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[?] Measuring antennas with a Vector Impedance Analyser

Discussion in 'Electronic Design' started by David Chapman, Jul 18, 2010.

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  1. I've been trying to optimise a quarter-wave antenna on a 433MHz
    transmitter using a T100 Vector Impedance Analyser.

    Attaching just the antenna directly to the T100 shows that it is
    already a good impedance match to 50 ohms, so I thought I'd check it out
    whilst it is actually attached to the hand-held case into which the
    transmitter is fitted.

    To do this I cut a length of RG174 co-ax cable (terminated in an SMA
    free plug) to be an exact half-wavelength long at 433MHz (remembering
    the velocity factor). I connected the cable to the T100's SMA connector
    and the other end of it to the SMA socket (other feed cable removed)
    mounted on the transmitter case. I then connected the antenna being
    tested to the SMA socket on the case. The transmitter case does offer a
    'ground-plane' of sorts, although it is nothing like a quarter-wave in

    I had thought that the antenna impedance shown at an exact
    half-wavelength along a 50 ohm co-ax cable would have been fairly
    similar to that of the first test (but just at 433MHz), but the second
    measurement bears no sensible relationship to the first, and I'm
    puzzling to understand why..

    No doubt it'll be immediately apparent to the experts in this NG that
    I'm not very familiar with the subtleties of antenna measurements. If
    someone who is knowledgeable about them reads this posting and would
    care to let me know what I'm doing wrong, I'd be most grateful to hear
    their comments and recommendations as to the correct way to make these

    TIA - Dave
  2. Baron

    Baron Guest

    David Chapman Inscribed thus:
    I would expect a resonant quarter wave to exhibit around 35 ohms
    assuming that it had an effective ground plane. Which your T100 is
    not !
    Thats only true if the load is purely resistive at the frequency where
    the cable is a half wavelength.
    As far as the quarter wave is concerned, against an infinite ground
    plane, ie one with a radius => 1/4 wave.
    Realistically is doesn't matter too much if the transmitter can feed the
    required power into it via the matching network without distress.
  3. ehsjr

    ehsjr Guest

    One "gotcha" is the "exact half wavelength long" connecting cable.
    Was it _constructed_ to 1/2 wave, or _measured_ (using your T100)
    as 1/2 wave after construction? *If* what I have heard is true,
    a short piece of coax cut from a large reel can depart quite a
    bit from nominal specs. "Quite a bit" = ?? but I was told "as
    much as 20%". It's hearsay, and I find it hard to believe, but
    the part about measuring the thing is not. Even assuming a
    perfect 66 vf and 50 (+/- 2) ohm z construction error can move it a
    bit from 1/2 wave

  4. Many thanks to those who took the trouble to reply (directly and via
    this NG) to my posting.

    All comments and suggestions have been duly noted - I now know quite a
    bit more about the tricky world of antenna measurements.

    ATB - Dave
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