DX Receiver — My #28 Radio
It finally had to come to this. A 660/46 litz wire crystal set. The litz wars have started. This is kind of a "litzkrieg". I imagine that we will need a Large Litz Non-Proliferation Treaty. Perhaps Owen Pool could monitor compliance?
This set is based mostly on the recent work that Jim Frederick did on his high performance crystal set. His is a variation of this earlier Australian design. The standout feature of Jim's set is the use of a differential variable capacitor to adjust how the diode is tapped in the circuit. A differential capacitor is a dual gang variable capacitor. The difference is as the capacitor is rotated, one side increases in capacitance while the other side decreases. These were used in the old days to make low drift oscillator circuits. Here, this capacitor is used as an rf voltage divider. As the capacitance between the high side of the coil and diode increase, the volume will tend to increase. But the selectivity decreases. If you need to tune sharp, make the coupling looser. The beauty of using a differential capacitor is that the detector tuning shifts very little as that circuit sees almost no net change in capacitance. Note how I mounted the capacitor. It stands on an insulator back from the front panel. The shaft coupling and the shaft extension are also non-metallic. If you don't take these steps, you will land in "hand capacitance hell".
The RF choke is to provide a DC path for the detector to work. It is a 25 millihenry choke. Smaller ones were tried but those are more for the short wave frequencies. Anything above a 10 mH choke will work fine. The output is made to work with an audio matching transformer. I use a 100,000 ohm to 1500 / 6000 ohm transformer. The transformer (and that 100k resistor and .01 capacitor) are not on the main board. I have one output circuit now for all my sets. The transformer drives my semi-homemade sound powered headphones. A crystal earphone may be used instead if a 100k - 220k ohm resistor is placed across the output connections.
There are two coils in this radio. The main detector coil is wound using a rook coil form. I used a 4 inch form. The coil is wound using the "One up, Two down" (1U2D) scheme. This coil has 47 turns (240 uH). It takes just about 50 feet according to my little spreadsheet program. Other less expensive litz wire may be used or even magnet wire. The coil efficiency and selectivity (Q)will be less. It depends on how crazy you want to get.
The antenna coil has 64 turns of 100/46 litz. You can also use 40/44 litz too. The inductance measures to 300 microhenries. The ferrite rod I used is 6 inches long and 3/8 inches in diameter. This is a rod I had in my collection for a long time. I am not sure of the material of this one but it was originally an AM antenna coil. Here is a recommended place to buy ferrite rods. Buy the Ferrite-61 material. The ferrite coil is mounted on a slide arrangement for variable coupling of the two coils. The closer the coils are together, the louder the signal, but you lose selectivity. You just can't have it both ways!
The variable capacitors are also important. I used ones that have a ceramic insulator, rather than the phenolic insulator. The circuit Q also depends on having good variable capacitors. I used styrene instead of Garolite® as I am not sure of the rf properties of the Garolite®.
This radio has a vernier dial on the main tuning capacitor so I could tune without missing stations. A half a turn tuning doesn't cut it with the big litz. It might be also handy to put a vernier on the input capacitor too. In lieu of the vernier dials, a small band spread capacitor could be placed in parallel with the main ones.
How does it perform? It does tune sharp and I can hear lots of stations. The band is at its worst during the summer months. This radio is the loudest set I have ever had. There are no blowtorch stations near by, so can hear all the locals during the day really well. At night I can hear stations all over the Northeast US. I can't wait for winter conditions (not the winter weather).
Earlier I built another one piece crystal set with an adjustable position coil.
Best wishes and good DX. Dave - N2DS