How much does inductance change?

[Silver Dollar]

I'm planning on making a "DD" coil for a PI detector and can figure the inductance for a circle
but how much will it change when formed into a "D". I saw that info one time but cannot find
it now.

I'm also thinking of making an Omega coil for the IDX. How much will the TX change when
bent into the Omega shape?

[Qiaozhi]

I'm planning on making a "DD" coil for a PI detector and can figure the inductance for a circle
but how much will it change when formed into a "D". I saw that info one time but cannot find
it now.

I'm also thinking of making an Omega coil for the IDX. How much will the TX change when
bent into the Omega shape?

For the D-shape, the best result is obtained by finding the average of the two radii and taking the mean value. In other words, take the length of the straight edge and add it to the distance across the middle, then divide by 4. Use that as the radius value to enter into the coil calculator.
The omega coil is more tricky, since the folded-back section subtracts inductance from the larger loop. Without knowing the mutual inductance between the two, it's difficult to estimate.

[Silver Dollar]

Well I did an experiment with a 10" coil. It read 300 uh when round and 280 uh in a "D" shape.
So based on that result I added one turn to my calculated inductance.

I wrapped a wire around my "D" and measured the length. Then computed the diameter of a
circle with that circumference (using this calculator http://www.onlineconversion.com/circlesolve.htm).
I then used the radius to calculate the inductance of a round coil and added one turn to "fix" the circle
to "D" difference.

I'm bumming though as I ran out of wire and only have one coil made...

Do both TX and RX have to be "fast" for a "DD" PI coil? I have magnet wire I could use for the RX
(I was going to make it exactly the same as TX..).

[Skippy]

Maybe you're thinking of Henrikras' experimental small Deus coil thread, where I suggested some adjustment factors.
I would say that the 'semi-circular' shaped coil (that makes up the common circular coil assembly) gives only a modest drop in inductance from the circular shape. A figure to work with is: about 97 - 98% of the inductance of a circular shaped coil of the same circumference.

[Davor]

You´ll see more change by compacting a coil than squeezing it into a D shape. The closest you can get by inductance estimation is by mere wire length for an equivalent circular coil.