Two Radios in one — Dave's #23 Crystal Radio
Here I take a new step in my crystal set building with this interesting radio. This crystal set uses two of my favorite circuits. It also uses a basket weave type coil. More on this later.
I would like to bring to your attention the circuit diagram below. There are two circuits that I have favored. First is the series-parallel tuning with one coil, such as in my set #12 and others. The selectivity is controlled by the series tuning capacitor. The circuit resonance is controlled by the parallel capacitor. By careful adjustment of the two capacitors, a good balance between selectivity and sensitivity can be achieved.
One bad feature of a single coil radio is the inadvertent pickup of shortwave stations. This is called "ghosting". A double coil receiver filters out this undesirable reception. By changing the switch position, another coil is placed in the circuit. This circuit acts like my #6 radio (in the series tuned switch position). The first capacitor becomes part of a series tuned circuit with the antenna coil. The rf is inductively coupled to the detector coil. My design allows for variable physical separation or coupling of the two coils. It is sensitivity vs. selectivity. Moving the coils closer does increase the volume, but the selectivity is impaired. There is no free lunch, as they say.
The basket weave coil that I mentioned earlier is called a rook coil. There is information there on how to obtain or build a coil form to wind these coils. It is very easy to make coils this way. However the coils are a little delicate and require some gluing. They do look pretty cool. The antenna coil is wound with 40 strand of 38 gauge litz and the detector coil is wound with 3/22/38 gauge litz. I chose the smaller litz for the antenna coil so there would be fewer windings, thus a smaller coil. The antenna coil has an inductance of 240 microhenries. The coil form diameter is slightly larger than 4 inches and has 43 turns of wire. The detector coil has 40 turns with a diode tap at 25 turns This coil measures around 210 uh.
Of special note is the little track I built to slide the antenna coil. I used the same method to hold the coils as in my #16 set. The wood blocks are 2 inches long and 1/2 inch square. The wooden dowel rods are 10 inches long and 3/16 inch in diameter. The diameter of the rod is just right for holding the coil. The antenna coil can be slid several inches to adjust the coupling of the two coils. If you are in the big city you might want to have the coils further apart, but if you are in the sticks, you might want to slide them closer. The only downside that I see to this arrangement is that a lot of adjusting could wear the litz coils. Perhaps some plastic tubing (soda straws) could be inserted between the dowel and the windings to eliminate this possible problem.
For switching the circuit, I used my familiar brass link technique. They just look neat. The front panel material is Garolite®. I use wide masking tape over the face to lay out where the holes should be drilled. This works very well. I use a forstner bit to make the half inch holes for the variable capacitors.
This radio tunes quite well. It looks like this one covers the whole band, but around here there is nothing below 660 khz, so I am not certain. The basket weave coils have low internal capacitance, so the tuning range is wider than with cylinder coils. The 38 gauge litz is somewhat large for this frequency, but the wire does work very well when wound on a rook coil form.
Go ahead and build this radio. I don't think you will be sorry that you did. Best wishes and good DX. Dave - N2DS