Seventy years ago this month, the April 1956 issue of Popular Electronics showed this extremely simple shortwave converter. It consisted of little more than a diode and tuned circuit, and the output went to the antenna terminals of an AM radio, or looped around the coil. (The loop antenna would pick up too many broadcast stations, so the author recommended replacing it with a small loopstick.)

The author doesn’t explain how it works, and I don’t know either, other than perhaps by brute force on strong signals. But according to the author, he was able to pick up foreign broadcast stations and amateurs on 40 and 80 meters. Even though it appeared in the April issue, I’m inclined to believe that it worked through brute force. We previously covered a similar idea for making prewar FM radios pick up signals on the new FM band.