Crystal Radio with Air Core Coil

This is the air-core-coil variant of my Ferrite Loopstick Crystal Radio.

Crystal Radio with Air Core Coil - Circuit diagram
The coil is 72 turns of 30 SWG enamelled copper wire, close-wound on a 2½" length of 2" diameter PVC pipe, with a tap at 12 turns.

On the primary side, a 200μH moulded inductor series-tunes the capacitive 60' random wire antenna to resonance at 612kHz (frequency of the lone 200kW local AM broadcast station). The secondary is parallel-tuned using a PVC gang condenser.

Crystal Radio with Air Core Coil
One half of an acrylic trinket box serves as the base/enclosure. A screw-type terminal strip is used to connect antenna, earth and headphones.

Reception, using sensitive balanced-armature headphones, is quite good.
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Ferrite Loopstick Crystal Radio

Here's my version of a ferrite loopstick crystal radio.

Ferrite Loopstick Crystal Radio Circuit
Parts used are a moulded inductor, a MW loopstick antenna, a PVC gang condenser and a germanium diode. The enclosure is a 3" x 3" x 1½" electrical bakelite box. Screw terminals are provided on the rear of the enclosure for connecting the antenna, earth and headphones. Headphones used are of the sensitive, balanced-armature type.

Inside the Ferrite Loopstick Crystal Radio
The loopstick is 125 turns of Litz wire close-wound on a 2" length of ferrite rod with a tap at 25 turns. The number of turns may vary depending on the diameter of the ferrite rod and its relative permeability.
The primary series-tuned circuit includes the 200μH moulded inductor and the capacitance of the 60' random wire antenna. The secondary is parallel-tuned.        

Ferrite Loopstick Crystal Radio
Reception of the local 612 kHz, 200 kW AM broadcast station is excellent.
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Permeability-tuned Crystal Radio

This simple permeability-tuned crystal radio is 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.  
                                      

A germanium diode is used as the detector in the series-fed version.

Permeability-tuned Crystal Radio - series-fed version
For the shunt-fed version it's a germanium transistor (with its base and emitter interconnected). 

Permeability-tuned Crystal Radio - shunt-fed version
An empty glue stick and a ferrite toroid are used for the tuning mechanism.

The toroid is fixed to the blue glue stick carrier using rubber adhesive. The coil is 60 turns of 30 SWG enamelled copper wire, close-wound on a homebrewed 1" diameter paper former which fits tightly on the glue stick body.

Tuning mechanism for Permeability-tuned Crystal Radio
Junk-box parts are used to assemble and wire the radio using a scrap wall wart enclosure as the base.

Permeability-tuned Crystal Radio Assembly
Reception of  the local 612 kHz, 200 kW AM broadcast station is quite good with just a 60' random-wire antenna.
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Unusual Crystal Radio Circuit

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 Circuit
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 good results obtained earlier in a series-tuned, shunt-fed crystal radio.

Parallel-tuned Crystal Radio Circuit with 
germanium transistor as detector
But the results were disappointing.

However, when the variable capacitor was wrongly connected across the headphones during trials, the germanium transistor worked as good as it did in the series-tuned circuit.

Unusual Parallel-tuned Crystal Radio Circuit
Hence this unusual circuit was adopted for the final version with a 35mm film canister serving as coil former as well as 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' random-wire antenna.
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Horn Speaker for a Crystal Radio

This horn speaker is an improvisation for my crystal radio through a speaker.

Horn Speaker
It requires only 2 additional parts - a dustbin and a food container - with the crystal radio speaker itself serving as the driver.

In order to simplify the mounting, the speaker is made to drive from its rear while its front is sealed by the food container.

Horn Speaker details
Rubber based adhesive is used for assembly.

Audio output is considerably more than with the speaker in its normal enclosure.
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Loudspeaking Crystal Radio

This is the speaker variant of my series-tuned crystal radio with an improved detector.

Loudspeaking Series-tuned Crystal Radio
Replacing balanced armature headphones (impedance around a few hundred ohms) with a low impedance speaker required the use of an output transformer.

A power transformer, with its inherent thick conductors, appeared to be an ideal substitute for a low-loss output transformer.

Trials with transformers rated 230V/24V-2A, 230V/24V-5A and 230V/24V-15A proved the last one to be the best.

Reception of the lone local 200kW 612kHz AM broadcast station, located 20km away, was quite good.

Antenna was a 60' random wire.

Performance was equally good in the parallel-tuned configuration.

Loudspeaking Parallel-tuned Crystal Radio
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Improved Detector for a Crystal Radio

After obtaining a significant improvement in performance by just adding an inductor to my crystal radio, the focus was on a replacement for the OA5 which would give a higher output.

OA5 diode
The base-emitter and base-collector junctions of germanium audio transistors like AC130, OC74, AC127, 2N61, AC188 and AD162 were tried out. However, they were all only as good as the OA5.

Germanium transistors
As luck would have it, when the 2N61 was being tried out, an accidental short between its base and emitter leads resulted in a tremendous increase in signal output. The other transistors too gave identical results with their base and emitter leads shorted.

Crystal Radio - Schematic
Shorting the base and collector leads gave similar, though not consistent, results.

With the OA5 in the crystal radio replaced by the base-emitter-shorted 2N61, the lone local broadcast station on 612 kHz is now loud enough to be heard faintly even at a distance from the headphones!

It is to be noted that germanium transistors, with base/emitter interconnection, outperform germanium diodes provided that the crystal radios are shunt-fed (series-tuned) and not series-fed (parallel-tuned).
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