Question about Q and capacitor/inductance ratio for concentric coils

[DavidB]

What seems to work the best when making concentric coils, less capacitance and more inductance or less inductance and more capacitance. I think less capacitance and more inductance would give a lower Q and broader tuning. There must be an optimum ratio or Q.

[ivconic]

What seems to work the best when making concentric coils, less capacitance and more inductance or less inductance and more capacitance. I think less capacitance and more inductance would give a lower Q and broader tuning. There must be an optimum ratio or Q.

I would take some original coil as reference. I suggest White's BM coil as those are suitable for broader tuning (as you said). TX=~0.5mH and RX=~36mH, TxC=~1uF. This coil can work nice from 5 to 9kHz
Next concentric coil suitable for experimenting and also broad tunable would be Tesoro concentric coil; TX=~1mH and RX=~15mH. This coil can work nice from 8 to 15kHz.
Probably the best "broad tunable" coil would be Fisher Quicksilver 28cm concentric coil (eliptical RX). TX=~1.1mH and RX=~6.6mH. Can work nice in range 5-20kHz (recently checked it on Troy Shadow X5 (19kHz) and with a bit of trimming it is working very nice!)

[DavidB]

Thanks Ivconic, that is exactly what I wanted to know. It makes sense that the xmit coil has a lower inductance than the receive also.

[Davor]

That would depend on Rx input impedance. Lower noise opamps usually have lower optimum impedance (and higher price tag).