Loop Crystal Set, #19 Crystal Radio
Happy New Year! This is my first radio of 2003. Well, sort of. I cheated a little. I took an existing homemade loop antenna and turned it into a crystal set.
This is one of the coolest sets I have built because you don't need an external antenna or ground for this to operate. But, you sure need good hearing.
Here is how to build one of these. The materials are as follows. You can use any wood that you like. I prefer to use oak. The base is a square piece of 3/4 inch oak. The size is around 9 x 9 inches. I would not go too much smaller as the loop would tip easily. I added a 4x4 inch piece of oak on top for that "deco" look. (You can tell that I have been spending too much time on ebay.
The loop is made up of a 32 inch long piece of 1/2 x 3/4 inch oak. This is the center mast. The arms are made of the same material, each 12 inches long. The black material in the center is Garolite® but you can use some thin wood. The capacitor and earphone terminals are also mounted on Garolite®.
There are a series of small 1/16 inch holes drilled through the loop to pass the wire. The top and the sides have each 15 holes, drilled 3/8 inch apart from each other and the end. For the bottom, measure 24 inches down from the top and drill the first hole. Drill 15 more holes going up the mast. There will be a total of 16 holes drilled in the bottom portion of the mast. The extra hole is because the windings start and stop at the bottom part of the mast. If you drill two more holes, each 1/4 inch above and below the end holes at the bottom, this will be a good way to anchor the wire ends by threading them around a couple of times. Look at the pictures and my loops page for more details.
Drill a 1/4 inch hole in the bottom of the mast and another hole in the center of the base. Use a 1/4 inch dowel rod to connect the mast to the base. Don't glue the rod because you may break it and need to replace it. If this happens, drill a small hole in the stuck dowel. Screw a small wood screw in and take a pair of pliers and yank out the broken rod.
There is about 68 feet of 20 gauge enameled magnet wire. A heavy litz can also be used for improved performance. It is best to have a big space (outdoors for me) when winding the loop. First measure and cut the wire to about 70 feet. Starting at the top hole of the bottom of the loop (confused?), thread the wire around the loop. You will have to stretch the wire back out and take the free end and thread it through a couple of the holes and pull it through. This will take you over an hour. It might help if someone handles the wire too. Be careful not to kink the wire.
After the loop is together, wire it as shown in the schematic. If you want, add another turn of wire on the outside or inside of the tuned loop for an external antenna and ground. You can also connect this single turn to an old radio to boost reception when you aren't using it as a crystal set. Use your imagination. These loops are pretty cheap to build but take a while to make them. If you come up with a cool loop design send it to me and I will put it on my visitors page.
My thanks to Gollum and my friend Mark in Holland for giving me the inspiration to make this really neat set. I have it on display where I work and the customers just love it. To improve the performance of a loop crystal radio, make the loop bigger. A bigger loop is in the planning stages now and it should be ready this spring.
I built another loop crystal radio, even bigger and better.
Best wishes from -- Dave N2DS