Video demonstration of loop antenna used for MF reception
Parts you'll need
Crystal sets require just a few basic parts; a coil, variable capacitor, germanium diode, earphone and a few other bits and bobs. Many can be salvaged or bought new. Links to examples are below.
Here you have a choice between a plastic dielectric variable capacitor and an air dielectric type. The plastic type is smaller, lighter and cheaper.
The air dielectric version is heavier but is easier to fit a nice knob to. The value isn't that critical but aim for one with a maximum of between 200 and about 500pF. The higher value will tune lower frequencies
(including outside the broadcast band). Removing turns from the coil raises the frequencies tuned, adding turns lowers them. You may need to make changes to cover all the stations in your area with your
particular variable capacitor.
In this project it's just lots of turns of hookup wire around a cardboard box. I use cheap stranded insulated wire.
Or you could use enamel covered wire from an old transformer, especially for crystal set versions that have their coil wound around a cardboard tube or ferrite rod instead.
The diode detects the signal of the desired station as selected by the tuning capacitor and coil described before, providing an output at audio frequencies. It needs to be a germanium diode -
the more common silicon diodes are much less sensitive. Output from it is converted to sound
by the earpiece or high impedance headphones. Depending on the earpiece or headphones you may also need a high value resistor (eg 220k) wired across them. Some crystal set circuits also feature a
capacitor (about 10nF) across the earphone to shunt unwanted radio signals to earth. While it departs from the crystal set spirit of 'free power radio' some builders use an audio amplifier module
(such as one using the popular LM386) to get more volume.
Two favourably reviewed books on radio construction to inform and inspire.
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