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
    43
    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

    New Member

    Sep 25, 2013
    19
    4
    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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