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.

It may be configured as parallel-tuned or series-tuned.

1. Parallel-tuned version 

Parallel-tuned Loop Antenna Crystal Radio

2. Series-tuned version

Series-tuned 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.

4-legged stool as Loop Antenna frame

Both the versions were tested with the loop oriented end-on 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.

Related post: Another Loop Antenna Crystal Radio
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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.

Crystal Radio 'S' Meter

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 significantly.

A voltmeter, discovered in my junk box, was used to build such a meter for my crystal radio which uses low-impedance balanced-armature phones.

Crystal Radio 'S' Meter - Schematic

On removing its series and shunt resistors, the resistance of the 1 mA 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.

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.

The result - an interesting weekend project and a handy crystal radio test aid.

Related post: Series-tuning for a Crystal Radio

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