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Originally posted by Nupi
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Typically, as the frequency range is higher the strands are thinner. Litz wire is only used up to a few MHz (about up to 3 MHz) in RF and high frequency power supplies. This tends to increase the Q of a coil as coil Q is calculated by dividing the inductance (XL) by the DCR. But at high frequencies the DCR is never reached as the current tends to flow on the outer area of the wire. By using many fine strands the actual RF wire resistance becomes closer the the DCR and then the Q is higher.
Making coils for a PI metal detector requires some practical design considerations. Coil stability and shielding are the key considerations. While dry air has the lowest dielectric constant and many high Q RF coils use special winding techniques where coils are woven into special shapes with air between the windings and wires are not run parallel and close together. These techniques, while useful in making very fast PI coils, may not show their full potential when shielding is added and the additional shield-to-coil capacitance off sets the low capacitance of the unshielded coil.
Using Teflon insulated wire is an easy Do-It-Yourself technique to make a stable coil with a shield spaced a few mm away from the coil using a low dielectric constant spacer material. If you use a 600V thicker Teflon wire insulation on stranded wire, you may need to add a few extra turns to get the desired coil inductance. Use polyethylene or Teflon spiral wrap to hold the coil strands together and keep them from making false signals. Add a second layer of spiral wrap then add the coil shield and then cover the whole assembly with another layer of larger diameter spiralwrap. These are just practical ways to make a good home made coil using commonly available parts. Your theory is correct but that only works well on a workbench at frequencies much higher than used on metal detectors.
Joseph J. Rogowski
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