Hero999
Banned
100nH for a self resonant frequency of 3MHz still sounds too low. I would think it's either resonating at a harmonic or the inductance is much higher.Yes, I get surprisingly good inductive coupling - on the bench I've had 90% coupling efficiency, driving the primary coil with a signal generator at 3 MHz.
The inductance of the coil I'm using was calculated using an online tool, not measured - maybe I put a number in wrong and it should be 100 nH, but it is a very tiny coil and 10 nH seemed intuitively correct to me. 3 MHz really is the optimal frequency (well, ok, it's really closer to 2.9 MHz) as tested with an RF sig gen and oscilloscope on the secondary coil.
An inductance of 10µH with a parasitic capacitance of 280pF would give a self resonant frequency of 3MHz. If the inductance is 100nH to resonate at 3MHz the parasitic capacitance would need to be 28nF which sounds far too high for a 100nH coil.
If it's resonating at the 10th harmonic (30MHz) then the parasitic capacitance would need to be 280pF which is more plausible but I don't think that's the case because there won't be much power at the 10th harmonic.
I think your inductance calculations are incorrect unless you've added another capacitor to the secondary.