Some thoughts on UHF Crystal Radios

Discussion in 'Wireless & RF Design' started by vu2nan, Feb 2, 2016.

  1. vu2nan

    Thread Starter Member

    Sep 11, 2014
    44
    10
    Diode capacitance

    Low capacitance diodes are a must for UHF crystal radios.

    It follows that they are to be so wired that the capacitance between their leads is kept to a minimum.

    This does not imply that the leads are to be cut to minimum length. In fact longer leads facilitate antenna-diode impedance matching.

    Antenna-diode impedance matching

    Stub matching is a must for maximizing the signal strength.

    [​IMG]
    Dipole with a single diode

    Long diode leads may be used to form the stub.

    [​IMG]
    Dipole with coil and diode

    A diode length of λ/4 gives a proper match.

    [​IMG]
    Folded dipole

    A circular stub is best for minimum capacitance between the leads.

    [​IMG]
    Dipole with a single diode / circular stub

    Regards,

    Nandu.
     
    Last edited by a moderator: Feb 2, 2016
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    a couple of ferite beads or an rf choke in the headphone leads should stop rf leakage through the headphones, and increase output.
     
  3. ronsoy2

    Member

    Sep 25, 2013
    36
    11
    Use a 1N21 microwave point contact diode. They will efficiently rectify millivolt signals up to past 10ghz. If you can't find any surplus, email me and I will send you a few.
     
  4. mohannatarajan

    New Member

    Nov 24, 2016
    2
    0
    What is the cost for your 1N21diode ?
     
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