Fisher 1280x Aquanaut

[Qiaozhi]

Thanks Ivconic for the information.
Anyway i can tell you that CZ-20 works two frequencies simultaniusly, 5Khz and 15Khz. So my concern is how to made a coil for it. I know i must respect inductance and resistance vales, but how is the coil set up. Are inside one or two pairs of coils for TX and RX?
Well this is a work that i have to do.
I think that a good choise to see what´s inside is to pass the coil thru an X-Ray machine.
Regards
Nelson

Have a look at the TX waveform on an oscilloscope. My guess that it's a 5KHz square wave, as 15KHz is the 3rd harmonic, and (as you know) an FFT will reveal that square waves are composed of a series of odd harmonics.

[nelson]

Ok, your rigth. I can see the picture now, so now is time to calculate a handmade coil for 18 cms in diameter.
I will take inductance and resistance values this weekend.
Take care
Nelson

No, it is ordinary coil, just as any other. Harmonics...that's the answer on your question. Coil is calculated and arranged to cover harmonics, in this case 5 and 15kHz.
One more thing you can do is to measure residual voltage at your coil, so to know in future how to balance your handmade coils.