Shortwave Crystal Radio

It was my first attempt at building a shortwave crystal radio.

Shortwave Crystal Radio
It's single-tuned and uses a 1N270 to drive vintage sound-powered phones.

The circuit was 'breadboarded' on an old fan regulator baseplate with the coil wound on a white plastic pill bottle.
Shortwave Crystal Radio - Schematic
A 2-section variable capacitor, with slow motion drive, was used for tuning. The 2 sections were connected in series for an effective maximum capacitance of 180 pF.

Using a 60' wire antenna and earth resulted in 'local strong medium wave station breakthrough' and hum.

With earth disconnected, the breakthrough and hum reduced considerably and it was possible to tune-in to a faint shortwave broadcast. It was from AIR Chennai, on 7380 kHz, located 270 km away.

Tuning further up, a weaker Oriental station was heard.

Severe fading was experienced with both the signals.

Similar results were obtained using a 1.5 m long telescopic antenna and earth.

On a later occasion the same stations came in louder, with no breakthrough and hum, using a 20 m inverted 'V' antenna.
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Portable AM Crystal Radio

Here's the portable version of my 'Series-tuned Crystal Radio'.

Portable AM Crystal Radio
The prototype was wired on a piece of perforated board.

M0UKD's 'Loaded Quarter Wave Antenna Inductance Calculator' gave a loading coil inductance of the order of 10 mH at 612 kHz (the frequency of the 200 kW AM station located 20 km away). Hence two 4.7 mH moulded inductors were connected in series at the feed point, to resonate the 0.75 m long telescopic antenna at that frequency. 

Portable AM Crystal Radio - Schematic
At first nothing was heard even on the shack roof. However, on moving around, a favourable spot was found, where the reception is indeed quite good. The spot is close to an overhead CATV cable working as a passive radiator. The cable runs exactly in the direction of the station like a Beverage antenna.

Encouraged by the results, the final version was built using the plastic front panel of a discarded radio. In this version OA79 diodes were used with equally good results.

Portable AM Crystal Radio - final version
This radio works quite well, unaided by passive radiators, up to 5 km away from the transmitter.

Related post: A Tuned Passive Radiator
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