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Will magnet affect a Bluetooth device's RF reception?

Discussion in 'Electronic Design' started by Bluetooth 9.99, May 30, 2007.

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  1. Hello All,

    I came across a problem with my Bluetooth portable device design: the
    antenna reception performance is super crapy! For some reason, I have
    to stick a 10mm x 5mm magnet onto the back of the device, and the
    lithium battery is placed inside the device's housing...
    I tested various antenna parameters, fast TIS, TRP, etc. but still
    could not identify the pitfall that made my RF link so damn poor!
    Now, in my wildest guess: will that piece of magnet deteriotate the
    Bluetooth RF link?
    Please shed your light on it.
    Thanks!

    Bluetooth 9.99
     
  2. mpm

    mpm Guest

    The electromagnetic wave incident on nearby metal will cause electric
    current to flow on the metal. In the near field, these currents will
    likely not be in quadrature. These currents subsequenty re-radiate
    more electromagnetic waves. (aka: "scattering")

    It is possible that the scattering itself is causing other problems
    (channel fades, etc..), or even simple ISI-type interference. More
    likely, it is detuning the Bluetooth transceiver, and its not making
    rated power (that a total guess?!) The receive path would likewise
    suffer, but here you're more concerned (generally) with max voltage,
    not max power. It may not suffer in direct proportion to the transmit
    side of the link. In fact, it could even be better than expected.

    In either case, it may be possible to add RF antenna folds to reduce
    the electromagnetic interference and scattering. This de-tuning
    approach would likely be most effective if you can discover, and
    detune the assembly at the offending resonant / harmonic frequencies.
    (I am presuming you are unable to move either / both the magnet and
    battery.?) And because it's Bluetooth and fairly high frequency, I'll
    also wager you don't have the real estate to try out very many
    solutions...

    I am not aware of any modeling software, but it "must" be out there
    somewhere??
    Otherwise, this might be one of those trial-and-error situations...
    It sounds like you're going to require the services of a really
    accurate spectrum analyzer / calibrated antenna setup, and maybe a
    network analyzer too.

    I would start by looking at the spectral mask and move things around
    to the extent you can. That will at least "confirm" if re-radiation
    is the culprit. (I think?) Hard to tell without much more info,
    etc.. But from what you say, it seems reasonable to focus on making
    the nearby metal transparent to the Bluetooth signal. Good luck. -
    mpm
     
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