Questions about Double D coils

I was wondering about how to get the coil inductance right with these coils i understand that with standard circular PI coil you have calculators to get the inductance right however i have also watched some youtube videos that show this inductance being changed when the coil is reshaped into a Double D formation how do you go about calculating any required extra length needed or is that shortening i forget if you measure it still connected to the spool of wire won't this affect the reading as in affect there would be 2 coils also a guide as to shaping would help.

I like the new clips on the IDXPRO this is of great interest to me as it discriminates and hope future videos include VDI and maybe even 2 tone sound would be great to see more clips on DD coil making.

Thankyou for all your efforts : Graham

[Qiaozhi]

I was wondering about how to get the coil inductance right with these coils i understand that with standard circular PI coil you have calculators to get the inductance right however i have also watched some youtube videos that show this inductance being changed when the coil is reshaped into a Double D formation how do you go about calculating any required extra length needed or is that shortening i forget if you measure it still connected to the spool of wire won't this affect the reading as in affect there would be 2 coils also a guide as to shaping would help.

This problem is mentioned on page 210 of "Inside the METAL DETECTOR".
"The coil calculator is intended for circular coils only, but it is often asked if it can be used to find the number of turns required for either a D-shaped or elliptical coil. A reasonable estimate can be found by taking the long and short diameters, adding them together, dividing by 4, and using this figure for the inner radius."

Also, this might be useful -> https://www.geotech1.com/forums/show...767#post126767

There will inevitably be a small change in loop inductance while overlapping due to mutual inductance between the two loops.

[Graham]

Thank you for the information given Guru im'e sure it will be of great help in determining the length of the wire required for Double D coils again thank you Graham

[Carl-NC]

Changing a round coil to a D coil will always reduce inductance, and typically by 10% or so. So I would simply wind a round coil with extra turns, reshape it, and then remove turns to get the final desired value. Keep in mind that inductance is ~proportional to turns-squared, so if you want a 10% higher inductance then increase the turns by 5%.

[dbanner]

I was wondering about how to get the coil inductance right with these coils i understand that with standard circular PI coil you have calculators to get the inductance right however i have also watched some youtube videos that show this inductance being changed when the coil is reshaped into a Double D formation how do you go about calculating any required extra length needed or is that shortening i forget if you measure it still connected to the spool of wire won't this affect the reading as in affect there would be 2 coils also a guide as to shaping would help.

I like the new clips on the IDXPRO this is of great interest to me as it discriminates and hope future videos include VDI and maybe even 2 tone sound would be great to see more clips on DD coil making.

Thankyou for all your efforts : Graham

Scraping a tap to measure inductance is fine. You're only measuring a single inductance from the end wire
However one should not be concerned with precise inductance values, just approximate the prescribed number of turns, get in the ballpark. Remember that hand wound coils are never going to be precise.
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Focus on resonance.