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7536Re: [loopantennas] Re: Feeding A Magnetic TX Loop

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  • toddroberts2001@aol.com
    Jun 3, 2010
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      In a message dated 6/3/2010 1:31:57 P.M. Eastern Daylight Time,
      jimdunstan@... writes:
      > > 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.


      Hi All,
      Thanks for the many ideas and suggestions on trying to feed a
      magloop next to the tuning capacitor. I had forgotten about the MFJ
      loop tuners that use a capacitive match right next to the tuning capacitor.

      It looks like the Gamma Match is not really suited to feed a loop with
      at a high-impedance point like right near the tuning cap, but works well at
      the
      low impedance point opposite the loop tuning cap. Plus by its nature one
      side
      of the Gamma Match is grounded. That would upset the balance of the loop
      if placed right next to the loop tuning cap.


      About 15 years ago I did successfully feed a balanced, ungrounded loop
      next to the tuning capacitor once
      with an unusual arrangement. I was using a QRP transmitter with a tuned
      tank coil and inductively coupled the tank coil into a corner of a magloop
      right
      next to the tuning capacitor. I made the tank coil out of about 3 turns of
      wire
      about 1 ft in diameter feeding a loop 10 ft square or 40 ft circumference.
      This
      was in the Medfer band about 1645KHz. The tank coil was resonated with its
      own tuning capacitor. Then I varied the spacing to the loop for best
      loading. So
      I was basically coupling one tuned coil into another larger tuned coil (the
      loop). The
      tuned tank coil of the Mosfet final was not a real high impedance like you
      would find in
      a tube final but I would guess was somewhere in the range of 250-300 ohms.

      This makes me think that perhaps stepping up the 50 ohm output of
      common coax feedline with a 1:2 or 1:3 ratio transformer to start with and
      feeding
      that into either a larger-size single-turn feed loop or perhaps a
      multi-turn
      feedloop would give a better match at the higher-impedance feedpoint next
      to
      the loop tuning capacitor? Then keep experimenting with the step-up turns
      ratio and
      size of the feedloop plus spacing to see if best match can be found?

      I will report back on this if I can get any success with this method over
      the summer.
      73 Todd WD4NGG
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