DD coil null

[Anonymous]

Hi folks, just wondering what undesirable characteristics I would encounter with a DD coil has not been correctly nulled. thanks, Peter.

 

[Anonymous]

Peter, it would depend on what kind of metal detector and how badly out of null the loop is. If the loop is really out of whack, it'll overload (saturate) the receiver electronics and the machine will be dead or nearly so. If the saturation is marginal it may still see targets in air testing but it won't get rid of ferrite (magnetite). If it's just a lousy loop but still functional, it may cost you a little bit of sensitivity and may overload too easily on ground minerals.
The foregoing applies to all induction balance loops, not just DD's.
--Dave J.

 

[Anonymous]

Thanks Dave, the coil in question is an 18" GP extreme coil. It is noisy on ground that has surface ironstone pebbles (or maybe it is magnetite?). Appears to handle most other ground types ok. I tried another of the same loop and it was quiet? Maybe it is only marginally out of null, lucky I don't use it often, I may even dissect it!

 

[Anonymous]

Sorry, Peter, I don't know enough about the GP to provide an informed opinion on your specific problem, other than to say the problem might not be a matter of null, but some other electrical characteristic.
If you're sure the problem is in the coil (your comparison with another supposedly identical coil would seem to confirm that), I'd say ask the manufacturer for a repair or replacement. Make sure you describe clearly to them the problem you observed and how you verified it was the coil that was bad.
--Dave J.