#62 Dual Litz Honeycomb Coils Crystal Radio
I hope that this crystal set is well received by all of you. My #62 radio combines the features of some of my earlier radios. It combines the one piece approach of my #48 set and the dual detector of one of my recent sets.
Most of my radios center around a theme, circuit or part. This one centers around a the coil.
The coil arrangement is a dual honeycomb coil, both mounted in an clear acrylic tube. So the coupling between the coils can be adjusted, one of them is on a pivot and can be turned with a knob. If better selectivity is desired, the antenna coil can be rotated perpendicular to the main coil. This reduces the coupling between the coils, making a sharper signal peak. The coils are made with 165 strand, 46 gauge litz wire. This circuit will also work with a spider or space wound coils. The litz can have 44 gauge or 46 gauge conductors, depending on your budget.
The three variable capacitors are dual gang types with 270 pf capacitance per section. These were obtained on the surplus market. I made them work very well in this radio. For example, the antenna input capacitor is one section at 270 pf. But at the top of the broadcast band, the adjustment can be a little touchy. By adding the second antenna terminal connection, which places the two sections in series, the maximum capacitance is cut in half. The capacitor next to it is used in parallel with the antenna coil. Both sections are tied in parallel for a maximum of 540 pf. The main detector tuning capacitor has one section directly connected across the tank coil, while the other section has 270 pf in series going to the coil. This gives a maximum capacitance of 400 pf. I am able to tune the entire band using the full range of the capacitor.
The capacitors also have a 2:1 gear arrangement. This makes the tuning a little nicer. The main detector tuning has a 6:1 vernier drive connected to the 2:1 gear, giving 6 full turns to turn the capacitor 180 degrees. What bliss!
There are two complete crystal sets in this project. The first detector is connected to the antenna coil and capacitors. The second detector is connected to the detector tank circuit. The audio outputs are wired to a front panel selector switch and routed to the audio output transformer.
The detectors use a modified Hobbydyne detector circuit. A small trimmer sets the loading on the tank circuits. This improves the match and the selectivity. I haven't actually checked it, but I believe that the loss of signal is minimal. In my #61 set, I did disconnect the detectors from the capacitor because I thought this would help reduce the losses when using the other detector. I found this not to be a problem because of the feather touch loading on the tank. So I left that feature out of this radio.
There is a parallel combination of a 47k ohm resistor and a .1uf capacitor. This is used to balance the dc resistance with the transformer impedance. This is done so that when strong signals come in, the tank won't be loaded due to the heavier conductance of the diode. This little circuit used for this purpose has been named the benny. I used a pair of FO-215 high performance germanium diodes in this radio. 1N34A diodes will also work.
I also added a 4 point brass link switch to select the impedance that closely matches the headphones used. This is in an out of the way place as it usually gets set only once. If you change headphones a lot, you might want to locate the switch on the front panel. If you never change headphones, just pick the right tap and wire it directly to the headphone jack.
The base is made from a 8x12 inch (20x30cm) piece of Garolite®. the thickness is 3/16 inches or about 4,5 mm. The front panel is 4-1/2 inches or 11,5 cm. I cut the front panel to give a little extra eye appeal to this radio. It was suggested that I could add length to the coupling adjustment and bring it to the front panel, but since I don't use plans or drawings when I build, it would have made the project a lot more difficult. I like how this turned out.
This radio is quite heavy and sets well on my table. The reception is very good. I sure like testing these sets during the winter. More signals. The selectivity is good, considering the components.
I haven't mentioned this in a while, I made the labels with a Brother P-Touch label maker that is connected to my computer. I try not to over label. I put enough on so you know what the controls are for. That's about it. If you have questions, please ask.
Good DX! Dave - N2DS