Dave's #33 Crystal Radio
Greetings once again crystal radio fans! Yes, I have been building my big sets with the Überlitz coils, but I still like building the medium performance sets too. This one at first use appears to be on the high end of the medium performance range.
This is a two coil receiver as one of my big sets. The schematic is very close. I like the two coil sets because their are nearly immune to shortwave interference. Since the antenna isn't hanging directly on the detector coil, a higher tank Q can be reached. The Q number of the coil has a lot to do with the selectivity and sensitivity of a crystal radio. My detector coil using 100/46 litz has an unloaded Q measured at 600. Some of the other spider coils I have wound with my 40/44 litz come in at just under 500. These better coils (compaired with solid wire coils) is is what upgrades this set from perhaps a low performance model up to the high end of medium performance. Someone recently asked me what the most improvement that one might make to a crystal set and I have to say it is the coil. But I am rambling.
Dave's #33 set uses a series tuned capacitor / coil arrangement. The coil measures around 350 microhenries. To reach the low end of the band, I added a switchable 330 pf capacitor. The value of the coil and capacitor depend on your antenna and ground. I design my input circuits for a 75 foot long wire. Start with a big coil. If you find that the stations only tune at the minimum value of the antenna tuning capacitor, then take off 10 turns from the antenna coil. Another feature of the antenna coil is it will rotate to vary the coupling between the two coils. This is important if you want to catch the weaker dx signals. When the open end of the tube is parallel with the spider coil, there is maximum coupling. At first use, the maximum setting to be an overcoupled situation and the selectivity is lousy. However this setting could be just right if you have a very short antenna. I found that I have a very good adjustment range with the way I placed the coils.
The detector coil is wound on an aol cd. Check out my spider coil page for some of the details. The inductance is 240 µH. This tunes well with the 365 pf capacitor to cover the entire band. If you have a 500 pf variable, you could reduce the inductance a little bit and the entire band would be covered.
Next is the detector circuit. I am using the same circuit as in my high performance sets. There are only two more parts required and the results pay off. The detector coil is not tapped, so a high impedance circuit exists at the top of the coil. This radio is made to be used with a transformer and sound powered headphones. You can get fair to good results with a crystal earphone and no transformer. A 47000 ohm resistor is half soldered so that if a crystal earphone is used, the other end can be easily soldered to the tie point.
The box is made from basswood (not spruce as the picture below would indicate). The front panel material is Garolite® but styrene can also be used. The cool labels are made with a Brother P-Touch labelmaker connected to my computer. Ok, that's about it for this installment. Be looking on my main page for website updates.
73 and good crystal DX. Dave - N2DS