Modular High Performance Antenna tuner and Detector
This is the Crystal Radio to End all Crystal Radios. Hello there. Can't you just feel the tension rising? With this new radio I shift gears again. Large litz wire, high quality variable capacitors, schottky diodes. It won't get much better than this. Last night I am proud to announce that I heard KMOX in St. Louis on this radio. That is my furthest DX ever. I am sure that the ionosphere was hot and the wind was blowing my way, but a heard station is a heard station. Enough bragging and on with the show.
This radio is built in modular form. Mike Peebles gave me the idea when he had his modular crystal set published in the Crystal Set Society newsletter. The first part built is the detector. This consists of the main tuned circuit with my 660/46 Überlitz Coil. This is of my own design. I am a big fan of litz spider coils, and this is a big litz spider coil. I believe I hit crystal set heaven with this coil. The detector unit also has a selectivity / sensitivity control using a differential capacitor. Besides all of this there is, of course, the detector diode itself. I made a board with a three position link switch and places for three diodes to be connected. This allows me to easily compare 3 diodes in the same set. After the diodes is an audio output transformer and RC network. The 27 mh choke provides a DC return path for the diode circuit.
The variable capacitor is an old one made by National Radio. I added a 6:1 vernier drive between the capacitor and the knob. I can tune this set accurately without missing stations. I made a dial scale from white styrene. The numbers are made by using a Brother P-Touch label maker.
The selectivity and sensitivity are handled by a differential capacitor and a piston trimmer. This system was first used in the Hobbydyne II circuit by Jim Frederick. The piston trimmer is for tracking adjustment. You adjust the piston trimmer until the main tuning needs no touch up after turning the differential capacitor. This is a breakthrough in weak signal reception on a crystal set.
The audio transformer matches the very high (100 k ohm +) detector circuit with the much lower impedance headphones. I am using a 100k to 1.5k ohm transformer. If you can find a high quality transformer, go for it.
My antenna tuning unit is the other section of this dx radio. The coil is made from 660/46 litz. The hub diameter of the coil is 3 inches. The coil form made from 1/16 inch styrene sheet is 6-1/2 inches wide and 7-1/2 inches tall. There are 33 turns on it for an inductance of 150 µH. The variable capacitor has 600 pf per section. Smaller ones can be used, however the low end of the band might not tune. You can get away with a 500 pf per section capacitor.
Earlier I had used a 150 micro henry coil made from 40/44 litz
wire. This worked quite well. One night very late, I scanned the dial and
with this set up I heard 40 stations. Not bad for a September night.
The two units are placed next to each other. The coil distance for good overall operation are about 15 inches apart. Careful adjustment of the antenna tuner and detector tuning are required. The Q of these types of coils have been measured at over 1000! This ain't your grand pappy's crystal set.
If you build one of these, let me know how it plays for you. Happy crystal set dx. Dave, N2DS