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Old 20th Nov 2017, 4:48 am   #3
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Wireless Self-Resonance Experiments

Al,

Interesting to read about these experiments.

I assume that the design of your apparatus, when it is finished, would have a coupling coil around the main Tank coil, and in this case you would be using large valves to drive the coupling coil ?

In any case, one interesting thing about inductively coupling to a tuned or Tank circuit, is the loading of the driver circuitry on the coupling winding, can significantly up-shift the value of the resonant frequency of the Tank coil. But the effect depends on how tightly the coupling between the coupling and Tank circuit is and the impedance of the source driving the coupling coil.

For well coupled coils the effect happens because the damping on the coupling coil neutralizes some of the inductance of the Tank coil and that is what raises the resonant frequency above what you might get with a free resonance test.

So for example the valve's plate resistance driving the coupling coil can play a role in the final resonant frequency value, even if this parameter not officially a "reactive circuit element".

One good example to demonstrate this, is attempting to determine the resonant frequency of a car's ignition coil secondary. If say you drive the primary from a very high Z source, say >100k, you get a secondary self resonance which is primarily determined by the secondary's self inductance and its distributed winding capacitance. However, if you use a low Z source loading the primary, this neutralizes most of the secondary's self inductance and you get a much higher apparent resonant frequency that represents a value of secondary's capacitance now tuning the leakage inductance instead, which is a much lower inductance than the main winding.

With other various loading of the primary and mutual inductance values in between, for other coupled resonant circuits, the secondary resonant frequency will be up-shifted some amount from what the tank circuit will resonate at on a free test, say when its excited into oscillation and you measure oscillatory decay without significantly loading it.

There is a great equation in Terman under coupled resonant circuits that explains this effect well. I'm not sure if it would relate to your setup. Do you have a sketch of what your valve driver, coupling coil & coupling arrangement to your main Tank circuit looks like ?

Hugo.

Last edited by Argus25; 20th Nov 2017 at 5:05 am. Reason: typo
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