The Triangle Redone — My #38 Crystal Radio
Another triangle radio! The last time I bought boxes, I received two triangular boxes. I used the smaller one to build my #34 set. I was very impressed with the way this radio turned out and I thought I would try a second time. This box is ten inches on a side. Since the box size was a lot larger, the building of the front panel wasn't as tight. Also, I was able to put a larger litz wire spider coil in the top. This worked perfect as I had just received a large spool of 165/46 litz wire. This size of litz is a good balance between having a high quality, high Q coil and not breaking the bank. I made this radio with the same circuit of some of my other successful radios, such as my #12 set. The two capacitors allow me to tune the radio using different antennas.
By setting the antenna capacitor near minimum and tuning with the detector capacitor gives me the best selectivity but the lowest selectivity. If you give up a little selectivity, you can gain some sensitivity.
Other features I have incorporated in this radio is an external diode connection point. I can experiment with different detector diodes, including a schottky diode. The diode is coupled via a small capacitor right off the top of the tank. This causes only light loading on the tank, so as to preserve the Q. This may not be such a big deal with this single coil set as I am sure the antenna connection destroys some of the high Q.
I did add a little wood piece to the back of the box to allow the lid to stand at a near vertical position. This allows for the radio to be displayed better on my shelf.
My detector circuit work best with a transformer to match the higher impedances of the detector output to a crystal earphone or magnetic earphones. A Bogen T725 transformer would help. I use a 100k to 1.5k old time transformer to connect to my sound powered headphones.
The performance looks pretty good, but the solar flares have really made radio listening difficult. One problem with a single coil set is that at the minimum detector capacitor setting, there can be some shortwave interference. By careful adjusting, most of that can be eliminated. Daytime reception isn't affected by this.
If you don't mind the angular cuts, this can be a fun radio to build and use. 73 and good crystal DX. Dave - N2DS