Bit of help please...

[mykaitch]

Failed last year to make a decent coil - used up a lot of wire.
I have now read everything I can find (esp Dave Emery's article).
Made Tx,Rx and Buck coil (as per D Emery) for concentric coil.
Did the math, even made a spreadsheet so I can feed figures in.
I can get down to about 10mV on the Rx coil (scope).
If I move the buck coil a lot off centre to the Rx coil I can get the figure lower.
What should I do in terms of windings to improve the null with the two coils on top of each other -as they should be?
Since (my Tessoro umaxII copy) the front end has a gain of 20 then my 10mV will be 200mV on the o/p of the first amp and that seems way too high?
What is the acceptable range for the Rx coil null (at the winding) ?
I have just trashed two more coils but I will not be beat!!!

General info:
Tx is 114 turns, 9.75 inches diam
Rx is 164 turns 5.25 inches diam
Buck is 32.5 turns around Rx.

Thanks everybody!

[Skippy]

You're mainly adjusting the zero degrees phase, I think, with that nulling adjustment. You need to introduce some 90 degrees, or 90 - 120 phase trimming, probably. Get a small low-conductivity target, eg. lead, cupro-nickel, like a 0.22 airgun pellet, and move it around the search-coil, see if there's a spot where the 10mV drops/reverses. You'll likely have to re-adjust the position of your nulling loop afterwards. Worth an experiment, anyway.