Dave's #57 Crystal Radio
Howdy friends! On this page, I proudly introduce you to my #57 radio. I haven't thought of a nickname for this one yet. This radio looks a lot like my #51 set. I had more of those variable capacitors just itching for a project.
The wood base is about 7x11 inches (18x28 cm). All the components are mounted on the 1/8 inch (3 mm) thick Garolite® chassis. This allowed for lower losses than if the capacitors were directly mounted to the wood. The chassis is raised by the use of ceramic insulators. The use of them is purely decorative. You might want to try some wood pillars.
Even though the bottom of the chassis is nearly out of view, I still wired it neatly. My mom used to tell me to change my underware on a regular basis. She said I never know when I might be in an accident. So, just in case, the under chassis looks good.
The coil is wound with magnet wire and is space wound. When a magnet wire coil is space wound, the Q goes up and the distributed capacitance goes down. If the distributed coil capacitance is low, then the minimum - maximum capacitance of the variable will have a greater frequency tuning range. Also, I have found with coils like this is the shortwave interference is reduced. This is why I make the spider coils. If I could fit a lathe in my kitchen, I might make more of these types. Spider coils are still my favorite.
The coil is tapped about 60% from the bottom for the diode. I generally don't like coil taps in crystal sets as the diode operates at a lower voltage, thus not in the sweet spot of operation. But tapping a coil is a simple way for impedance matching of the tank, detector and audio output parts of this set. And after all, this is meant as a simple crystal set.
The circuit shown below is my favorite single coil configuration. It works well and you can control the selectivity / sensitivity by adjusting the ratio of the two capacitors. Adjusting the antenna capacitor for less capacitance will increase the selectivity but reduce the sensitivity. In crystal sets, and in life, there is no free lunch. Darned physics anyway!
Not counting the base wood preparation, this set only took me a Sunday afternoon to finish. The weather was lousy anyway, so a nice crystal set build is a good way to cheer me up. I hope your project building helps you too.
Guten Empfang wünscht Dave - N2DS