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coax stub and series section line transformers

Discussion in 'Electronic Design' started by kell, Jun 16, 2007.

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  1. kell

    kell Guest

    Looking at this wifi antenna design:
    http://wireless.gumph.org/articles/homemadeomni.html
    got me thinking of a series of articles Jerry Sevick wrote for CQ
    magazine about twenty years ago showing how to do impedance-matching
    using pieces of coax cut and soldered together, often reversing the
    center conductor and sheath but I don't remember it in any detail. I
    tried googling and found a couple of papers
    http://www.tuc.nrao.edu/~demerson/weller/weller.pdf
    http://www.qsl.net/aa3rl/tlcalc1.html
    but still can't reconstruct the understanding I got from those CQ
    articles twenty years ago.
    Can anybody explain how that wifi antenna works, or have a link with
    some introductory explanation of how stub and series matching works?
     
  2. ehsjr

    ehsjr Guest

    Maybe this will help:
    http://www.highfrequencyelectronics.com/Archives/Feb04/HFE0204_Sevick.pdf

    Ed
     
  3. kell

    kell Guest

    I saw that paper. It addresses wide-band transformers wound on
    ferrite. I'm talking about frequency-dependent transformers that are
    typically built from bits of coax at certain fractions of a
    wavelength, cut and soldered together in certain configurations.
    You're probably familiar with closed and open quarter-wave stubs, for
    example. That sort of thing, but more general; 4:1 and 9:1 impedance
    transformers made from pieces of coax, used at a fixed frequency or a
    narrow band. Sevick's articles went into it in depth. The antenna I
    gave the link to is made the same way. Take a look at it.
     
  4. Tom Bruhns

    Tom Bruhns Guest

    Assuming you mean the first link you posted, that's a typical "coaxial
    collinear." The points where center and outer reverse are feedpoints
    to a string of dipoles; the half-wave (accounting for the propagation
    velocity of the line used to make them) forces the feed voltages to be
    in-phase. The currents that result are nearly in phase. The parallel
    combination of all the feedpoints is generally a fairly low impedance,
    but not 50 ohms, and some matching is required. It's also useful to
    decouple the feedline from the antenna.

    As for impedance matching...you can match between any two dissipative
    impedances using line segments (or between any two non-dissipative
    impedances using a lossless line segment). A Smith chart is (for
    many) a nice way to visualize the matching, and there are computer
    programs that automate the calculations of a Smith chart and free you
    to concentrate on the visualization part. I use WinSmith, but there
    are several others that will do the job, too. I guess I view it
    rather like the old saw, "give a man a fish and you feed him for a
    meal; teach him to fish and you feed him for a lifetime" (or at least
    give him an excuse to waste weekends trying to catch them...). I'd
    rather point you to tutorials on use of Smith charts or the like than
    to tell you how to do some specific matching job. Along the way you
    may come to understand that you can control the bandwidth of the
    matching over quite a range, too...and you can mix line segments with
    lumped components as well.

    Cheers,
    Tom
     
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