The Nail Board Test And Sensitivity

[Geotech]

14 hours ago, mcjtom said:

Which bring me to a probably related question: with double-D coil, if a flat coin is close enough to the coil there are 3 beeps in one sweep.  When you stand up the same coin (the plane of the coin perpendicular to sweep direction now) it beeps twice, just like longitudinal nails.

Magnetic fields form enclosed loops of flux; here is what it looks like for a coil:

The flux has direction which is equivalent to polarity; let's call the downward flux through the center to be "positive" and the returning flux around the outside to be "negative." This is obviously the case for a TX coil but the field received by the RX coil has the same polarity: flux running through the center of the RX coil is positive and flux that runs to the outside of the coil is negative. Now put the TX & RX coils together in a DD coil and mark these regions:

Basic math:
(+) x (+) = (+)
(+) x (-) = (-)
(-) x (-) = (+)

So the overlap region with +TX and +RX will produce a positive signal, but so will targets outside the perimeter of the coil although they will be weak. The areas inside one coil but outside the other produce negative signals that the detector ignores.