Another Parallel-tuned Air-core Coil Crystal Radio

A 7" long, 2" diameter PVC pipe doubles as coil former and enclosure for this crystal radio. A couple of white plastic cosmetic jar covers serve as the end caps.

Parallel-tuned Air-core Coil 
Crystal Radio

The coil is (280 + 40) turns of 30 SWG enamelled copper wire, close-wound as shown. The PVC variable capacitor is mounted on one of the end caps.

The OA79 diode feeds a pair of sensitive balanced-armature phones.

Parallel-tuned Air-core Coil
Crystal Radio - Schematic

Using a wire antenna 60' long, reception of the 612 kHz - 200 kW local AM broadcast station is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 350 μA.

Related post: Colourful Parallel-tuned Air-core Coil Crystal Radio

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Makeshift Crystal Radio Output Transformer

This project was part of my initial attempts at homebrewing an output transformer for my  'Loudspeaker Crystal Radio'.

An air-core crossover network inductor and a ferrite-core one (from the junk box) appeared suitable as the primary and secondary windings of the transformer.

An Air-core & a Ferrite Core Crossover Inductor

The large number of turns of the air-core inductor as primary and the small number of the ferrite one as secondary would give a step-down ratio. With luck, the open ferrite core would enable reasonable coupling.

 Simple coupling of the two inductors

Testing was simple, with the air-core inductor hooked up to the crystal radio and the ferrite-core one (placed within) connected  to the speaker.

The result was encouraging enough for further attempts which culminated in a proper 'Crystal Radio Output Transformer'.
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