Build Project: Universal/Progressive RF Coil Winder for Theremins

I'm still trying to figure out a good gear ratio for the progressive motion based on some more pink thread winding tests (although I am changing colors so that my thread theft is less noticeable).  With some starter gears that I've printed I will get about 1:28.125 reduction, so with the lead screw's 18 threads per inch it would take about 506 turns to make the winding traverse an inch (and add to that the width of the reversing helix).

506 turns of a single layer coil in #36 wire would be about 2.4" long, and in #28 it would be 6.4" long.  This size reduction to 1" long is possible because we are going vertical with a multi-layer wind which has advantages and disadvantages.  The capacitance between adjacent turns is reduced.  But if you compare the capacitance between some well-separated turns on a single layer coil (for example, turns 1 and 20) with those same turns on a progressive-universal wind, they will be closer together on the latter.  So the schematic of the inductor with all the parasitics will look different between the two types, with different values of parasitics between closely and widely spaced turns. 

Will something be gained?  Certainly the P/U wind will give more inductance in a given volume than a single layer coil, which is the whole point.  I will have some control over how much spread I can get per turn, so end-to-end capacitance can probably be managed.  The self-resonant frequency is bound to drop, but within reason that should be acceptable.  I'm hoping that the ability to increase wire gauge or even use some thicker Litz wire without affecting the length of the coil past the optimum aspect ratio (length/diameter) will result in smaller coils that still perform well at RF frequencies.

This isn't what I'm building this winder for, but here is a test of what it will do so far.  This is a random number of turns (until the wire broke on me) of #42 wire on a .25" ferrite core.  The honeycomb wind is holding together without binder, although I expect that I will need some to go higher in a narrow section like this.  This particular coil is almost exactly 1.25 mH.

In the meantime I'm frantically designing and printing parts for the progressive gear train so that I can wrap this up and get back to other business.  It should be noted that behind every successful printed part lies a bin of those that didn't make the cut.  I do wish that this stuff could be easily recycled.

Some of the duds:

"This isn't what I'm building this winder for, but here is a test of what it will do so far.  This is a random number of turns (until the wire broke on me) of #42 wire on a .25" ferrite core.  The honeycomb wind is holding together without binder, although I expect that I will need some to go higher in a narrow section like this.  This particular coil is almost exactly 1.25 mH."  - pitts8rh

Gorgeous!  But #42 wire, man that sounds like absolute torture.  If your winder and your sanity can handle that, then you could probably make fairly tiny air cores.  I imagine a lot of this will be trial-and-error / seat-of-the-pants type engineering, but I wouldn't be surprised at all if min self-C / max self resonance freq. largely comes down to how long the total thing ends up being (all donut widths + their spacings).

Examining an EW choke, the windings are right next to each other and pretty tight, flipping to the next layer as they hit the edge of the donut, and the winding angle is such that 1/2 turn goes from one side of the donut to the other (a ~1:1 ratio?).  Donut cross section is 3mm square, spacing between donuts roughly 1.5mm (1/2 the donut width) with the core ends hanging out 5mm (~2x the donut width).  The phenolic or cardboard core spacer is maybe 1mm thick.  I wonder how universal these ratios might end up being?  I wonder what their optimization goals were for these chokes?  It's pretty hard to find manufactured coils that work well for Theremin EQ use.

"It should be noted that behind every successful printed part lies a bin of those that didn't make the cut.  I do wish that this stuff could be easily recycled."

I concur, my scrap box grows fuller by the hour and I don't know what to do with it all.  Supercharge a pasta maker and turn it back into filament?

"But #42 wire, man that sounds like absolute torture.  If your winder and your sanity can handle that, then you could probably make fairly tiny air cores.  I imagine a lot of this will be trial-and-error / seat-of-the-pants type engineering, but I wouldn't be surprised at all if min self-C / max self resonance freq. largely comes down to how long the total thing ends up being (all donut widths + their spacings)" - Dewster

The test with fine wire showed me that I need some shock absorption in the feed path.  The wire reel has enough inertia that the wire takes the brunt of the hand-cranking fluctuations.  I think a springy wire roller in the path will do the job.

Regarding SRF, I don't have any real motivation to try to get too small and push the issue.  I would just like to get the D-Lev inductors reduced a little.

"Examining an EW choke, the windings are right next to each other and pretty tight, flipping to the next layer as they hit the edge of the donut, and the winding angle is such that 1/2 turn goes from one side of the donut to the other (a ~1:1 ratio?)"

I think that's actually the ratio of 39 to 38, 40, 42, or as I have it now, 44.  It has to be something other than 1:1 so that the windings are staggered every time around.

This really is a learning experience because I can hardly grasp what the universal wind with progressive movement added will look like, but if I do it right it should end up looking like a Melodia inductor.  Although I will say that the winding in the picture below looks not so much like a uniform progressive wind but more like regular honeycomb sections separated with discrete steps, sort of like a low-profile pi-section coil with the sections almost touching.  From what I can tell if I engage the progressive movement gears I get a smooth wind from end to end without any banding that you see here.  I don't know what I'm looking at, really.

"This really is a learning experience because I can hardly grasp what the universal wind with progressive movement added will look like, but if I do it right it should end up looking like a Melodia inductor.  Although I will say that the winding in the picture below looks not so much like a uniform progressive wind but more like regular honeycomb sections separated with discrete steps, sort of like a low-profile pi-section coil with the sections almost touching.  From what I can tell if I engage the progressive movement gears I get a smooth wind from end to end without any banding that you see here.  I don't know what I'm looking at, really."  - pitts8rh

That thing is crazy!  On the upper left it looks like there is a light brown former / insulator underneath, and the windings ran past it?  If so, then the inductance might be somewhere between a single and dual layer solenoid?

If you do one long progressive donut, won't the windings end up spaced tighter in one direction than the other?  And one end will have overlap and the other won't, which perhaps explains the Melodia EQ inductor looking sparse on one end (they started there)?

[EDIT] I have to wonder if there is any significant electrical difference between donuts of simple single layers on top of each other and a tight zig-zag type winding that leaves no air in there (like the EW chokes).  If there isn't much phase difference between layers then it doesn't seem like it matters much how you do them.  Could it be that the zig-zag is mainly utilized in order to form a donut that structurally stands on its own two feet without the need for bobbin sidewalls to keep the end winding from spilling off?  If this is the case, then a bobbin might be the most straightforward way to go if one is DIYing.  That would be an interesting experiment, electrically test a single bobbin donut vs. a single tight zig-zag donut.  On a bobbin with simple layers just use thicker wire or thicker insulation to gain "air" space volume.

   
Nice work, I remembered a similar machine called ‘Kreuzspulenwickelmaschine’, which uses a different principle (control-disk in heart shape), from my archives of vintage Radio / TV Service magazines and found it: German Magazine Funkschau, 1953, Number 14, Pages 265 to 267

It is currently available here: https://nvhrbiblio.nl/biblio/tijdschrift/Funkschau/1952/Funkschau%201952_14_OCR.pdf

Hope I can realize it one day using 3D printing. In the article it is mentioned, that only fabric covered RF litz wire (not enamelled wire) would work correctly with this machine.

Some times ago I found this on YT and was fascinated about the result: https://www.youtube.com/watch?v=tg67CvbcHaU

Upto 39 mH air core, 5.1 pF self-capacitance, 357 kHz resonant frequency. 

I don't think that the Melodia coil shown above uses reciprocating or reversing helical winds in separate narrow pi sections.  I'm guessing that while it does have discrete sections as the banding would suggest, the method of winding those sections is different.  It may be something as simple as winding each little 4mm (guessing) section like its own multilayer coil - forward, back, forward, then move to the new section and repeat.  I don't see any honeycomb winds in that picture - some other method that I probably won't be able to duplicate is being used.

I still think that my best shot is to either try a continuous honeycomb wind that is always spilling off one side of the wire pile as it progresses, or simply wind a long multi-section inductor, sort of like the Etherwave 2.5, 5, and 10mH inductors except with lower sections and closer spacing.  Fortunately the other Melodia coils are plain old honeycomb winds. My winder can do these, but the inductor pictured is something else..

"It is currently available here: [url=https://nvhrbiblio.nl/biblio/tijdschrift/Funkschau
Hope I can realize it one day using 3D printing. In the article it is mentioned, that only fabric covered RF litz wire (not enamelled wire) would work correctly with this machine." -Neutrodon

I can't read it but I recognize the pictures.  It's the same basic design (without the lead screw feature) except that it uses rubber-tired wheels against disks for the right-angle rotation, which at the same time allows a continuously variable ratio between the reciprocating motion and the coil rotation.  My winder has bevel gears with four ratios, or more if I wanted to print more gears.  The down side of the rubber tire arrangement is if you have to back up to correct a bad layer the wire feeder may get out of sync because of tire slippage.

There is a little booklet (out of print now) for a similar type of coil winder.  It was written by Dave Gingery who also wrote a series of books on everything from setting up a backyard metalcasting foundry to building your own metal lathe.

"Some times ago I found this on YT and was fascinated about the result: https://www.youtube.com/watch?v=tg67CvbcHaU
Upto 39 mH air core, 5.1 pF self-capacitance, 357 kHz resonant frequency"  - JPascal

That video makes me think zig-zag wound donuts are more structural than electrical - a clever way to do a "virtual bobbin" with the winder?

"That video makes me think zig-zag wound donuts are more structural than electrical - a clever way to do a "virtual bobbin" with the winder?" - Dewster

It's true that this type of winding is structural in that way, but I suspect it's used primarily for electrical performance.  The honeycomb wind puts distance between any parallel runs of wire, and a given volume of HC winding contains less copper than a tightly wound coil - because of all the air space.  I remember seeing old coils or transformers wound on plastic bobbins that still used the universal/honeycomb winding pattern.  I'm thinking of coils that I ran across in old TVs and radios, sometimes in the little rectangular metal shield cans.  I regret throwing out all of these parts over the years that I would now love to inspect more closely.  All of those nice tunable coil forms and big antenna coils of Litz wire that could have been re-purposed...

I just re-read this article that I downloaded a while back that has some useful information about winding RF coils.  It's old - written at a time when capacitors were condensers, but a few of the methods could be used to make interesting-looking theremin coils:

https://worldradiohistory.com/BOOKSHELF-ARH/Author-Groups/Osterhoudt/HB-6.pdf