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7534Re: [loopantennas] Re: Feeding A Magnetic TX Loop

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  • Jim Dunstan
    Jun 3, 2010
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      At 01:41 AM 6/3/2010 +0000, you wrote:
      >
      >
      >
      >
      >--- In
      ><mailto:loopantennas%40yahoogroups.com>loopantennas@yahoogroups.com,
      >toddroberts2001@... wrote:
      > >
      > > Hi All,
      > > I am wondering if anyone has tried locating the feed loop
      > > or Gamma match next to the tuning capacitor and if
      > > they had success with this arrangement?
      > >>
      > > 73 Todd WD4NGG
      > >
      >
      >The portion of the loop opposite the capacitor is the only real
      >place to get a low impedance feed point which the gamma match
      >requires. You would have difficulty feeding the loop anywhere
      >around the capacitor because of the very high voltages there.

      Hi Irv,

      It is not really difficult to couple to the loop at the tuning
      capacitor. The circuitry is simple ... the problem is as you say ...
      dealing with the high RF voltages. It also relates to the amount of power
      you run. If you run QRP no problem.

      >Believe me, it would greatly simplify the construction of the
      >magloops if the feedpoint and capacitor could be colocated -- thats
      >why you never see it in magloop articles.

      The reason you don't see it done is that it requires non-traditional methods.


      MFJ made several versions of a Mag Loop Tuner that operated in this
      fashion. Simply connect the loop to the 2 stand-off insulators. the tuner
      acted as the base as well as the tuner and it could sit on the table next
      to the rig. I didn't particular like the idea of the mag loop sitting a
      couple of feet away from me ... even though I was only operating 10 watts
      output.

      I extended the distance. The version I built separated the loop from the
      tuning capacitor/coupling unit by about 8 feet using home brew parallel
      feed line.The loop was about 3 ft in diameter and I ran 10 watts output (I
      used a Kenwood TS-50 running at the low power output setting) no
      problem. it worked just as well as the same loop fed in the more
      traditional way at the low impedance side. If I fed the same loop at the
      low impedance side I could easily run 50 watts .... but then I had the
      inconvenience of not being able to tune the loop and adjust coupling from
      the operating position. By the way ... the loop was fastened to the
      balcony railing and the tuner/coupler was just inside the window to the
      apartment. Both loop arrangements had the same efficiency (e.g.
      output). The difference was that feeding the high impedance point
      dramatically lowers the 'destruction' point hi hi.

      It is not a matter of will it work. It works! ... It is just not easily
      done with traditional components. Modern day Amateurs love coax .... and
      coax is incompatible with high impedance feed. If Mag loops were popular
      back in the 1930's I am sure that is how it would have been done (that is
      tuning and feed at the same point).


      Jim, VE3CI


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