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White's Dual Field Search Coil Patent

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As introduced on the White's pulse induction detectors beginning in 2008.



A new dual field search coil for pulse induction metal detectors has multiple coplanar wire coils of different diameters connected in series with the output of a pulse source, overcoming the loss of target size resolution associated with a single coil search loop. Small objects are sensed by an internal small coil and larger objects are sensed by the larger outside coil, and the overall depth of target sensing remains similar to that of a single coil construction. The smaller coil is isolated from the pulse source by the inductance of the larger coil and will ring at a frequency determined by its own parameters. To prevent the smaller coil from ringing, a second damping resistor is connected across the terminals of the smaller coil at the junction of the large and small coils making up the modified search loop.

White's Dual Field Search Coil

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The dual field TDI coils haven't received the smoothest response.  For starters they didn't work well in Australia's mineralized conditions which led to the release of mono coils ("Aussie mono" or "Oz" versions).  Possibly related, their 7.5"/3" smaller dual field coil received a fair amount of negative reviews.

Was this ever resolved?  White's continues to market the larger dual field coil in the US in both of their (only remaining) TDI models, so it seems they, at least, feel they've weathered the storm.

As a sidelight, there are many aftermarket coils that have been made in both Australia and US that work on all(?) of the TDI models.

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There was nothing to resolve. The Dual Field is a hotter coil. Hotter in small gold but that also means more reactive to ground. Turns out the Dual Field was a little too hot for the worst Australia ground, so they went regular mono for the Oz machines. I never had issues with the dual field coils and most people don’t. I loved the little 7.5” dual field. Even in Australia there is plenty of ground you can run the dual field on with no problem. I would not opt for a mono on ground capable of handling the dual field. Everything in detector land is a trade, and while the mono is less reactive to ground it is also less sensitive to small gold.

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