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Question about antenna lengh

Discussion in 'General Electronics Discussion' started by samy555, Sep 3, 2012.

  1. samy555

    samy555

    63
    0
    May 11, 2010
    Hi
    to the following circuit, from: http://www.circuitdiagram.org/simple-am-receiver.html

    [​IMG]

    How much should be the length of the antenna if the recieved frequency = 1MHz?
    Why the circuit Ground must be connected to public ground within the home?
    Are there errors or defects in the circuit?
    thanks
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,137
    1,846
    Nov 17, 2011
    The optimum length of an antenna depends on the wavelength. The equation for teh wavelength is:
    l = c/f
    where
    l (lambda) = wavelength
    c = speed of light
    f = frequency

    At 1MHz that's a wavelength of ~ 300m. An antenna with 300 m wire is impractical. Even at 1/4 lambda (a typical antenna design length) you'd need 75 meters. Still impractical.

    Use a loop antenna or loopstick antenna.

    Earthing is required to improve reception.
    I use the following memory hook: The sender emits energy in the form of radio waves. This energy is picked up by the reciever's antenna. If the receiver's antenna were completely isolated the energy potential at the sender would be depleted whereas atz the receiving site an energy potential would build up. By referring sender and receiver to the same potential (ground), this potential difference is eliminated.
    Note that this explanation is not physically 100% correct. It serves me for visualization only.
     
  3. duke37

    duke37

    5,211
    718
    Jan 9, 2011
    The OA91 rectifies the incoming signal but there is nowhere for the current to go so, if it works at all, it will be due to leakage in the diode.

    The transistor can be used as a detector so the OA91 can be eliminated together with some capacitors.

    The antenna will damp the tuned circuit, you could try connecting to some taps on the coil.
     
  4. JMW

    JMW

    88
    2
    Jan 30, 2012
    Samy:
    A long wire is just that, a long wire. This seems to be little more than an amplified crystal set from days gone by. It looks as if you will be receiving your local AM radio stations; as long as practicable is all that is needed, a nearby tree, or out the window will be fine for most work. A good ground is essential as another pointed out. If you go the "tree" route or something similar, disconnect it when not in use. This can be become a lightning magnet.
     
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