Harmonic Interference from a Crystal Radio

One day, in spite of my Minimalist Converter being disconnected, harmonics of the local 612 kHz broadcast station were still being received in the shack receiver.

That was indeed surprising. Then, after some thought, realisation dawned that the Crystal Radio on the shack table could be the cause.

This was confirmed by disappearance of signals when the the ground connection to the crystal radio was removed.

Crystal Radio - Schematic

Then, on a feeling that the minimalist converter would display identical behaviour, its output was next connected to ground.

Minimalist Converter - Output grounded

The signals returned.

It was my first-hand experience of harmonic interference!
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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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