Crystal radio
Building a simple AM crystal radio is, or ought to be, a basic survival skill.
This circuit [1] shows an antenna tuned with an inductor of 240μH and a variable capacitor of 365 pF.
The resonant frequency for this tuning with the variable capacitor set at 365 pF starts at about
- f = 1 / 2π√(LC) =~ 537.73472 kilohertz
If the variable capacitance is reduced to 36.5 pF, then the resonant frequency will be increased to 1700.46649 kilohertz, the other end of the popular AM broadcast radio band, which is about exactly half a decade wide, spanning a ratio of √10 on the radio spectrum.
Notes
The circuit shows a ground. It would not typically be considered necessary to physically connect the circuit to the earth at that point, but, as this is an unamplified radio receiver powered solely by the energy of broadcast radio waves hitting the antenna, it may be possible and convenient to draw more power from the antenna if the circuit is physically grounded at the other end of the inductor coil. Otherwise the other pole of a dipole antenna might be connected at that point.
The listening device could be any basic earbud or one of a pair of good quality earbuds salvaged if the other is damaged or broken. Special high impedance earbuds are sold specifically for crystal radio sets.
Crystal diodes have been made out of razor blades, pencil leads, safety pins, etc. The original "crystal" was a galena (lead or silver sulfide) crystal with the anode soldered in place with an alloy of bismuth, lead, tin and cadmium called "Wood’s Metal." A normal tin-lead solder alloy or even leaded or lead-free silver-bearing solder is probably adequate, since cadmium is even more toxic than lead.
A very fine sharp wire called a “cat’s whisker” is allowed to touch the surface of the galena crystal where it will allow electrons to pass into the crystal but it will not be able to pick up any electrons from the crystal. (The solder alloy bonded to the other side of the crystal will pick up and conduct electrons freely.) Commercially available “cat’s whisker” diodes are typically encased in clear glass and sold as “fast recovery” diodes.
There are many interesting ways to make inductors and variable or fixed capacitors.
- ↑ All About Circuits. Textbook // Semiconductors // Practical Analog Semiconductor Circuits // Radio Circuits. https://www.allaboutcircuits.com/textbook/semiconductors/chpt-9/radio-circuits/
