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I have hand-wrapped a couple of air core tube coils/solenoids with 50 gauge wire (0.001 inch diameter). My goal is to obtain a resonant frequency of ~10 khz without the use of a capacitor and to keep the coil length (not the length of wire used) to ~3 inches. The first coil I wrapped is far too fast at a resonant frequency of ~148 khz. I then wrapped another coil but this time completed two layers. For the double layer coil I made sure to maintain the wrapping orientation using the right hand rule for both layers (i.e. the first layer was wrapped over-to-under and the second layer continued this over-to-under pattern). The second layer was started at the end of the first layer and was wrapped back towards the first layers starting point. Both layers are the same continuous piece of wire.

Here is the oscilloscope reading of the coil with one layer:enter image description here

Works as expected but just too fast. So I decided to add more turns with a second layer and got this (on two separate wrappings of the coil):enter image description here

Channel 1 (yellow) is just the gate signal for a transistor and is left in the image as it is the trigger for the scope, Channel 2 (purple) is what I'm after. Besides the coil change there is no other change to the circuit or the microcontroller program controlling it. There is a layer of regular scotch tape between the two wrappings. Another interesting point is that after (tearfully) dismantling the first two-layer attempt I had the idea to remove only the outer layer and hook up the inner layer to the o-scope. This "rang" as expected and looked very much like the first image posted here.

On a more human note I'd like to point out that hand winding each coil with such small wire is a process that takes many hours and leads to strained eyes and hand cramps :) Since the length of the wrapped coil is ~3 inches and each wrap only accounts for ~.001 inch of that 3 inches, this process took around 6000 wraps... and this not counting screw ups! If it took less time and effort I would just try the other obvious configurations before coming to StackExchange with blistered, trembling fingers and crossed eyes. But it does take so long and so here I am.

Where did I go wrong?

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    \$\begingroup\$ In my opinion, reading the coplexity of described winding of a simple coil you somehow wrapped the wire in the wrong direction. Why so much complications? You do spin a spindle and wind the wire all the time in same direction. \$\endgroup\$ – Marko Buršič Aug 27 '20 at 20:33
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    \$\begingroup\$ I hope you wrapped these with a rotating spindle \$\endgroup\$ – Voltage Spike Aug 27 '20 at 20:52
  • \$\begingroup\$ Over, under, through -- what? You've confused me, which makes me suspect you overcomplicated things and outsmarted yourself. If you got to the end of your coil and either flipped the coil or reversed your hand direction, you did it wrong. You could scramble-wind the coil and as long as you didn't reverse your hand direction or flip the coil then all the turns are the same way. \$\endgroup\$ – TimWescott Aug 27 '20 at 21:04
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    \$\begingroup\$ Second layer has capacitance to the first layer. Try making a single layer 6 inches long. (Might get you down to about 80kHz) \$\endgroup\$ – user_1818839 Aug 27 '20 at 21:21
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    \$\begingroup\$ Don't kid yourself -- you ARE using capacitance, but it's the self-capacitance (parasitic) of the coil itself. The single-layer coil functions partly as a coil in the usual sense, and partly as a transmission line. When you add the second layer in the manner you describe, you're creating extra end-to-end capacitance that partly shorts out sections of the transmission line, creating the mess you see on the scope. \$\endgroup\$ – Dave Tweed Aug 27 '20 at 21:47
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Answering because the real question is starting to emerge in the comments. These will go away eventually, so please edit the question to ask the real question.

How do I design an inductor to hit a specific self- resonant frequency without thermal drift?

Now...

You'll get temperature drift in the coil itself. Thermal expansion will change its dimensions, which will change the inductance.

Take your nominal coil design and enter its parameters in any online calculator for air-cored inductance. Or look up the formulae/approximations in books. And calculate the inductance.

Look up the thermal COE (coefficient of expansion) of copper and re-calculate those nominal dimensions for (say) a 10C temperature increase : recalculate the inductance. You can now get a thermal coefficient for your inductance by dividing the ratio Lhot/Lcold by 10.

Next: Old radio ham trick : use a network of capacitors to create a capacitance with the opposite thermal coefficient to cancel out most of that drift.

Then you can use a more reasonable coil and save your fingers. And use fatter wire, decreasing R and increasing Q if that would help performance for whatever the application is.

Your 3000 turn coil appears to have a Q about 4, estimated from the rate of decay of the sinusoid (to about 1/e in about 4 cycles). Not surprising with such high resistance wire.

Get hold of Pat Hawker (G3VA)'s book "Amateur Radio Techniques" if you can find it

Or maybe something from the late 1920s when big open inductors and very simple circuits were SOTA, and people really cared about good resonant circuits before "supersonic heterodyne" (superhet) techniques made them unnecessary for radio. "The Manual of Modern Radio (John Scott-Taggart, 1933) is probably too late. "The Admiralty Handbook of Wireless Telegraphy" (1925 edition) may be a better bet.

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    \$\begingroup\$ Excellent reply, thanks! Plenty to read up on and learn from. I upvoted your post, but I don't have enough rep for it to show. Also, that is a top notch cat. \$\endgroup\$ – Justin Kennedy Aug 28 '20 at 19:32

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