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983Re: [loopantennas] Re: Transmitting loop

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  • vze3v8dt@verizon.net
    Aug 2, 2005
      Hello, Richards. Good luck with your studies.

      In general large SWR is a bad thing, however there are things that can
      be done to deal with it, some more efficiently than others. Generally,
      a blanket statement about something excludes some possibilities which do
      work okay. An example with a high SWR is a center fed Zepp that is fed
      with twin lead transmission line, like 300 to 600 or more ohms
      characteristic impedance. At some frequencies there will be a good
      match between the antenna (load) and the transmission line. The next
      problem with that is that most rigs are set up with 50 ohm coaxial
      output (source impedance) so some method to convert from 50 ohms to 300
      - 600 ohms is needed. Sometimes people use baluns to do this, however
      they also have limitations and issues. For example, they introduce an
      extra loss element and at high power that generates heat, and so at high
      enough power levels with enough heat the ferrite or powdered core of a
      balun saturates causing the balun to become nonlinear and not work very
      well (as well as generating harmonics?). Maybe another way instead of
      using a balun is to use an antenna tuner (although most of them that
      have balanced line terminals also have baluns). I like my Johnson KW
      Matchbox tuner which doesn't have a balun and is quite efficient (at
      least on the "standard" (pre-WARC79) ham bands from 80 to 10m. Now,
      take for example an antenna impedance that is not the same as the
      characteristic impedance of the transmission line, then there is an
      SWR. The main problem with that is loss of signal in the transmission
      line as the reflected wave goes between from the antenna and the tuner
      and back again (and again and again ...). In coax this is a problem
      because the losses are high however using balanced feeders (twin lead)
      the losses are surprisingly small. So, it becomes an engineering
      problem, trying to minimize losses with tradeoffs of different
      architectures for a given design parameters. In general resonant
      antennas work very well and should be fed with coax or could be fed with
      balanced feeders and an appropriate impedance transformation at some
      point. Non-Resonant antennas should NOT be fed with coax (unless some
      tricks are done, I suppose), but would be better fed with balanced
      (twin-lead) feeders and use of a good "transmatch". Oh, by the way, the
      term "antenna tuner" is generally a false nomenclature unless it is
      actually employed at the end of the feedline at the antenna. However,
      most "antenna tuners" people use in their hamshacks actually help the
      transmitter match the load at that point. The only place there is a low
      SWR then is between the radio and the tuner, a matter of a few feet, but
      there could still be high SWR on the long run from the tuner to the
      antenna.

      You can learn a great deal more about this if you pick up a book called
      "Reflections" by Walt Maxwell, W2DU. He used to be the department head
      of the antenna group at a company that was at one time called RCA where
      communications and research satellites were developed and built. He
      retired before I was employed there in 1990, but it was evident that he
      was still greatly respected. The original version of the book was
      published by ARRL but later they ignored the science and tended to more
      folklore so they dropped his publication. It is again published in
      softcover as "Reflections II" and I think is in the 2nd or 3rd printing
      the last I knew and published now by WorldRadio Books.

      73,

      Mark, NK8Q


      Richards wrote:

      >I am a novice just studying for the first exams.... I gather you
      >guys are in agreement on one thing...
      >
      >Whether or not you fry your output transistor, having that big
      >standing wave really sucks... right?
      >
      >///////////// Richards /////////////////
      >===========================================
      >
      >dldorrance wrote:
      >
      >
      >
      >>Hank, thanks for the distinction. However, even if the wave is
      >>re-reflected (back and forth around the speed of light about
      >>
      >>
      >>
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      >
      >If you've got links, post them in the Links section!
      >http://groups.yahoo.com/group/loopantennas/links
      >
      >For uploading images, I prefer the Files section since Photos only allows everyone (except the uploader and moderators) to see a max of 300x400.
      >http://groups.yahoo.com/group/loopantennas/files
      >
      >Put them in the appropriate folder, or create one.
      >Yahoo! Groups Links
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