It has been a few months since I last visited this topic. Sorry about that. Family matters. However, let us look at how a crystal radio works. For those who’ve been licensed hams for any length of time, this is really simple, but for others, this borders on magic, but we can do this.
Let us start with a basic assumption. Our assumption is that we are surrounded by electromagnetic energy in a wide variety of frequencies and modulations. It doesn’t matter if it is your computer, your mobile device, or something, you are surrounded by RF generating devices, and this makes our basic assumption true. If you want to say that you are miles from civilization and thus it is impossible to be surrounded by RF energy, not only are you wrong, but I’m going to wonder how you can even read this.
This is where it gets interesting, and I will do my best to keep it simple.
A crystal radio, much like any other radio, uses an antenna to pick up an RF signal. That signal is then sent to the antenna coil. The antenna coil converts that energy into a very low level of AC current, which is then fed into either a germanium diode, or cat’s whisker detector (galena or pyrite crystal with a thin wire barely contacting it). This forces the current to flow in only one direction, and rectifies the signal. The rectified signal then either goes to the earphones, creating something you can hear, or into a variable capacitor, and giving you the ability to select what you hear, by varying the amount of energy stored between the detector and the earphone. The earphone transforms that energy into an audio signal, and for the sake of being somewhat technical, it is about 400 to 2000 Ohms.
Realistically, this is an oversimplification of the whole thing. However, for the purposes of this blog, simpler is better. I will be revisiting this topic again as I prepare for the actual build process. Next time, I will build a simple “popsicle stick” antenna for detecting RF energy. Also, I will be using an actual diode for the detector, as a cat’s whisker detector is really awesome, but a royal pain to maintain a signal with.