Coil and target experiments

[Gwil]

On page 89 is an experiment showing what happens when various targets are presented to a search coil. Trying to understand what was happening made my brain hurt, but after drawing a few rough phasor diagrams I arrived at a possible explanation. (If anyone's interested there are numerous websites dealing with the subject of phasor diagrams)

Anyway, here are my thoughts. Any comments?

(Sorry about the poor quality. They looked better on my screen)

[WM6]

Did you read this patent?:

http://www.eudem.vub.ac.be/publicati.../US4486713.pdf

[Gwil]

Yes, I am familiar with this patent. The signal phase information in it is presented in a very strange way, with leading and lagging signals going in the opposite directions to the normal convention. Also, the "mineral" signal is shown shifted by 90 degrees from the transmit coil voltage. It depends on exactly what signal is meant, but the emf induced in the receive coil by a high permeability material is close to being in phase with the transmit voltage (or 180 degrees away, depending on how you connect the coil).

[WM6]

Thanks for starting very interesting thematic.

Assume that you with "constant signal present" mean TX signal?

[Gwil]

That's right, a small constant signal in the receive coil due to the transmit coil magnetic field. If the coil was perfectly nulled there would be no constant signal, but in this coil there is a signal present.