I'm winding a coil for a home built regen radio, and I need to know how much wire I need. The coil form is a 2 inch mailing tube. Main winding is meant to be litz wire, but I'm using 28 gauge magnet wire, and is 66 turns. The second winding is meant to be 8 turns of 30 gauge wire, but I'm using 28 gauge magnet wire again. Third winding is 20 turns of 30 gauge, but again I'm using 28 gauge.
Radio schematic (at bottom of page)
page)
Surely you where taught basic mathematics in school, such as the circumference of a circle?
Pi times the diameter gives the circumference. Multiply by the number of turns and add a few percent because it's not in perfect circles.
Bob
Take a piece of string, surely you have some of that around, and make one winding on the coil form. Then measure that, and multiply by the number of turns. Add a bit extra for leads and "just in case".
Michael
Every dimensional change you make to the design will require a compensating turns change. The trick is to either make one as directed, measure its inductive properties, and than adjust the turns count on your modification to have about the same measured properties, or make calculations or an educated guess of how much you have to change the turns count to compensate for the dimensional changes you have made. At the very least, you should have an idea which way the turns count much change to be a correction in the right direction.
If your mailing tube has the same outside diameter as the author's, that is one variable eliminated.
The author says his Litz wire is the same size as 28 AWG, so this coil should have about the same length as his 66 turn one. Its inductance will be just a little less, because the solid wire repels the flux from inside some of the wire cross section with eddy current, but this is a tiny part of the total flux, so maybe a single additional turn will compensate for that. The effect is proportional to frequency, so there is no count that will work exactly like the Litz coil would. Your coil will also have a lower Q because it is absorbing a little of the RF energy into eddy current losses.
The second winding is meant to be 8 turns of 30 gauge
Your winding will be quite a bit longer than the author's, so will have lower inductance (less well coupled turns), but, again, an extra turn might compensate, or put it just a bit closer to the tuned winding so that the mid point of this winding is about the same distance to the midpoint of the tuned coil as is the case with the author's design. It won't take much of a correction, and the author may not have done it the best way possible, so your changes might even make it better. Too many variables. You might have to make more than one and compare them in experimental trials.
Again, your coil will have more axial length than the author's, but it is not tuned, nor part of a feedback gain ratio, so its inductance is not critical. I'm guessing this one doesn't matter so much. It also might work better if this coil were closer or further from the tuned one. I doubt if the author optimized it in any way. It is probably just something that worked.
-- Regards, John Popelish
Fifty feet. Why don't you purchase a 1/4 lb spool of 36 gauge magnet wire. Parallel seven strands to make the 28 gauge litz wire and parallel four strands to make the 30 gauge litz wire. In litz wire, the strands are insulated from each other and connected together only at the ends.
To make the 28 ga.stretch out the seven strands about forty feet. Use nails in boards at each end to keep the strands from tangling. With an electric drill twist up the strands from one end into the litz bundle. Keep the bundle loose, do not twist too tightly. Use this forty foot length to wind the 66 turn winding. Tin the ends together into a single group for connection.
Repeat with about 18 feet of four strands to make the 30ga. Litz wire.
Note, you can use more strands of thinner wire to make the litz bundles, but thinner wire is more expensive and harder to handle. For example, you could use 17 strands of 40 gauge wire to make the 28 ga. bundle but that's a bit harder and not necessary for your application.
winding 1 = 2*3.1416*66/12 = ?feet winding 2 = 2*3.1416*20/12 = ?feet
so you need to do this simple math. Add them together, then add a couple feet for the fly leads and a little extra, just in case there is some variation in the tube's diameter.
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The form size and turns count gives you a length; but magnet wire is sold by the pound, so using 28 gauge wire you look up... it's half a pound per thousand feet.
The minimum purchase quantity is about a thousand feet. That should be enough.
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