Dave goes Postal, part II — #37 Crystal Set
Oops. I did it again. Sitting around on a Sunday morning getting over my EDT to EST fall time jet lag, I decided to do a little cleaning up around the abode. I came across some painted mailing tubes and some end caps left over from a spring scouting project. Hmmmm. Didn't I just see an article written by my friend Dan Petersen, W7OIL that used something like this? Yes!, just in the July 2003 Crystal Set Society Newsletter. BTW, if you aren't a subscriber, please do. It is a very nice newsletter and it helps to support the Rap N' Tap crystal radio forum.
My set is a copy of Dan's The Old Salt radio. He describes it as a Telefunken crystal set. The Germans really knew how to build radios. This radio used a triple coil arrangement, all very tightly coupled. The coils are not connected to each other. The construction is very easy and is suitable for a scout troop, or a Parent ~ Child building experience. The most expensive part is the variable capacitor. You will also need a 3 inch mailing tube with a white end cap. If you are building a lot of sets, those end caps are available by the bag. I used some of my knurled nuts and brass screws to make the connections to the outside world. Otherwise there isn't much to this.
The mailing tube is 5-1/2 inches long and wound on it is the main coil of 54 turns of 20 gauge wire close wound. On each end of the coil, I wrapped one turn of 3/4 inch masking tape. I then wound two equal coils of 15 turns using some 30 gauge kynar coated wire. I then put some Q dope cement to hold the wires in place. Check out the diagram for further details.
How does this perform? It actually isn't too bad for the daytime listening. The tuning is somewhat sharper than I had expected. I use a 70 foot wire for an antenna. My first night time reception wasn't too wonderful though. I had a lot of shortwave ghosting interference. Also the broadcast band was in very bad shape too. But for the low cost and simple construction, it can't be beat.
Earlier I built another crystal radio using a mailing tube as a coil form.
That's about it on this project. Best wishes ~ Dave