Permeability-tuned Crystal Radio

Discussion in 'Wireless & RF Design' started by vu2nan, Oct 18, 2015.

  1. vu2nan

    Thread Starter Member

    Sep 11, 2014
    43
    10
    This simple, permeability-tuned crystal radio was wired using a variable inductor, a germanium diode/transistor and a pair of sensitive DLR No.5 I.T.B.A.5 S balanced-armature headphones.

    A germanium diode was used as the detector in the series-fed version.

    [​IMG]
    Permeability-tuned Crystal Radio - series-fed version

    For the shunt-fed version it was a germanium transistor (with its base and emitter interconnected).

    [​IMG]
    Permeability-tuned Crystal Radio - shunt-fed version

    An empty glue stick and a ferrite toroid were used for the tuning mechanism.

    The toroid was fixed to the blue glue stick carrier using rubber adhesive. The coil was 60 turns of 30 SWG enamelled copper wire, close-wound on a homebrewed 1" diameter paper former which fit tightly on the glue stick body.

    [​IMG]
    Tuning mechanism for Permeability-tuned Crystal Radio

    Junk-box parts were used to assemble and wire the radio using a scrap wall wart enclosure as the base.

    [​IMG]
    Permeability-tuned Crystal Radio Assembly

    Reception of the local 612 kHz, 200 kW AM broadcast station was quite good, with just a 60' long wire antenna. Headphone current, measured using a 1mA FSD 60 Ω meter, was 300 μA.

    During subsequent trials, it was concluded that a shunt-fed, permeability-tuned crystal radio, with a fixed series-capacitor, could be easily tweaked for best performance.

    [​IMG]
    Permeability-tuned Crystal Radio
    with fixed series-capacitor - Schematic

    Hence another unit was built, using an empty lip salve stick container as coil former/tuning mechanism. A temporary coil, consisting of 120 turns of 30 SWG enamelled copper wire was close-wound on the 5/8 " diameter lip salve stick body. A 1" length of ferrite rod was glued on to the lip salve stick carrier. A value of 330 pF was chosen for the tubular ceramic capacitor, as a 365 pF variable would be normally set around that value, to receive a station at 612 kHz. A germanium transistor, with its base and emitter interconnected, was used as the detector. After a number of trials, it was found that best reception of the local station was obtained when the number of turns was reduced to 90, keeping the ferrite rod 90% inside the coil.

    The temporary coil was then replaced with a proper one, having 90 turns of 30 SWG enamelled copper wire, close-wound and taped.

    Assembly/wiring was on a discarded blister pack.

    [​IMG]
    Permeability-tuned Crystal Radio
    with fixed series-capacitor

    Excellent reception of the local station was obtained with the same sensitive balanced-armature phones and 60' long wire antenna. Headphone current indicated by the 1mA FSD 60 Ω meter was 600 μA.

    Regards,

    Nandu.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    Where does permeability come into the picture? From the title, I expected something to vary the permeability of the tuning inductor.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    Thank you. I just learned something.
     
  4. sirch2

    Well-Known Member

    Jan 21, 2013
    1,008
    351
    Great idea, thanks for posting
     
  5. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi,

    I also expected to see something else as i think Dick C. did, such as a DC current being used to vary the actual permeability of the entire core. This is just sliding the core in and out of the coil form, which changes the permeability of the entire construction, but that is not new at all and has been used for as long as transistor radios have been around. So you change the permeability of the construction but you also change the physical construction, which is not just changing the permeability. If you changed the permeability by changing the DC current thought the coil, that would be purely electrical and so would be more interesting i think. You could also try using a separate electromagnet close to the core end to try to change the slope of the BH curve and thus change the operating permeability.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    Yes very clever....a Collins PTO for 50 cents. :)
     
  7. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi again,

    You are very welcome, and i look forward to your next experiment with this too.
    I also like your original idea because of the screw type mechanism that actually moves the core.
     
  8. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    Few are old enough to remember these. The silver tip is connected to the tuning slug. One would slide the rod to which the tip was connected to tune the receiver.

    [​IMG]

    [​IMG]

    These coexisted with pocket transistor radios for a few years then disappeared from the market.
     
  9. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi,

    Wow, i am guessing 1960 ish?
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    another example of the permeability tuned radio was the "Rocket Radio" yo pulled and pushed a rod in and out of the end of the plastic "rocket" a fairly simple crystal radio, still available.
     
  11. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi,

    Isnt that what Dick C. posted in post #12 ?
     
  12. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Didn't EVERYONE have one of these once?
     
  13. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    On this topic, sometime in the late 1970's Stan Horton at Prem Magnetics designed an adjustable linearity coil for CRT displays. A linearity coil is a a saturable inductor with a given inductance vs current slope and was usually adjusted with a permanent magnet to provide magnetic bias to the core. Stan placed a solenoid winding with its field perpendicular to the field from the inductor part of the linearity coil. Varying the current in the bias winding was analogous to mechanicallly adjusting the permanent magnet. This was true adjustment of the permeability of the core, and was ideal for adjustment with a microcontroller. Unfortunately, it was way ahead of its time as microcontrollers were still several years in the future.
     
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