Coil Values

[Daves]

1) Nothing, that will work as well. Tesoro has a coil with LTX=6mH and LRX=6mH.

2) For a single frequency design you usually bandwith-limit the preamp to the signal of interest.

So some kind of band pass filter?
Additionally: I seem to remember a comment where you mentioned that if tx and rx are the same, id calculations will be done incorrectly. Is such an error possible?

[Carl-NC]

Typical VLF preamp:


R1*C1 forms the low pole, R2*C2 form the high pole. You may see the coil unloaded, loaded with a resistor, or resonated with a cap. If it's resonated with a cap, you usually don't want to resonate it at the exact TX frequency because that produces a steep phase slope that makes TID difficult.

[ivconic]

I don't know how engineers originally came up with coil specs. Looking at really old coil designs, it appears they bought into the fantasy of transformer gain because some of them had an extremely high LRX. And then there is one of the Tesoro coils where LTX=6mH and LRX=6mH. No idea how they landed there.

I've told this story before: the White's V3 was required to use DFX coils, where LTX=540uH and LRX=15.7mH. It quickly became obvious that LRX was too high for 22.5kHz but we were not allowed to change it, so we had to add "tweak" components inside the coil to make it work. It also made for a very thick winding in the 4x6DD shooter coil that was difficult to bake out, those coils went bad quite often, sometimes just sitting in inventory. One day I was playing with an Explorer and decide to put the Explorer coil on the V3. The LTX was about the same and the LRX was 1/30th that of the V3, so I had to boost the preamp by 5.5X. The V3 had digital calibration so once I put the coil on all I had to do was re-calibrate. It worked perfectly, same performance.

Cool story!