The Arachniodyne I. My #20 Crystal Set
Welcome to my #20 page. This radio is a dual spider coil radio. The antenna input coil is wound with 70 turns of 40/44 litz for an inductance of 350 µH. The inductance of this coil was selected due to the antenna I was using. My antenna is a 75 foot piece of wire about 10 feet over the ground. If I use my 80 meter dipole (or one half of it), I found that the inductance had to be smaller. You may have to experiment with this for your own use. The spider form AOL cd) is attached to a dowel that can be pulled out or pushed in from the front panel. This varies the coupling between the antenna and detector coils.
The detector coil is a biggie. The coil is made from about 58 feet of660/46 litz wire. The form is made from a 1/16 inch styrene sheet. The piece is 8 inches wide and 8½ inches tall. The inside diameter is 3 inches and this form has 11 slits. After winding, adjust the positions of the wires so they aren't touching an adjacent wire. This will keep the coil capacitance to a minimum. The inductance is 240 µH.
For most efficient operation, the coil is not tapped. With the use of the detector circuit shown plus an audio matching transformer, the high impedance is dealt with properly. There is a small variable capacitor that couples from the top of the coil to the diode. This adjusts the impedance and the signal level. It is possible to come up with the best sensitivity while maintaining selectivity. A 27 mh rf choke is used as a dc ground return path. A 330 pf capacitor removes the rf from the circuit and the audio is sent to a transformer.
The RC network shown on the diagram maintains a high dc resistance in the detector circuit for minimum audio distortion. Some use a 500k pot to adjust for the least distortion. I found that around here, that setting just didn't change much, so I put in a fixed resistor equal to the transformer impedance.
The variable capacitors should be of high quality. Ceramic insulators on the stator will give you a high Q circuit. Be sure to look for a rotor wiper on the capacitor too. These capacitors are few and far between but if you can find them, that just adds to the performance of your radio. I don't think that my Remler capacitors are that wonderful but I like how they look. A vernier dial mounted on the antenna and detector capacitor will help you not miss stations. The small 25 pf coupling capacitor should be mounted back from the front panel with an insulated coupling so that hand capacitance doesn't upset the settings.
About the name "Arachniodyne". My friend Fred Wise in MD came up with that! Great name Fred! This radio is kind of expensive to build, so keep some money stashed away for this project. BTW, I built another variable coupling coil dx crystal set radio. Best wishes from -- Dave N2DS