Jane Russell Radio — a.k.a. #41 Crystal Radio
I have never built a radio with more shape than this. This is not some mickey mouse radio. This radio is a high performance set suitable as an entry in the Crystal Radio Contest in the "Hobby Class" section. Since this is a single coil set, a little more finesse is needed to pick out the weak stations buried among the blowtorch senders. I used mammarial efforts in building this set!
I call this a high performance radio because several of my conditions have been met. The coil is made up of the highest quality 660 strand, 46 gauge litz wire. Also, this coil is wound on a homemade spider coil form. The distributed capacitance is very low, so this helps the sharpness and the tuning range. The coil is similar to my Überlitz ® coil but wound to about 170 µH.
The variable capacitors are also high quality. There are ceramic studs to insulate the stator which makes for a very low rf loss capacitor. In a high tank impedance situation, having low loss components around the coil is very important. Connected to the capacitors are vernier dials. If the set tunes sharper, having slow tuning is a must. Stations can be peaked very easily. The number scales also help to reset the dials to particular setting.
A schottky diode becomes important in a high performance radio. Since the high Q coil is not tapped, a very high impedance exists at the top of the tank. The advantage of this selective circuit can be ruined by using a regular detector circuit and diode. I used a low capacitance trimmer, schottky diode and a 27mh choke. This combination performs very well.
Last but not least, since the impedances are very high, much higher than a 2000 ohm set of headphones, a transformer should be used to get a closer match to the detector and tank. Ideally, a 100,000 ohm or higher input, heavy iron transformer, matched down to your headphones is what the doctor is ordering here. If you can't find one of those, a Bogen T725 speaker matching transformer will do you. If you can find some good sound powered headphones, this will be the icing on the cake! Examples of transformer coupling can be found on my audio transformers page.
Here is how to use this type of crystal set. The selectivity (4-30 pf) capacitor should be set to the mid range to begin with. If you would like higher selectivity, reduce the value. If you want more volume, increase the value. If you want, this could be a front panel adjustment, but make sure to set the capacitor back a ways from the front panel and use an insulated shaft to connect to the knob. Otherwise you will upset the tuning when you move your hand near the knob.
When tuning the band, I like to start at the top end. I set both capacitors at the minimum value. Then I adjust the antenna capacitor and listen for the stations. As the capacitance is increased, the received frequency will lower. Keeping the antenna capacitor at a minimum value, decreases the likelyhood of bad selectivity and short wave ghosts. You won't be able to tune the band with the detector capacitor at the minimum setting. Next move this capacitor about 10% higher in capacitance. Readjust the antenna capacitor to minimum and work down the band again. When you are tuned to a station, rocking both capacitors for the best volume with least interference will sharpen the tuning. Once you find the optimum settings, write down the dial settings so you can later return to the station. Changing the antenna will change the dial settings.
73 and good crystal DX. Dave - N2DS