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  1. #1
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    "Crystal set" radio works only when connected to earth

    Hello,
    I made a crystal radio {no batteries, just a ferrite rod antenna, adjustable air dielectric capacitor, and a switching diode (for demodulation) and a crystal microphone.}
    I put the earpiece in my ear and heard nothing, it did not work. Then I connected the ground of the circuit to earth ground ( which I got from a mains plug socket) ....and it then worked well.
    Why was this?
    Why does electromagnetic noise choose to flow away to earth?

    If i had been on an aeroplane, (obviously with no earth conection), then to what would i have had to connect it to get it to work?

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    Re: "Crystal set" radio works only when connected to earth

    Think of the circuit as a 'floating' receiver, it doesn't care where it's signal comes from as long as it's there. What you did was attach an antenna to inject a signal, perhaps not at the optimal spot but there nevertheless. To fully utilize it as an Earth you have to attach both Earth and antenna. In effect your ear was probably the nearest to an antenna it saw, at least it created an RF potential difference between two points in the circuit.

    What 'switching diode' did you use? Ideally you should use a Ge diode or at least a small signal Shottky type.

    As for the aircraft, well , put any receiver in a metal box and it won't pick up anything!

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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  3. #3
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    Re: "Crystal set" radio works only when connected to earth

    "What is the sound of one hand clapping?"



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    Re: "Crystal set" radio works only when connected to earth

    Then I connected the ground of the circuit to earth ground ( which I got from a mains plug socket)
    Consider a flat wire of length lambda/2 (or even lambda/4) stretched in the direction of propagation of the radio signal. You can see that the two ends will be at different potentials and by adding a tuned circuit, we can extract energy from the wave.

    As the wire length gets shorter, the potential difference between the two ends gets reduced. But what happens if you fix one end at a fixed potential? The free end will respond to all the waves floating by... (the current flows through the tuned tank)

    In an aeroplane, you will connect the antenna end (the tip) and the ground to two furthest points outside the metal enclosure.



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    Re: "Crystal set" radio works only when connected to earth

    The confusing point is the claim of using a ferrite rod antenna. If it works, radio operation won't depend on an earth connection.

    I guess, calculation will clarify, that the power received by the ferrite rod is too low to operate a crystal radio, or the circuit is inappropriate in other regards.



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  6. #6
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    Re: "Crystal set" radio works only when connected to earth

    The confusing point is the claim of using a ferrite rod antenna
    My humble opinion is that nothing beats a good old wire stretched OUTDOORS horizontally (but as high as possible) as an antenna for medium and short wave radio.

    I however do not like to use the power point earth connection as the ground; water mains are far better. And even with a ferrite rod, you will need a decent antenna to listen to some music!

    The real problem is to get a decent high impedance headphones. The output for the crystal radio is not good enough to drive a low impedance headphone.



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