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Antenna effective length

Discussion in 'Electronic Design' started by przemek klosowski, Nov 5, 2008.

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  1. I helped a friend with a remote flash trigger to extend the range. The device
    operates at 433 MHz, and as-built has a 1.25" PCB stub serving as an antenna.
    Since the wavelength is 27.26", it's really poorly matched and the range petered
    out at under ten meters. We soldered a coiled insulated wire of length 12.4 in
    (lambda/2 minus the PCB trace length) to the end of the built-in antenna,
    and the range increased tremendously, by a factor of ten. The coiled wire fit
    in the original enclosure, so it worked out very well. However, after I thought
    about it I realized that I don't understand why this thing works.

    My question is: what is the effective length of the antenna when it is coiled
    up? Since the E-M field extends outside of the antenna conductor, shouldn't
    the effective length be the actual length of the coil, rather than the length
    of the wire in the coil? Is there a simple, back-of-the-envelope argument
    on how the antenna length/configuration helps to couple the E-M radiation out?
  2. MooseFET

    MooseFET Guest

    An element that is under 1/4 wavelength looks capacitive. If you
    think of your coil of wire as an inductor in series with short
    element, you will begin to get a handle on what happens when an open
    ended coil is used.
  3. Guest

    The short whips look like a tiny capacitor in series with a small
    radiation resistance. That series capacitive reactance is way bigger
    than the bit of resistance that radiates the actual power, so by
    potential divider effect most of the available drive voltage is lost
    across the capacitance leaving only a tiny part to feed the radiation
    resistance, hence little transmitted output power.
    Seeing as not a lot can be done about the radiation resistance item,
    the next best thing is to try and resonate out the series capacitance
    using a series L. This (essentially) leaves only the radiation
    resistance part, to power match to the transmitter. (about 4ohms?)
    In your case ( 433mc), I make your whip at say 0.2pF which has a
    reactance of 1700ohms. Thus you need 1700ohms of series XL to resonate
    out the series XC. This is a series inductor of about 0.6uH, which by
    chance? works out about 10 turns 1/4" using about a foot of wire.
    Looks like you guessed the coil right!.

    I've no interest in aerials but have used a couple of equations at
    the top of ...
    makes for useful reading.
    There's heaps of info' out there but the subject invites a feeding
    frenzy for mathematicians, hence rare to find any back-of-fag-packet
  4. JeffM

    JeffM Guest

    ....even more on
  5. Almost, thanks. Assuming near-direct connection to the transmitter's
    output balun (such as in a handheld, where a short whip is desired)
    there should be no additional difficulty, right?

    But if you need to run co-ax, you'd need a balun at each end?
    Just making sure I have this straight.

    Clifford Heath, VK3CLF.
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