Coil freq. help

[" Alexandru Marian" <sestert@xxxxxxxxx>]

Hello List,
I am building a metal detector. 
It is based on 2 coils, with equal frequency. When the search coil 
will pass over metal, this will change its fixed freq. and the system 
will scream. 
The search coil will have 15 turns of 0.4mm, 6inch diameter, should 
be 70kHz. The second coil...it just says "take a small former, wind 
some 150 turns, you might need thiner wire" 
I will probably not be able to find an osciloscope to emasure the 
second coil, so that looks like a big problem. 
It means ill need the math, freq. function of coil diameter, turns, 
wire thickness and core. Ill need some screw for the core, moving it 
should allow me 70kHz +- 250Hz.
I supose that when the 2 frequencies are very close, the sistem will 
not make too much sounds... 

Thanks in advance for any ideeas,
Alex