Variable/fixed tuning — #22 Crystal Radio
This is another of my revised design radios. This set previously had a cylinder coil with a lot of taps. It was a very simple radio that worked ok. But as I tested it and compared it with my other single coil sets, I found that this one had a lot of shortwave ghosts. A shortwave ghost is undesired signal reception in the lower shortwave bands due to the set design, antenna and ground systems. This is a night time problem mostly and doesn't occur with dual coil crystal sets.
After careful consideration, I decided to scrap that design in favor of a modified version of my #12 radio. The circuit is nearly the same but I used fixed capacitors instead of a second variable cap. This allowed me to use keep the tap switch.
The coil is wound on an old AOL cd. The procedures are described on my spider coil form page. I am using some 40 strand, 38 gauge litz wire for this project. This wire fills the AOL disc pretty well for a 140 µH coil. This is the right size coil for a circuit of this type. I selected the fixed capacitors to give me a good overall tuning range. I don't think there are any gaps.
Instead of a tap switch, you can use a rotary switch. If this switch were used, more capacitors can be used for finer tuning.
Happy xtal listening. Dave, N2DS.