and are connected to one 5 turn winding on a 1.5 inch diameter painted card- board tube. This provides the signal, absorbed by the main tuned coil, with 30 turns of wire on the same form. The is tapped 20 turns from the bottom. I used 23 AWG insulated wire, also a junk box find. Wire much larger in diameter wire will work better, but the coil dimensions would need to be changed. I used a crystal type earphone in this design, which requires a resistor be placed across it to provide a path for the diode circuit to work. Crystal or piezo phones are very high resistance devices (20 megaohms being typical) and act as an open The MT Special Dual Coil Shortwave Crystal circuit. Magnetic style earphones don’t require Set Schematic. this shunt resistor as the phones themselves provide relatively low resistance path needed The antenna is connected to a tap on the first Rebuilt second shortwave crystal radio, now a coil. This tap can be placed about 25% up tube radio. to complete the detector circuit. The variable is 22 pF with from the bottom of the coil for longer outside ceramic insulators, making it a low loss style, antenna connections. If your antenna is much good for this project. Nothing is critical in this shorter, the tap can be as high as 50% up from radio, so look and see what you have before the bottom. purchasing items. The coil distances should be made adjustable if possible, but they’ll generally be less than 3 inches apart. The diode can be placed on a tap Turning Failure into about half way up the coil. This should offer Success reasonable performance and a good match to the Bogen T725 . These transform- Sometimes a project doesn’t work out ers are available from electronic parts distribu- and it ends up quite different. Such was the tors, or online hobby radio parts sellers such case with my second shortwave crystal radio, as Peebles Originals (http://peeblesoriginals. which I affectionately named #58. (http:// com). Mike Peebles also offers high impedance makearadio.com/crystal/58.php ) As part made from piezo elements. If you One Tube Radio Schematic. of an online auction treasure, I ended up with wish to make your own piezophones, I have an “oscillation transformer,” an old spark gap some extra elements available. the coil may not allow the diode and the tuned component. This looked like a great I calculated the and winding circuit to operate as efficiently as possible. coil to build another shortwave crystal radio. values for two ranges. The largest There is a happy point tapping somewhere in After measuring the coil, and pairing it with a coil will allow coverage of the 75 and 40 meter the middle of the coil. , I found the radio would tune ham bands as well as several popular shortwave Using sensitive high impedance head- from about 6 to 10 MHz. That was perfect for broadcast bands. The smaller coil moves the phones is a must. Use well adjusted balanced the best evening listening bands, those being frequency range higher to tune the 31 and 25 armature phones if you can find them. Piezo the 49, 41 and 31 meter broadcast bands. meter shortwave bands but leaves out the 75 element phones also work well and can be To add to the old time look and feel of this meter ham band. cheaply built. I wrote an article some time ago radio, I added a cat-whisker detector. After building, connect the radio to your about this on my website. (http://makearadio. This looked like the perfect eye candy to show antenna and ground and wait for the sun to com/misc-stuff/piezophones.php) off to everyone on my website. Unfortunately, go down. The tuning controls won’t be real Also, use an audio frequency matching the radio didn’t work worth a hoot. Maybe it sensitive, but should be adjusted to the same transformer between the detector and head- was the low sunspot number, time of year or dial position to begin. Once you hear a sta- phones. The tuned circuit loading is reduced whatever. It did work but not with the expected tion, rock the two back and forth and the diode can be tapped at a higher position performance. to peak the signal. Note the shaft positions as on the coil, away from the grounded end. The Then I had the epiphany of turning this those relative positions will remain about the resistor in parallel with a capacitor equalizes beauty in to a one tube regenerative shortwave same throughout the tuning range. This is how the impedance vs. resistance values for less receiver. I took out the detector, made a few radio listeners in the mid 1920s operated their loading on the tuned circuit. other changes to the chassis board and added Atwater-Kent sets before some smart person A Bogen model T725 transformer is an a big 1625 transmitting tube. This saved my invented the ganged variable capacitor. inexpensive matching transformer with many project! I had hoped for a shortwave crystal I hope you enjoyed building your short- connection combinations. I’ve built a lot of set but was quite happy to have a cool looking wave crystal set. Make sure to show it off to crystal sets using this part. You may be lucky one-tube radio. everyone because, whether they are young or enough to find one of the old time audio match- old, they will be amazed at your scientific and ing that have a much higher input My Suggested Radio mechanical expertise. Let me know how your impedance. project works. Design My DIY Shortwave Crystal I designed a dual coil shortwave crystal About the author: set especially for Monitoring Times readers. I Dave Schmarder N2DS operates Dave’s Radios promise that this radio will bring you hours of Homemade Radios at http://makearadio.com. Several years ago I put together a little building and operating fun. There is a lot of He may be reached at [email protected]. shortwave crystal radio. It was built in a plastic latitude in how you build it. Before purchasing Dave is also webmaster for peeblesoriginals. box. All the parts were from my junk box, so anything, have a look in your junk box for parts com, a source for radio parts and kits for the cost was nil and the fun factor large. This that are close. crystal sets and tube radios. radio uses a dual coil arrangement where the The schematic shown is a dual coil set.

10 MONITORING TIMES October 2012