The Crystal Loop! — #30 Radio
I wasn't sure where I was going to put this on my website. It is a loop, and it is a crystal set. Since my crystal set section is more organized than the loops, I will put it here.
This loop serves several purposes. It is a stand alone crystal radio. The stations have to be fairly close by to hear anything. I can hear the local 5 kw daytime station that is 13 miles from my house. To help the reception during the poor summer propagation months, an outside antenna and ground can be connected to the coupling coil.
Loop antennas make very good AM antennas for old time radios. If you like listening to AM radio and the radio doesn't have a loop, you should make one of these. If you have young ones around the house, a loop antenna is almost as fascinating as giving a kid a flashlight.
The third purpose for this set is a wave trap for use with other crystal sets. Connect the coupling loop in your antenna line and you have an instant wave trap. This is true with all my dual coil loops.
This loop is 30 inches wide and 36 inches tall, somewhat bigger than my other loops. I wanted more pickup surface area. Refer to my loops page about drilling the holes for the wires. My loop has 12 turns on the tuning coil. This comes out to 125 microhenries as measured on my AADE L/C Meter. With a dual 365 pf variable capacitor, the entire band, 1700 to 540 khz can be tuned. The litz wire I used has 660 strands of #46 gauge wire. However, you may want to use a different wire. Besides being very expensive, it is somewhat hard to handle. I ended up with a lot of kinks in the wire. I second person to help handle the wire during winding is necessary. Loop winding experience is also helpful. A solid wire or a larger gauge wire litz might be helpful. But I sure like this 660/46 litz for performance! The coupling loop is two turns of 40/38 litz wire.
Check out the pictures to see how I did the mechanical work on my crystal loop. The crystal radio tuner uses a circuit that will help maintain the high Q of the coil. For the highest Q, try to find a variable capacitor that has ceramic insulators on the stator. A vernier tuning system would be also helpful if you plan on using this mostly as a crystal set. I use my outboard audio transformer and sound powered headphones to listen. If you don't have these items, a crystal headphone with a 100 k ohm resistor connected across the headphone terminals will get you by.If you want a high quality AM tuner, connect the output directly to an amplifier.
The connections are 5 way binding posts, made by Johnson. Other connectors can be used, but these looked right for this project. Besides my little construction glitches, this set came out very well. This stands proudly with my other loop antennas. Perhaps in the summer time I should put an electric charge on them and use them to kill the insects flying around here. Wow, a fourth use!
If this set is a little big for you, check out an earlier loop antenna crystal radio I built.
Best wishes and good DX. Dave - N2DS