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)