Series-tuned Ferrite Loopstick Crystal Radio

This series-tuned crystal radio is housed in a plastic pencil box.

Series-tuned Ferrite Loopstick 
Crystal Radio
It has 2 windings of 30 SWG enamelled copper wire on a 6" long, 3/8 " diameter ferrite rod. The primary has 10 turns and the secondary 90, both close-wound.

Series-tuned Ferrite Loopstick
Crystal Radio - schematic
The primary series-tuned circuit, which includes the 200μH moulded inductor, the 3600 pf capacitor and the capacitance of the 60' wire antenna, is resonant at 612 kHz (the frequency of the local 200 kW station, located 20km away). The secondary is also series-tuned using a 500pf PVC variable capacitor. The detector, a germanium transistor with its base and emitter connected, shunt-feeds sensitive balanced-armature phones. Reception of the local station is quite good, with a headphone current of 625 μA (measured using a 1mA FSD 60 Ω meter).

Series-tuned Ferrite Loopstick
Crystal Radio - inside view
Reception is as good without an external antenna and earth, when the radio is held close to a CATV cable running over the shack and working as a passive radiator. Signal strength is maximum when the ferrite rod is laterally oriented with respect to it (with headphone current measuring around 350 μA).

This radio also works quite well, even up to 5 km away from the transmitter, unaided by external antennas, earth and passive radiators.

Related post: Series-tuned Air-core Coil Crystal Radio
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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.

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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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Crystal Radio Output Transformer

The large size of the power transformer, used as an output transformer for my 'Loudspeaker 
Crystal Radio', was good enough reason to attempt homebrewing a relatively small one.

A search through the junk box yielded a 47mm diameter, Siemens N22 pot core set.

Pot Core used for Crystal Radio Output Transformer
Winding was for an autotransformer, having 500 turns of 30 SWG enamelled copper wire (the maximum number that could be accommodated), with output taps at 50 and 100 turns.


Crystal Radio Output Transformer - winding scheme
The 50-turn tap was to match the existing 3 Ω speaker and the100-turn one a provision for speakers of higher impedance.

A small PCB, also from the junk box, provided termination points for the winding.

Crystal Radio Output Transformer- inside view
A couple of reworked white plastic cosmetic jar bodies lent themselves as enclosure halves.

Crystal Radio Output Transformer
Tests indicated that, inspite of its small size, the output transformer's performance is comparable, though not equal, to that of the power transformer.

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Colourful Parallel-tuned Air-core Coil Crystal Radio

This is the colourful version of my 'Parallel-tuned Air Core Coil Crystal Radio'.

Colourful Crystal Radio
The coil is 75turns of hook-up wire, close-wound on a 2 ⅝" diameter plastic bottle, with a tap at 35 turns.

The 35 turn primary series-tunes the capacitive 60' wire antenna to resonance at 612kHz (frequency of the lone 200kW local AM broadcast station). The secondary is parallel-tuned using a 500+500 pF PVC gang condenser.

Colourful Crystal Radio - Schematic
The plastic bottle serves as the coil former cum enclosure. A screw-type terminal strip is used to connect antenna, earth and headphones.

Reception, using sensitive balanced-armature headphones, is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 400 μA.

Related post: Parallel-tuned Ferrite Toroid Crystal Radio
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Parallel-tuned Ferrite Toroid Crystal Radio

This is the toroid coil variant of my 'Ferrite Loopstick Crystal Radio'.

Parallel-tuned Ferrite Toroid Crystal Radio
A ferrite toroid, measuring OD 16.5mm, ID 10.5mm and H 10.5mm, is used. The coil is 25 turns of hook-up wire with a tap at 5 turns. On the primary side, a 200μH moulded inductor series-tunes the capacitive 60' long 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.

Parallel-tuned Ferrite Toroid Crystal Radio - Schematic
A small plastic trinket box serves as the enclosure. A screw-type terminal strip is used to connect antenna, earth and headphones.

Rear of Crystal Radio with Ferrite Toroid Coil
Reception, using sensitive balanced-armature headphones, is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 275 μA.

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

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

Parallel-tuned Air-core Coil Crystal Radio
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.

The primary winding, along with the 200μH moulded inductor and 1000 pf capacitor, series-tunes the capacitive 60' 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.

Parallel-tuned Air-cor Coil Crystal Radio Schematic
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. Headphone current, measured using a 1mA FSD 60 Ω meter, is 350 μA.

Related post: Ferrite Loopstick Crystal Radio
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