This simple, permeability-tuned crystal radio was wired using a variable inductor, a germanium diode/transistor and a pair of sensitive DLR No.5 I.T.B.A.5 S balanced-armature headphones.
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| Permeability-tuned Crystal Radio |
A germanium diode was used as the detector in the series-fed version.
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Permeability-tuned, Series-fed
Crystal Radio - Schematic |
For the shunt-fed version it was a germanium transistor (with its base and emitter interconnected).
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Permeability-tuned, Shunt-fed
Crystal Radio - Schematic |
An empty glue stick and a ferrite toroid were used for the tuning mechanism.
The toroid was fixed to the blue glue stick carrier using rubber adhesive. The coil was 60 turns of 30 SWG enamelled copper wire, close-wound on a homebrewed 1" diameter paper former which fit tightly on the glue stick body.
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Tuning mechanism for
Permeability-tuned Crystal Radio |
Junk-box parts were used to assemble and wire the radio using a scrap wall wart enclosure as the base.
Reception of the local 612 kHz, 200 kW AM broadcast station was quite good, with just a 60' wire antenna. Headphone current, measured using a 1mA FSD 60 Ω meter, was 350 μA.
During subsequent trials, it was concluded that a shunt-fed, permeability-tuned crystal radio, with a fixed series-capacitor, could be easily tweaked for best performance.
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Permeability-tuned Crystal Radio
with fixed series-capacitor - Schematic |
Hence another unit was built, using an empty lip salve stick container as coil former/tuning mechanism. A temporary coil, consisting of 120 turns of 30 SWG enamelled copper wire was close-wound on the 5/8 " diameter lip salve stick body. A 1" length of ferrite rod was glued on to the lip salve stick carrier. A value of 330 pF was chosen for the tubular ceramic capacitor, as a 365 pF variable would be normally set around that value, to receive a station at 612 kHz. A germanium transistor, with its base and emitter interconnected, was used as the detector. After a number of trials, it was found that best reception of the local station was obtained when the number of turns was reduced to 90, keeping the ferrite rod 90% inside the coil.
The temporary coil was then replaced with a proper one, having 90 turns of 30 SWG enamelled copper wire, close-wound and taped.
Assembly/wiring was on a discarded blister pack.
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Permeability-tuned Crystal Radio
with fixed series-capacitor |
Excellent reception of the local station was obtained with the same sensitive balanced-armature phones and 60' long wire antenna. Headphone current indicated by the 1mA FSD 60 Ω meter was 800 μA.
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