Testing the TK Tar Baby

[SeaMonkey]

The "hash" you saw in the waveshape across the
load may be an important clue.  Spurious oscillations
are the bug-a-boo of RF Power Amplifiers and in
most cases are not at all desirable.

Your case may be an exception if it seems that the
"hash" is present while it's operating in the strange
but powerful mode.

Can you fiddle with the sweep on your scope while
it's in that mode to try and capture the "envelope"
of the hash modulation?

[MileHigh]

TK:

I can offer you a theory with moderately high confidence for the super-bright mode.  It was more or less confirmed in the earlier comments.

I am assuming that this was built with the intention that both resonators would have the same frequency.  If course that's not possible without tuning and many types of caps have a tolerance of +/-20%.

In normal mode the transmitter is generating the near-field and the receiver is picking it up and lighting the bulb.  You are off of the resonant frequency of the receiver so everything acts like a normal filter.  The Q-factor for the receiver is not so narrow, so you get the bulb lighting up.

When it jumps into super-bright mode something interesting is happening.  The receiver LC resonator wants to resonate at its center frequency and it's response to the external excitation from the transmitter is being "felt" by the transmitter.  This dynamic instantaneous impedance due to the receiver as felt by the transmitter gets stronger as you bring the two loops together.

What eventually happens is that the resonant frequency of the receiving LC resonator is "forced" onto your LC transmitting oscillator and the LC transmitting oscillator starts to slave to the LC receiver.  The LC transmitting oscillator is not running at its natural frequency at all, but the nature of the "load," the instantaneous impedance of the LC receiver, gets the transmitting oscillator to sync up to the load.  Once they are synced up, then you have true resonant near-field energy transfer - the same thing that they have for certain laptops and cell phones where you just put them on top of a special mat.

When you move the LC receiver away from the LC transmitter, the "forcing power" of the synchronization goes away until eventually the main transmitting LC oscillator "breaks free" and goes back to its natural operating frequency.

I think that's the way the cookie crumbles.  The transmitter falls under the influence of the receiver and starts to oscillate in sympathy to the receiver.

MileHigh

[TinselKoala]

....
Can you fiddle with the sweep on your scope while
it's in that mode to try and capture the "envelope"
of the hash modulation?

Yes, I've done that. I'll take a photo in a few minutes. It's not what I usually call "hash": it's a regular little waveform superposed on the main sine wave, not a "noise" kind of thing at all.

Hmmm..... interesting.

It looks like the "hash" is smoother when the circuit is in the brilliant mode, and more pronounced in the "regular" mode.
See the photos below. Timebase is at 0.2 microseconds per division, vertical amp is at 10 v/div, scope is across the terminals where the transmitting loop attaches (which is the drains of the mosfets, one on each side).
The first one is in the "normal" mode, the second one is the superbrilliant mode. For some reason I don't see the voltage change this time. Or rather I do but I'm starting to understand the distance dependence of the voltages and frequencies... sometimes.

[MileHigh]

I suppose that I should add, so why does the light get so bright?

The receiver LC oscillator is now being excited at its dead-on natural resonant frequency.  This looks like a very low impedance to the transmitter and as a result this also looks like a very low impedance to the battery that is powering the transmitting oscillator.

The bulb connected to the LC receiver is soaking up and burning off the maximum power that can be transferred through space for the near-field transmitter-receiver pair.

So when everybody syncs up the impedance of the circuit is quite low and as a result a lot of power get's transmitted to the bulb.

MileHigh

[MileHigh]

TK:

The "hash" looks like just another sine wave superimposed on the main sine wave.  That would mean that somewhere else in the circuit there is a secondary very-high-frequency resonator that is also being stimulated.  It could be LC-anything, stray capacitance, stray inductance, coupled with the MOSFET capacitances, who knows.  I don't think it is having any affect on your circuit.

MileHigh