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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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 balanced-armature 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 first 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.

Related post: Minimalist UHF Crystal Radio
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