Double Litz! — Dave's #21 Radio
After 20 comes 21. It is too bad I am all out of those Remler capacitors. Ok, on to 21. This crystal radio features two coils. One tunes the top half of the broadcast band and the other coil tunes the bottom part. This was done to increase efficiency and control the bandwidth a little bit better. I also wanted to see if having separate coils was better than tapping a single coil. Right now the jury is out on this matter. I write these pages soon after I build them and wait until the evening to test them.
I won't spend much time on the construction details as these are well described in my other sets. I did something that might be of note with this set and that involves the rounded edges on the front panel. The contour of that edge matches the knob size. I traced the knob edge along the corner as a cutting guide. I cut the edge with a hacksaw at a 45 degree angle and then cut two more times at 30 and 60 degrees. I then file and finally sand the edges. Before I cut that edge, I drew a 45 degree line from each corner down through the center part of the Garolite® panel. Then I placed the tuning shaft of the capacitors on these lines, 4 inches up from the bottom of the panel. This gives this radio a better visual flow. By the way, before I do any measuring and cutting on the panel, I cover it with masking tape and mark my hole positions on the tape. I am not sure if I had mentioned that before.
The coils have a tap in the middle for the diode connection. I found that a 50% tap was a good compromise between volume and tuning sharpness (selectivity). I checked this out with my #20 set. This applies only with these type of coils and circuit.
The low frequency coil has a total of 42 turns with a measured inductance of 220 microhenries. The high frequency coil has a total of 30 turns with a total inductance of 140 microhenries. If you use a different size of wire, the number of windings will be different. With the 3/22/38 litz wire that I use, the holes on the low frequency coil are at 1-1/16 inch intervals (2-1/8 inches end to end). The high frequency coil has holes placed at 3/4 inch intervals (1-1/2 inch total distance). I use a Styrene 4 inch sewer pipe coupler as a coil form. The outside diameter is 4-1/2 inches. This pipe is available from Genova Products (# S40140) or from Home Depot. Don't buy PVC couplers, only styrene.
The basic circuit is like my #17 and other sets. This is my favorite crystal set circuit. The only problem with this circuit is because of the single coil construction, there can be some shortwave leakage. A trap tuned to 6 mhz should take care of this problem.
I write these pages so that you might get an idea or two for building your own crystal set. The main thing is that you have fun doing it. I have not had this much fun with radio in 20 years, so I am happy to share all this joy with you. Best wishes from -- Dave N2DS