Radio Electronics: Foxhole Radios - dummies

Radio Electronics: Foxhole Radios

By Doug Lowe

In World War II, electronically-inclined GIs would often build their own crystal radios from whatever materials they could scrounge up. These were called foxhole radios.

Wire for making antennas and coils wasn’t too hard to come by, and super-sensitive headphones weren’t hard to scrounge. But crystals for making the detector part of the circuit were another story. So the GIs came up with a cleverly improvised solution: They used razor blades, pencil lead, and safety pins.

To build the detector for a foxhole radio, the razor blade must be made of blue steel. Most modern blades aren’t, but you can fix that by placing the blade in a metal vice and blasting it with a propane torch until it glows red hot. Let it cool before you handle it!

Note that it also doesn’t hurt if the razor blade is a bit rusty; the oxide in the rust actually helps.

To build the detector, first glue the razor blade to a piece of wood. Sharpen the pencil, then cut it short (1/2″ is long enough). Bend open the safety pin to about 90° and jam the pointed end of the pin into the lead at the end of the pencil that you cut off.

Then nail or screw the flat end of the safety pin to the board, positioned so that the tip of the pencil sits on the razor blade.

Connect one wire to the razor blade and the other to the safety pin and wire it into your circuit right where the germanium diode would go. Then, hook your radio up to the antenna and ground, put the earphone in your ear, and drag the pencil tip around to different parts of the razor blade until you hear a signal.

This type of detector is very finicky, so you might have to try different angles and positions, and you might have to try different razor blades or pencils. But once you get it to work, you’ll be delighted that you were able to make a radio out of an old razor blade, a safety pin, and a pencil.