Series-tuned Air-core Coil Crystal Radio

A conventional parallel-tuned crystal radio was recently put together using a pair of sensitive DLR No.5 I.T.B.A.5 S balanced-armature headphones.

Conventional Parallel-tuned Crystal Radio - Schematic
To further improve its performance, a germanium transistor (with its base and emitter interconnected) was used as a substitute for the OA5. This was based on the good results obtained earlier with a series-tuned, shunt-fed crystal radio.

Parallel-tuned Series-fed Crystal Radio with
Germanium Transistor as detector - Schematic
But the results were disappointing.

However, when the circuit was reconfigured as series-tuned and shunt-fed, there was a dramatic improvement in performance.

Series-tuned, Shunt-fed Crystal Radio - Schematic
A 35mm film canister doubled as coil former and enclosure for the PVC variable capacitor and germanium transistor.

Crystal Radio using a 35mm film canister
A screw-type terminal strip was used for connecting the antenna (yellow), earth (blue) and headphone (red & blue) leads.

The local 612 kHz, 200 kW AM broadcast station came in real loud with just a 60' wire antenna. Headphone current, measured using a 1mA FSD 60 Ω meter, was 700 μA.

Subsequently, an improved, link-coupled, fixed-tuned version was built. The 50 turn link winding series-tunes the capacitive wire antenna to resonance at 612 kHz.

Link-coupled, fixed-tuned version - schematic
A cosmetic jar was used as the enclosure/base and a polythene can body as the coil former.

Link-coupled, fixed-tuned version
Performance was quite good with headphone current measuring 825 μA.  ______________________________