RAYMOND INGRAHAM said:
Cibola questions, 1.if a coil is a basically a antenna (yes ?) shouldnt the cable be the same impedance (or swr in radio terms )
1. if a coil is basically an antenna...(yes?)...
We can stop there. No.
A very common and often quoted misconception (and over simplification) is that the coil operates as an antenna.
Kinda...sorta...but not really.
(Although the overall coil does need to be built to a specific impedance,)
In reality...(electronically) the search coil has far more in common with a transformer.
What we call transmit and receive windings in the search coil act as the primary and secondary of a transformer to create a constant/stable Induction Balance field.
Then that electromagnetic field is passed over, say a coin, the field induces an eddy current in the target that unbalances the field.,,allowing us to 'sense' the target.
Another way to put it...in radio terms...is that an antenna operates in a 'far field' mode of operation. Transformers and metal detectors operate in a 'near field' mode.
An excellent resource for curious electronic enthusiasts is the book "Inside the Metal Detector" by Overton and Moreland...
It will answer more technical questions about the theory of metal detector operation than you thought you wanted to know.

mike