This experiment may interest you.
(Mods, please can you change thread title to 'Wireless Self-Resonance Experiments'? - thank you. Also is it possible to rotate my sketch please?!)
Some of you will of course be familiar with this, as professionals electronics engineers. But for those of us, who, like me, had only previously investigated self resonance by calculation, or from inductance measurements taken with a bridge or a meter, the method may appear counter-intuitive and even unlikely, since it it involves measuring scattered radiation effects from an open circuit inductor coupled to an induction loop. Yes, you read correctly: without connecting to it at all.
I am
directly quoting from the following paper, which inspired the set-up shown in my photos. The author is David Knight G3YNH
The author explains:
''The connectionless measurement method turns out to be straightforward because the electromagnetic field around a resonating inductor is extensive, and the E-field magnification effect is so great that the scattered signal tends to swamp the electric component of the excitation field. Complete electrical isolation is, of course, impossible; but situations involving either minimal disturbance or quantifiable disturbance are not difficult to achieve. The basic technique is that of exciting the coil using an induction loop, and sampling the field using either a small dipole or another loop.''
David Knight expands the theoretical basis of his work at length, and uses various glow tubes to indicate visually the effects of the integer multiples of a half-wavelength.
The author uses a transceiver as a signal source into a coupling loop. I don't have one; see instead my photos showing a a home-made 20W modulator, connected to an induction loop.
This has a low inductance and is non-resonant at the source frequency, which is variable between 190-390KHz. The induction loop is around 20cm away from the inductor under test. It is the outer coil of a variometer, 5 turns of 3mm dia wire, to a length of 5cm. The inner (litz) winding is not connected.
The long red coil (inductor under test) is my main Tesla coil inductor - the secondary coil - and is resting on a reasonable insulator. Ideally it should be suspended totally. Its ends are not connected to anything.
The inductance is approx 25mH. Using the wireless method described, with the coil open-circuit, the measured self-resonant frequency, detected by a scope probe hanging in the air at the distal end of the tube (and again, not connected to the coil) is around 220KHz, which is 100KHz lower than the measurement I made by close-coupling the coil to a tuned circuit, the other day.
This previous measurement is mentioned in post no 113 on my thread about constructing a large valve Tesla coil. This may not be accurate as I'm not yet replicating the method of sampling the frequency shown.
The author goes on to explain how:
A coil exhibits self-resonance because a wave travelling along the helix is reflected at the impedance discontinuities that occur at the ends of the wire. Resonance occurs when the wave gets back to its starting point in phase with itself, and a corresponding standing-wave pattern develops. A very strong response is obtained when the wire-length approaches an electrical half-wavelength. This is the fundamental self-resonance frequency (SRF), generally simulated, with moderate accuracy, by representing the coil as a lumped inductance in parallel with a capacitance (the 'self-capacitance'). That this is just a representation, with little to do with the physics of the processes occurring in the coil, becomes obvious when we note that there is also a series of overtone resonances.
I hope to go on to investigate some spectral effects as described by David, as well as to properly replicate his test rig.
Please note, if it isn't obvious, that very large voltages indeed may be induced across the inductor under test, even when excited by low powers of RF (my home-made signal source has about 20W output and runs off a low-voltage bench power supply at 30V)- not a hazard to us, but enough to mess with sensitive measuring equipment - and any capacitors used in the later experiments should be kV rated types.