Coil winding

[GaD]

If we was looking for a 2cm diameter X 1mm thick piece of gold or silver, using PI or IB, at say 200mm below the suface in normal uncontaminated soil...what do folk think about coils suitable for the above?
eg. Coil 'weave' being scrambled or unscrambled? Wire choice? Shielding? Insulation?
Coil parallel or series configurations?
Its about time we nailed down a few specific do's/don't-do's regarding coil design.
Cheers
GaD
PS someone should start a coil forum, or write a book!

[Dave Johnson]

Re: Coil winding

A coin 8 inches deep is a typical slightly difficult situation for a commercial "coinshooting" metal detector. Nearly every manufacturer of such products furnishes them with a coil between 7 1/2 and 9 1/2 inches (19 to 24 cm) diameter. A nominal 10 inch (25 cm) elliptical coil has about the same surface area as a 7 1/2 inch (19 cm) diameter round coil, which makes them roughly equivalent in basic sensitivity. So, there is an industry consensus on optimum diameter for the typical situation. Nearly all manufacturers offer coils of other diameters and shapes because not every situation or user is "typical".

Regarding the other things you asked about, there are no "best" answers. It depends on the circuit to which the coil is being connected, the preferred manufacturing method, and what problems besides detecting a thin coin at 8 inch depth the manufacturer is trying to optimize. ......So-- "nailing down the specific do's/don'ts" is not a doable task. Each manufacturer has their own rules of thumb, and that list of rules is not the same as that of the other manufacturers.

For an experimenter, my best advice is to do pulse induction, not VLF-IB, because PI is much more forgiving of variations in coil parameters. A good VLF-IB coil is a near-impossibility for an experimenter to build-- they're difficult even for the manufacturers.

--Dave J.