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7595Re: Feeding A Magnetic TX Loop

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  • Irv F
    Jun 12, 2010
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      Ah Yes -- now I recall the capacitive tuning/matching
      network using two caps, one a dual section. That's what
      MFJ uses in their loop tuners. I may just
      build one up someday. I run an FT-817ND and always was
      an advocate of low power. It sure simplifies capacitor
      procurement ;-)

      As a result of recent illness I have given up my house
      which was ham heaven and moved into an assisted living
      residence where towers and wires are out of the question,
      but a loop will provide a very satisfactory antenna.
      I've got most of the materials ready and just need spring
      to arrive -- it's late this year in Calgary!

      Irv VE6BP

      --- In loopantennas@yahoogroups.com, Jim Dunstan <jimdunstan@...> wrote:
      >
      > 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
      >
      >
      > [Non-text portions of this message have been removed]
      >
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