# Parameters of a metal detector coil

I built metal detector called MiniPulse Plus (it is a kit. comes with calibration instructions but says nothing about the coil, except that it should be between 300-500uH). A circuit sends a pulse into the coil (58uS width, at 1KHz), it waits for coil's magnetic field to decay, then it uses the same coil to listen for signals coming back from the ground (generated by possible buried metal items). A 470ohm resistor mounted on the coil help its magnetic field to fade quicker. The resistance of the coil in my case is 3 ohm, 24 turns, gauge 0.45, diameter 23cm, enamel Cu wire.

I want to know what are the most important parameters for the coil.

For example, the size of the coil (diameter). Will a 20cm coil be better than a 10cm coil (while both have 400uH).
Since the coil operates at only 1KHz, does the wire gauge matter?
Should I use wire with thick insulation? (some recommend speaker wire because the distance between the cooper wire will be big, decreasing this way coil's capacity)

• Are you done with this Q and A now @Ultralisk? Nov 21, 2020 at 15:23

A larger diameter coil will produce the same magnetic flux density as a smaller coil but at a greater distance. This is important. So a 20cm coil is better than a 10cm coil.

The symmetry is such that all the terms in this element are constant except the distance element dL, which when integrated just gives the circumference of the circle. The magnetic field is then: -

As distance from the plane of the current loop increases, z dominates and flux density tends to be an inverse cube law with distance Z. Data/pictures taken from here

As for the wire, there will be a pulse of current associated with the applied voltage pulse and this may be in the order of an amp so choose an appropriate wire gauge so that dc losses are not significant.

Regards insulation, basic copper wire with enamel is perfectly fine.

• The resistance of the coil is 3 ohm. I think is not big. May 1, 2015 at 18:45
• Also people speak a lot about using speaker wire (thick insulation) to decrease the internal capacity of the coil... which I think... makes sense. May 1, 2015 at 18:47
• At 1kHz operating frequency the self resonance of the coil (hundreds of kHz I'd estimate) caused by self-capacitance will have minimal effect but please do state what the coil construction is. May 1, 2015 at 18:52

Theoretically a coil with x cm diameter would get 5 times this diameter in distance to detect items not less than 5 % of the area size of the coil.

so your 20cm coil at best you can detect 1 square cm surface of a metal at only 1 meter max.

is not any easy task to make a such a system. mostly you can get to 3 time the size of the coil.