Physics of the Radio Without a Battery — the Crystal Radio
It’s also called a foxhole radio. The idea is that you can build a radio from some pretty basic parts and then use this to listen to nearby radio stations. It doesn’t even need a battery. If you have been watching Masters of the Air (on Apple TV), you can see one of these radios in episode 7 (that’s all I’m going to say about the show to avoid spoilers).
There’s a bunch of cool physics in the operation of this kind of radio. Let’s go over the basics.
What is a Radio Wave?
If you want to listen to the radio, you need to know what a radio detects. The answer is light. Well, it’s a type of light — it’s an electromagnetic wave. This is an oscillating electric and magnetic field that’s created by the radio station. It’s just like visible light except that it has a MUCH larger wavelength. In the USA, you can pick up AM radio on frequencies between 540 kiloHertz and 1700 kHz. This corresponds to a wavelength between 555 meters and 176 meters.
But really, it’s the oscillating frequency of the electric field in a radio wave that’s import if you want to build a radio from scrap. I’ll get to that soon.
