"Ideal" coil inductance for PI

[Searchy]

Thank you all (especially Carl) very much for your comments.
Think I understood the compromise now.

[green]

Assuming a mono coil, there are a lot of factors. On the TX side fewer turns produces a stronger magnetic field (the higher amps outruns the fewer turns) and has lower C. But on the RX side more turns produce a stronger EMF, but also increase the C which pushes out the sample delay. Increasing the turns helps the RX EMF more than it hurts the TX field, so you go as high as you can to achieve the optimum balance between TX field, RX EMF, and sample delay. 300uH seems to be what the industry settled on some time ago, and it may or may not be optimum depending on the application.

on the RX side more turns produce a stronger EMF. More turns, more target signal. I'm guessing more noise. Does S/N change at the Rx coil with more turns?

[Carl-NC]

on the RX side more turns produce a stronger EMF. More turns, more target signal. I'm guessing more noise. Does S/N change at the Rx coil with more turns?

In general, more turns gives you proportionately stronger target signal, ground signal, EMI, and coil resistance thermal noise. Using less turns results in a weaker signal that has to be further amplified in the preamp, which is almost always (thermal) noisier than adding RX turns. This is true in any metal detector design.

[Jim in Idaho]

I'm no engineer, but in my testing on the 83% copper nodules I'm looking for with the Delta Pulse, an inductance around 600uH seems to be optimal with a 24" coil. I'm using 17 turns of 22ga. wire.
Jim