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near-field magnetic comm technolgy

Discussion in 'Electronic Design' started by Carey Fisher - NCS, Nov 9, 2003.

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    Interesting short range headset, also in EE Times (Nov 3, p79). Anyone have
    an idea where to start in designing something like this? Would you treat
    the "antennas" like (large) air-core transformers? This is the only link
    I've found by Googling - anyone know of others?
  2. I read in that Carey Fisher - NCS>) about 'near-field magnetic comm technolgy', on Sun, 9 Nov
    The above URL seems to give conflicting information.

    "The foneGEAR chord free headset "near field" magnetic communications –
    induction - supports 25 hours of talk time on one AA battery and runs on
    the low frequency 13.5MHz band."

    13.5 MHz isn't 'the low frequency band', and its wavelength is 22 m, so
    the near field radius is about 3.5 m. So the transmission is not purely
    by induction unless the range is really short.

    "The firm told the INQUIRER that you would have to get really close to
    someone using a similar device before you'd experience interference."

    Well, maybe. It depends on whether different systems operate on
    sufficiently different carrier frequencies.

    "The Department of Defense (DOD) and NASA are both interested in the
    technology, which is non-progagating."

    It can't really be non-propagating, AFAICS.

    "The firm said that unlike radio frequency (RF) technologies, the power
    generated is about 100 nanowatts. The range of the headset is about four

    It IS radio-frequency technology. 13.5 MHz isn't audio, even for bats.

    Of course, the URL text may have been subjected to journalistic

    I'll be surprised if 4 feet is enough range to be practically useful. I
    have radio-frequency headphones for assisted hearing of TV and radio
    without disturbing the neighbours, and I find the range of around 10 m
    is about right.

    Furthermore, *baseband audio* induction communication systems have been
    around for many decades.
  3. The corresponding article in EETimes is:

    Seems to me they don't REALLY mean RF vs Magnetic Field since, as you point
    out, 13+ MHz is Radio Frequency. They must mean Electromagnetic vs
    Magnetic. So they are using the magnetic field alone with no electric field
    component. Why do editors of a magazine directed at EE professionals make
    mistakes like this?

    This is still interesting as it has applications for headset use in a RF
    (EM!) rich environment with reduced EMI issues. So I'm still interested in
    any info anyone has about designing stuff like this. I'll try some more
    Googling also.
  4. onestone

    onestone Guest

    This sounds like the Libertylink Chip from Aura. It is magnetic
    Induction for the comms. Range is rated at 1.5mtr in the spec sheet I
    have. There are very definite though obscure applications in military roles.

  5. Good link. Thanks!!!
  6. I read in that Carey Fisher - NCS>) about 'near-field magnetic comm technolgy', on Sun, 9 Nov
    Because they are editors, not electronic engineers. Even editors who
    have some technical knowledge often have big gaps and unusual opinions.
    The sending antenna is typically an air-cored coil of fairly large
    dimensions e.g. 150 mm square. The receiving antenna may well be on a
    ferrite rod. Both antennas, of course, have figure-of-eight directional
    responses. It requires heroic methods (angular diversity reception) to
    overcome this.

    You can do this with baseband audio, as well, and a lot of design
    information is available. But the r.f. solution probably requires much
    less power, especially if you use one of the high-efficiency versions of
    amplitude modulation. OTOH, spectrum management authorities may not let
    you use the 13.5 MHz band (or any other ISM band) for speech
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