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433Mhz rf sheilding

Discussion in 'Electronic Design' started by [email protected], May 12, 2005.

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  1. Guest

    Hi to all.
    I am using an old microwave oven to sheild a 433.92Mhz receiver , for
    testing purposes , and have a few questions.

    If I put a transmitter(433Mhz) in the microwave and close the door
    the signal is quite well attenuated , 30-40Db or so on the spectrum
    analyser.The analyser just has a piece of wire as an antenna , and when
    you open and close the door you can see the obvious attenustion of the
    signal.This is more of a reletave measurement than absolute.
    Surely the microwave should block all of the signal , seing that the
    wavelength is much longer than a 2.54Ghz signal , which I hope it
    blocks totally!!
    These transmitters are only about 10mW or so , compared the the
    microwave signal in the 100's of watts range!!

    There are no wires comming out of the microwave to "carry" the signal
    out.

    Any ideas / suggestions would be appreciated.
    Cheers
    Rob
     
  2. OBones

    OBones Guest

    Shielding of microwaves is far from perfect.
     
  3. On 12 May 2005 02:20:53 -0700, in sci.electronics.design
    the shielding only has to shield 2.54G for safety. what it does at
    other frequencies is more or less irrelevant


    martin

    After the first death, there is no other.
    (Dylan Thomas)
     
  4. colin

    colin Guest

    microwave ovens rely heavily on a feromagnetic strip around the door seal
    (usualy a half inch brown strip of plastic), this is very efective at
    microwave frequencies as it has a very high absorption, however i dont think
    it is likly to be very efective at much lower frequencies i think it may
    also be tuned to the frequency used although im not sure, i know if you
    remove it or if it becomes damaged (dirt such as fat acumaleted on it can
    become hot and melt it) a heck of a lot of microwave energy can escape, the
    gap arond the door is plenty long enough to let 433 mhz out, maybe if you
    want to improve it you could add the metalic springy contact sealing strip
    used on case joints to seal agaisnt emi to the oven door, although its not
    worth risking tampering with a microwave oven if theres any chance its
    liklely to be used again. i used to work on some of the first electronic
    controlers for microwave ovens. the radiation outside the ovens we were
    using at moderate distance was actualy remarkably low, we did have some
    problems inside the case with pcb tracks on some models wich just hapened to
    be just the right length to act as tuned anteneas tho.

    I would sugest an ordinary metal instrument case would be better.

    Colin =^.^=
     
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