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Although crystal radios are designed to detect AM, they also frequently detect FM fairly well which is in the 100 MHz range.
Around 1906, researchers discovered that certain metallic minerals, such as galena, could be used to detect signals. These devices were called 'crystal detectors'.
In the early 1920s Russia, devastated by civil war, young scientist Oleg Losev was experimenting with applying voltage biases to various kinds of crystals, with purpose to refine the reception. The result was astonishing - with a zincyte (zinc oxide) crystal he gained amplification. This was negative resistance phenomenon, decades before the tunnel diode. After the first experiments, he built regenerative and superheterodyne receivers, and even transmitters.
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