FM Crystal Radio Receivers

The notion of `crystal radio` is strongly associated with huge antennas and radio broadcasting on long and medium bands, in this article, the author describes the experimentally tested detector circuits of VHF receivers designed to listening to a FM stations. The very possibility of receiving VHF FM detector was discovered accidentally. One day I
FM Crystal Radio Receivers - schematic

was walking in the Terletskiy park in Moscow, Novogireevo, I decided to listen to the broadcast - I had a simple crystal set without resonant tank ( this circuit is described in the "Radio", 2001, – 1, Fig. 3 ). The receiver had a telescopic antenna with length of about 1. 4 m. Wonder whether it is possible to receive radio broadcast with this short antenna It was possible to hear, but weakly, simultaneous operation of two stations. But what is surprised me is the volume of receiving was rise and fall periodically almost to zero after every 5. 7 m, and it was different for each radio station! It is known that in the LW and MW bands, where the wavelengths are hundreds of meters, it is impossible. I had to stop at the point of receive with maximum volume of one of the stations and listen attentively. It turned out - this is "Radio Nostalgie", 100. 5 MHz, broadcasting from the near city Balashikha. There were no line of sight between antennas. How does the FM transmission could be received by using the AM detector Further calculations and experiments shows that it is quite possible and is not depends on the receiver. A simple portable FM crystal receiver is made exactly the same way as an indicator of the electric field, but instead of measuring device it is necessary to connect a high-impedance headphones. It makes sense to add an adjustment of coupling between the detector circuit and the resonant tank to adjust the maximum...

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