Loop Antenna Crystal Radio

A loop antenna crystal radio is of interest on account of its selectivity and portability. But, with respect to signal strength, it is no match for a crystal radio using an external vertical wire antenna and earth.

If signal strength could be considerably improved with increased loop size, then its portability would be compromised.

As usual, 2 configurations are possible - parallel-tuned-series-fed and series-tuned-shunt-fed.

1. Parallel-tuned-series-fed version 

Parallel-tuned-series-fed Loop Antenna Crystal Radio
2. Series-tuned-shunt-fed version

Series-tuned-shunt-fed Loop Antenna Crystal Radio
The loop antenna was rigged up using a 4-legged stool as a makeshift frame. The stool was laid on its side on the table, making it convenient to wind the loop on its legs.

The 4-legged stool used as a makeshift Loop Antenna frame
Both the versions were tested with the plane of the loop oriented towards the local 612 kHz, 200 kW AM broadcast station.

Signal strength was the same with both, though a far cry from another crystal radio using a vertical wire antenna and earth.
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Current-operated 'S' Meter for a Crystal Radio

An 'S' meter, for a crystal radio with high-impedance phones, is generally a high-resistance microammeter connected across the phones. In other words, a sensitive voltmeter that does not bypass headphone current significantly.

On the other hand, for a crystal radio with low-impedance phones, it's a low resistance microammeter connected in series with the phones. That is, a sensitive ammeter that does not drop headphone current signnificantly. A voltmeter, discovered in my junkbox, was used to build such a meter for my crystal radio which uses low-impedance balanced-armature phones.

On removing its series and shunt resistors, the resistance of the 1mA FSD movement turned out to be only 60 Ω and hence quite suitable for the purpose. The existing graduations on the meter face were removed and replaced with graduations 0 -10 using a lead pencil.

Crystal Radio S-Meter - Schematic
The meter was already mounted on a square bakelite flange. The flange mounting holes served as terminal mounting holes, obviating the need for an enclosure.

Crystal Radio S-Meter
The result - an interesting weekend project and a handy crystal radio test aid.
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UHF Crystal Radio

A crystal radio was wired up to check for reception of VHF/UHF signals in the vicinity of the shack.

VHF / UHF Crystal Radio - Schematic
Using a gold bonded Germanium diode 1N65, a 500 mm whip antenna and sound-powered phones, no signals were audible either in the shack or on the shack roof.

However, using a 250 mm whip, a weak digital signal was detected while moving on the shack roof. Position on the roof and direction in which the whip was pointed were quite critical.

It was presumed that the signals were from a nearby cellphone tower.

A 3 Element Yagi Beam for 33 cm was made using a broken snare drum stick for the boom and 20 SWG bare copper wire for the elements.

UHF Yagi Beam Crystal Radio  - Schematic
The coil was wound integral with the driven element. 

UHF Yagi Beam Crystal Radio
With this setup a higher signal strength was obtained.

A second Yagi Beam was then rigged up, with another diode providing the return path for the audio, instead of the coil.
UHF Yagi Beam Crystal Radio
with additional diode - Schematic
The signal strength was not as good. During trials, a chance shorting of  the second diode gave a considerable increase in signal strength.

Single-diode UHF Yagi Beam
Crystal Radio - Schematic
The diode and capacitor were hence discarded. Performance of this single-diode UHF crystal radio is quite good.

Single-diode UHF Yagi Beam Crystal Radio 
A folded dipole version, mounted on a terminal block, followed.

UHF Folded Dipole Crystal Radio - Schematic
It obviated the need for a coil / additional diode and worked quite well too.

UHF Folded Dipole Crystal Radio
The terminal block arrangement made it convenient to replace the folded dipole with a full wave loop.
UHF Full Wave Loop Crystal Radio
Likewise with an AWX antenna.

UHF AWX Antenna Crystal Radio
Results were as good as with the folded dipole.
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Series-tuned, Shunt-fed Crystal Radio with Ferrite-Core Coil

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

Series-tuned, Shunt-fed 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, Shunt-fed Crystal Radio - Schematic
The primary series-tuned circuit, which includes the 200μH moulded inductor and the capacitance of the 60' long 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.

Series-tuned, Shunt-fed Crystal Radio - Inside view
Reception of the local station is quite good with headphone current being 400 μA (measured using a 1mA FSD 60 Ω meter).
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Different Parallel-tuned 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.

Another Crystal Radio Variant
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.

Another Crystal Radio Variant - Schematic
The OA79 diode feeds a pair of sensitive balanced-armature phones.

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 Loudspeaking Crystal Radio.

An air-core crossover network inductor and a ferrite-core one (from the junkbox) 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 Loudspeaking 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 accomodated), 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, for its size, the output transformer's performance is comparable, though not equal, to that of the power transformer.
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