on it to tune the inductance of it. If you wanted to characterize it i suppose you could connect a network analyzer across the terminals of the sparkgap. This would let you see the primary side resonance with the sparkgap conducting. Its when the sparkgap conducts that the energy is being moved into the tesla coil so that's when it is important. The open sparkgap state is there just to charge up the capacitors for the next injection of energy.