For use in an oscillator circuit, a good, high Q coil with minimum self-capacitance allows for a wide tuning range with a given variable capacitor, and for receiving purposes, maximum selectivity and peak voltage. There are a number of forms of this, such as basket wound, spider wound, etc. but a "Universal Wound" is more compact.
Factory universal wound coils are done on a machine that pulls the wire from left to right as the coil core is turned. It is set up so that a little more than one revolution is made before the wire makes a complete left to right, and back again, pass. This causes each layer of wire to cross the previous layer at an angle which minimizes the coil's own self capacitance. Self capacitance is lower than in a side-by-side wound (also called a solenoid wound coil) because in the latter, each winding has a bit of difference in voltage than the one laying tightly next to it. This is not always a problem in a tuned circuit because the parallel (or series) capacitor can be adjusted to include the coil's self capacitance into the total, and solenoid wound coils, properly made, can also be quite efficient. The problem of self capacitance stands out in tunable resonant circuits where a standard variable capacitor cannot tune a single homemade coil across, for example, the entire AM broadcast band because the coil itself carries much of the total circuit capacitance and remains there unchanging with turning of the tuning capacitor. This "swamps out" part of the change of circuit capacitance.
There are times, therefore, that it is advantageous to be able to wind your own high Q, low capacitance tuning or oscillator coil, and sometimes only a coil of cylindrical form will physically fit into the space available.
This example coil is not the ideal proportions (which usually ends up look like a tire) for maximum efficiency, and it would also be improved with larger diameter wire for less resistance, and/or litz wire for the frequencies it is useful for (below 2MHz). Nevertheless, in parallel with 250pF capacitance where its measured resonance was 916kHz, it has a Q factor of about 60. It was made of 30 gauge magnet wire and consists of 132 turns on a 3/4" diameter dowel, with a length of about 5/8". When connected in parallel with a typical 365pF AM Band tuning capacitor it resonated at 740kHz minimum and 2.8MHz maximum.
A wooden dowel is wrapped with waxed paper and that is taped down at the ends. Then a piece of ordinary white paper is wrapped around and trimmed to exactly on turn's length. It should be marked at even intervals and numbered as in Figure 1 below, though you do not have to use 12 pins per side. The idea is to use it for a template to mark where the pin holes are to be drilled, and then number the pins' holes on the waxed paper so that a number 1 on one side of the dowel is half a turn of the dowel from another number 1. Do this for both sides. I chose to go from numbers 1 through 6 and 1 through 6 again on each (left and right) side, but you could just go from 1 through 12, too. I just think this way is easier when it is time to wind it with the wire. The main thing is to be able to know at a glance when you are halfway around the dowel from the last pin you wrapped the wire around, because you would be winding from, say, number 1 to the next number 1 that is halfway around the dowel and on the opposite side.
Small holes are then drilled, evenly spaced, around the circumference on both sides of where the coil is to be wound. A drill press is handy for this, but a hand drill can do a neat enough job of it with a little care. Wires are cut (paper clips work well) and hammered into the little holes. If they fit too loosely, a little white glue will hold them. Do not use epoxy or anything that will overly grip the metal wires because you will need to remove them to remove the coil from the dowel. You should be able to reuse the form you make multiple times. (See Figure 2.)
The wire spool can he held on a piece of pipe in a vice so that it will turn freely to feed the wire. (See figure 3.) Some pieces of tape that can be picked up with your free hand is a good idea so that you can tape the loose end of the wire to the dowel, temporarily, when you need to take a little break.
Simply study the picture series below and take your time winding. Do remember to keep tension on the wire with your hand so that it lays straight, and mark down your turn count, at least every complete row. In the example coil, since there are 12 pins per side, there are 12 turns to wrap each pin once (which is what I mean by "a row.") It actually goes pretty fast when you get the hang of it. I stabilize mine with candle wax. As always, be very careful with hot wax or with heating flammable wax if you choose to do that.
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3/13/2023 12:58:45 pm
Brilliant! I'll be trying this myself very soon. Thanks for sharing the idea.
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Rob Tracy is the operator of Rob's Radio-Active, LLC. Parts and vintage stereo, radio, audio, and test equipment sales and restoration services.