I have an air coupled transformer consisting of a primary L C resonant circuit fed with feedback transistors and a secondary L C circuit feeding a bridge recitfier to power a small circuit over an air gap. It is almost identical to this circuit: https://en.wikipedia.org/wiki/Royer_oscillator
I also use a FET shorting the rectifier on the secondary side that is triggered occasionally to pass a signal back to the primary circuit. It looks like this example, the secondary coil and resonance capcitor is on the left, in this case a forked light barrier is used as source of the signal to be transmitted: The lowered resistance leads to a small change (about 2%) in frequency which can be sensed on the primary side. That worked well in several places, but if the constant power consumtion is higher, it sometimes fails: While the signal is triggered, it tends to overload the primary circuit, giving an trashed non-sine waveform.
However, by changing the capacitance C4 mounted to the secondary resonator, the frequency can be changed easily over a broad range of about 30% without much additional power consumption.
So I wonder if there any way to change the AC capacitance with the FET or some other simple circuit?