Crystal Radio, Capacitive Tuning (Update with Annotations Available!)

The BEST crystal radio I have used or built! This radio works very well. I've tried many crystal radio designs with variable capacitor tuning and none worked. (I've got a video on a failed one.) The starting point for this model came from a 1997 ARRL article (based on a design from the 1930's by Elmer G. Osterhoudt) and I've made changes in both parts and construction.

This is now one of my 2 favorite crystal radios. The other is the modified Boy Scout radio. The BSA radio is simpler to construct, cheaper - requires only 2 purchased parts, easier to operate / tune, does not require a significant ground. It gives a loud output. On the downside selectivity is not good; weaker signals are often buried under crosstalk from stronger stations. The modified ARRL 1997 radio is more expensive, and more complex to build. It requires a good ground. Where this radio shines is in clarity of reception, i.e. selectivity - it tunes very precisely and even the weakest signals can be heard without crosstalk. The output is only slightly weaker than the BSA radio. Overall a very good radio.

If you like radio and / or electronics, you might consider joining ARRL (Amateur Radio Relay League). www.arrl.org

MATERIALS
- 1N34a germanium diode (can NOT use substitutes)
- Piezoelectric earphone (can NOT use substitutes)
- 2 x 450 pF variable capacitors (can be air or mica types)
- 10K ohm resistor (can vary to suit your radio)
- Inductor: approx 146 microhenries. Core - 4 inch diameter PVC pipe. Wire - 32 turns, 35 feet (10.668 m) of 0.5mm diameter (23-24 wire gauge) enamel wire. 1.1 feet per turn. 1.1 feet = 33.582 cms. 35 feet = 10.668 meters. Circumference of 1.09 feet = diameter of 4.16349 inches.
3D Printable Knobs For This Radio: https://www.thingiverse.com/thing:619...

ANTENNA
The best antenna would be 152.4m (500 feet) long. A "good" antenna would be 30.48m (100 feet). The video uses an antenna of only 12 meters (40 feet) and it works OK.
WARNING - Never run antennas over or near power lines or electric wires. Serious injury or death may result. If you don't know, learn or don't do it.

ORIGINAL ARTICLE
"The Crystal Radio", Dave Evison, W7DE, QST December 1997, pp. 56-57. This article's design is based on one from the 1930's by Elmer G. Osterhoudt

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