Antenna Length importance?

Discussion in 'Wireless & RF Design' started by Shelton, Mar 13, 2008.

  1. Shelton

    Thread Starter Member

    Mar 13, 2008
    19
    0
    Hi All

    I have built the circuit on this website


    http://braincambre500.freeservers.com/rss(1)(1)(1).htm


    and the circuit diagram is here:


    http://braincambre500.freeservers.com/rss schematic.gif


    My question is this:


    When I increase the antenna length to say 30cms, from the recommeded
    6inchs or 15cms, the circuit does not work. Why is this? I know the
    antenna captures RF signals and this current is fed into the circuit,
    but I dont know why the circuit stops working - is it something to do
    with overloading the circuit ?


    Thanks for your time
    Shelton.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
  3. Shelton

    Thread Starter Member

    Mar 13, 2008
    19
    0
    Hi

    Thanks for your answer.

    But If i increase the antenna to half wave length - the circuit should still operate correctly right ?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Since the specified antenna is less than half wave length, we can deduce the designer has added inductance in series. You might experiment with L1 turns and tap.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The radio circuit is a super-regen.
    If the antenna is too efficient then stong signals overload the super-regen.
    If the antenna is not the short length in the design then the FET might oscillate too much.
     
  6. Shelton

    Thread Starter Member

    Mar 13, 2008
    19
    0
    Ok thanks Audioguru and the others - yes i think audioguru is quite right as it is a super regen circuit and the antenna if too long can overload the oscillation.

    Thanks all!
     
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