Series-tuning for a Crystal Radio

Adding series-tuning was an afterthought to improve the performance of my first crystal radio.

Ferri Loopstick & PVC Gang Condenser

The loopstick (90 turns of Litz wire, close-wound on a 2" length of  ¼" ferrite rod) and variable (a 500+500 pF PVC gang condenser) were wired on a bread board and connected in series with the

antenna lead-in.

Series-tuned Crystal Radio - Schematic

With the plates of the condenser nearly fully meshed, the only local broadcast station on 612 kHz was received with a considerable increase in volume.

However, both the loopstick and variable were too big for the crystal radio enclosure.

Trials, with a fixed 1000pf capacitor in series with available fixed inductors, were unsuccessful.

Ultimately, a 200 μH moulded inductor was found to work quite well, with the 60' wire antenna itself providing the capacitance.

Moulded Inductor

It was easy to solder it right at the antenna socket.

Moulded Inductor-tuned Crystal Radio - Schematic

Related post: My first Crystal Radio
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Minimalist Converter

A simple converter was homebrewed in order to listen to a strong 612 kHz local station using the 1.5 - 30 MHz shack receiver.

Simple Converter - Schematic

Since the 1.612 MHz received signal was noisy, a 2.5 mH RF choke was connected across the output - to no avail. Then, on an impulse, it was connected across the OA90 diode. The result was an enormous increase in signal strength.

Tuning up later, with the converter inadvertently switched off, an equally strong signal was found on 1.836 MHz.

A real effective tripler had been stumbled upon, making the oscillator redundant.

Minimalist Converter - Schematic

So out it went, leaving only diode and choke to do the job.

The result - a chance minimalist project!

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