co-axial coils

[Frank]

could anyone give me imformation on how they are constructed

[Carl]

Frank,

Coaxial coils are made by stacking 3 same-sized coils in a cylindrical row, spaced slightly apart. Think of donuts stacked 3 high. Coaxial literally means "along the same axis".

Usually (probably always), the center coil is transmit and the 2 outer coils are receive, and the receive coils are connected out-of-phase. Therefore, they nominally receive the same signal from the transmit coil, but since they're out-of-phase the signals cancel. When you bring a target close to the coils it creates an imbalance in the out-of-phase signals so now there is a net signal at the receiver.

Coax coils are usually thicker than other coil types. I think the early Garrett VLF detectors used them, as did the C&G 'cat detectors. You can also make the 2 receive coils smaller than the transmit coil, probably gives better ground rejection.

Regards,
Carl

[Frank]

Carl
Many thanks for that imformation.But do you need to null the receive coils,and how do they compare to a DD,and concentric.
Regards Frank

[Eric Foster]

Hi Carl and Frank,

A big plus for coaxial coils is that they cancel power line and other low frequency noise.

Eric.

[Carl]

Yes, they must be nulled. If both RX coils have the same # of windings then they simply need to be equally spaced from the TX coil and connected out-of-phase for nulling.

Eric is right, coax coils cancel external interference better because both coils are affected equally but oppositely, thus canceling the effect. Compared to concentric and DD, I don't thing the coax gives as much depth unless you substantially space the RX coils, which makes the overall search head thicker. That's because a target will affect the closest RX coil but will also, to a lesser amount, affect the farther RX coil which creates some cancelation. Coils with only 1 RX winding don't have this problem, even concentric coils that have a secondary RX winding since it's a much "weaker" coil.

- Carl

[Frank]

Thanks Carl
One further question,how do they handle mineralization compared to the DD

Regards Frank

[Carl]

I'm not sure. Apparently not as good since I think no one is making coax coils these days. You could try asking Jack Gifford, he's the "G" in the old C&G Technology that made the 'cat series, he now runs Tesoro.

- Carl