Crystal Radio Antennas

A medium wave broadcast station antenna system generally comprises a ¼ λ vertical element
and earth.

It would not be wrong to infer that such an antenna would be ideal for a crystal radio.

Crystal Radio Antenna Arrangement

However, practical reasons necessitate use of a much shorter element, with a suitable loading
coil making it resonant at the desired frequency.

Good results would be obtained with an element not shorter than 60 feet.

Related post: My first Antenna
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Crystal Radio / Piezo Earpiece Interfacing

Here's how to interface a piezo earpiece with a crystal radio.

Parallel-tuned, series-fed radio:

The earpiece is interfaced through a 33 kΩ shunt resistor. The resistor provides a path for the
detector current. The voltage drop across the resistor drives the earpiece.

Series-tuned, shunt-fed radio:   
   

The earpiece is interfaced through a 2.5 mH RF Choke to prevent its capacitance from short-circuiting the detector. The voltage drop across the detector drives the earpiece.

Related post: Makeshift Piezo Earphones
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