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Transmitting on all frequencies?

  1. Jun 19, 2007 #1
    i know you've heard this in a movie or something, usually when a ship is sinking or the last survivor of the human race is looking for other survivors. considering a transmitter is just a driven LRC circuit oscillating at a certain frequency, is this possible? obviously you could have a bunch of different transmitters going at the same time or broadcast, change the frequency of oscillations and retransmit but could you broadcast on several freqs at once?
     
  2. jcsd
  3. Jun 19, 2007 #2
    An ideal pulse such as described by the dirac delta function is composed of infinite frequencies. You can check that by taking its fourier transform.

    But practically it's impossible to achieve an ideal pulse with infinite amplitude, and 0 period, but you can get a bit close.

    Another way is to generate harmonics. By passing a radio signal through a nonlinear device such as a transistor amplifier, the output will be composed of the fundemental frequency and its multiples. Its possible to mess around with the nonlinear device to produce all kinds of subharmonics, and mixing products.
     
  4. Jun 19, 2007 #3

    xez

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    You can broadcast on several frequencies at once,
    there's the Fourier transform relationship between a waveform and the frequencies that make up the waveform.

    So in fact anything that isn't an infinite duration pure
    sine wave occupies a spread of frequencies.

    If you transmit very sharp impulses they'll cover a wide
    range of frequencies, and the old "spark gap" transmitters
    worked in just such a way to generate RF at some useful
    frequency range without very precise control of oscillator
    tuning in the transmitter.

    To really cover a wide range of frequencies there are
    "ultra wideband" modulation techniques. To cover
    several sets of individual frequencies at once you can
    just generate the waveform of each of those transmissions
    and add them together coming out of one broadband
    antenna.
     
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