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Impedance Question

Discussion in 'General Electronics' started by MbumboTatum, May 26, 2004.

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  1. MbumboTatum

    MbumboTatum Guest

    I don't so much want to know more about the theory of impedance, as its
    practical applications. I would like to learn more.

    Some examples. I take an older cell phone antenna (from 5 years old technology)
    and I want to use it to work on my scanner or my CB receiver. I take a couple
    of dual end alligator clipped wires and connect it up as best I can. I get the
    distinct impression that I am losing signal strength...cause the impedance
    don't match up.

    I'm thinking that electronics hobbyists or technicians know how to best make
    such things work. That perhaps they immediately resort to "jumping" some sort
    of inductor coil or resistor either in parallel or in series, to closely match
    impedance. That they likely know to do this (what to use and where) from years
    of past practice?
  2. Externet

    Externet Guest

    Why do you say impedances do not match up?
    What is the impedance of each? Antenna, alligator wires and connection point ?
    Why do you say that a jumper or a resistor will make impedances match?
  3. Mike Berger

    Mike Berger Guest

    The length of the antenna element is directly related to the
    frequencies it's intended to receive. If you use a cell phone
    antenna designed for 800 MHz, it's not going to work too well
    at CB (27 MHz) or scanner (40-470 MHz) frequencies.

    The physical size of the antenna might not give you a clue as to
    what frequency it's for. The flexible rubber antennas are usually
    wound in a helical coil around a form, then covered with a rubber
    sleeve. So the actual antenna is electrically much longer than its
    physical appearance.

    Antennas are electrically lengthened or shortened to match the exact
    wavelength by adding coils or capacitors (or a capacitive stub).
    A coil makes the antenna look longer, and a capacitor makes it look

    Impedance is based on the design of the antenna itself, and needs to
    be matched by an external matching network for best performance.

    Adding coils or capacitors to the antenna will match it electrically
    to the right frequency, but won't perform as well as an antenna cut
    to the right length. A center or top loading coil works better than
    a base loaded coil, but can be unwieldy.

    The American Radio Relay League's Radio Amateur's Handbook has all
    the information you'll ever want to know about antennas. Your local
    library probably has a copy.
  4. Gary Lecomte

    Gary Lecomte Guest

    There are Formulas for calculating this.
    But first you need to know what the antenna is designed for and at
    what impedance, as well as what the transmitter is, frequency and
    other parameters.

    The cell phone is usually a Much Higher Frequency than the 27 Mhz of
    the CB. Typically a Coil will be need in Series with the antenna. But
    That antenna being so small will never work well anyway, even with a
    proper inductor.

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