I have at my disposal some PVC pipe measuring 1.055" O.D., and some 14 gauge stranded THHN wire. I need a 100 microhenry coil for an antenna project.

I have looked at several online calculators but a lot of them either ask for data that I am not sure how to find, or are aimed at finding the impedance of an existing coil, rather than calculating the number of turns on a specific sized form with a specific type of wire to achieve a desired inductance. And some of these calculators are in metric units requiring an additional conversion.

I have one of these ARRL single layer coil winding calculators somewhere - not sure whether I can get my hands on it quickly, but I will take a look.

ARRL coil winding calculator


Or maybe I could try the equation at the end of this article:

Homebrew your own inductors


It would be an approximation of an approximation, since I don't think that these formulas expect stranded wire, but I will look up the diameter of #14 stranded THHN and add that to the diameter of the coil form and see what I get. Are you planning to strip the insulation from the wire? If not, that would also enter into the calculation, just a bit.

I was planning to keep the insulation so they could be close wound without shorting.

I'm getting numbers that don't quite add up - just extrapolating from the example at the bottom of the article, I would have guessed that about a 12-inch long coil with 260 turns would do it.

However, from THHN specs with a PVC jacket, the diameter of #14 THHN insulated is about 0.127" (127 mils).

But you can only get about 100 turns of that wire on a 12-inch long form.

So if my math is correct, 260 turns would actually take just over 30 inches of form length.

Which may violate some of the assumptions that went into the formula.

So I'm guessing that a larger diameter form would be needed.

I'll give this some more thought.

ETA:

Just doing a sanity check here - looking at a B&W coil chart at coil chart (airdux - wound on a polycarbonate rod), they have an example of 92 uH with a 2-inch diameter form with #16 wire and a 10-inch coil length, at 10 turns per inch, for a total of 100 turns.

Another data point - the B&W Miniductor chart at Coil chart (those are more like air-wound, with plastic strips holding the wires in place):

They have an example coil with an inductance of 90 uH, with a 10-inch overall length, using #14 wire, at 8 turns per inch, on a 2.5-inch diameter form.

So I'm thinking that a larger diameter form and about a foot of coil length will be required.

Would you be able to go to a larger diameter form?

I can. The next larger PVC I have is 2.370" O.D.

I will try plugging that diameter into the formula and see what that gives.

If my math is correct, using the formula at the bottom of that article, I get 88 turns on an 11-inch long form of 2.370" diameter:

N = sqr( 100 [ (18 * 2.497) + (40 * 11)  ]) / 2.497
 
N = sqr( 100 [ (44.946) + (440)  ]) / 2.497

N = sqr( 100 [ (485)  ]) / 2.497


N = sqr( 48500) / 2.497


N = 88 turns


And, as a sanity check, 88 turns of wire would take up 88 * 0.127" = 11.2 inches long

So maybe the form would need to be slightly longer than 11 inches, with the wire wrapped tightly in a single layer.

Thanks for helping with this! I have enough PVC to try it out.

Looks like you got enough links posted. Keep in mind that the formulas will bring you near that number. It may not be close enough. Leave enough space on both sides of the coil so there is room to spread the turns apart or bring them closer together to fine tune the coil.
If you plan to have a capacitor in parallel to the coil, you will most likely need to fine tune the coil anyway due to high capacitor tolerances. You'll need to measure the resonance frequency anyway of you are building traps.

Try measuring dimensions in metric. It is a whole a lot easier to make calculations without having to remember weird multipliers.

These calculators work for me.

http://www.k7mem.com/Ind_Coil_Design.html

http://www.k7mem.com/Ind_Coil_Ind_Calc.html