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Re: Feeding A Magnetic TX Loop

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  • Irv F
    ... 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
    Message 1 of 12 , Jun 2, 2010
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      --- In 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?
      >
      > This would simplify tuning/matching the loop and
      > simplify construction a bit if the tuning cap and
      > feedpoint could be located close together, especially for a
      > larger size loop. This is compared to the normal method
      > of locating the tuning capacitor at the opposite end of the loop
      > away from the feedpoint. This requires locating the tuning capacitor at
      > the highest point of the loop and complicates the mechanical
      > construction especially for a larger circumference loop like a 70
      > ft circumference loop for 80 meters.
      >
      > I know it will unbalance the loop if the gamma
      > match ground point is next to the tuning capacitor
      > but I don't see any reason why it wouldn't work.
      >
      > Unbalancing the loop should mainly effect the
      > sharpness of the null direction but for transmitting
      > most people want the antenna to be more omnidirectional.
      >
      > I plan to experiment with this feed method over the next
      > few months and will report on how it works for me. My test
      > loop will be a square loop 30-40ft circumference for 40
      > meters only for highest efficiency.
      >
      > 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.

      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.

      I'm in the process of building a loop now. Due to medical reasons
      I had to sell my house and move into a residence where I could get
      nursing assistance. I have a nice patio, and that's where the loop
      is going to sit. I just picked up a Yaesu ND-817 and am looking
      to get back on the air again. I'll propably build two loops in order to cover 80 thru 10M with a reasonably degree of efficiency.

      Irv VE6BP
    • capntripps2000
      That s quite interesting and is what I observed when I tried to feed a loop next to the tuning capacitor; the gamma feed needed to be increased to a size much
      Message 2 of 12 , Jun 2, 2010
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        That's quite interesting and is what I observed when I tried to feed a loop next to the tuning capacitor; the gamma feed needed to be increased to a size much larger than typical and I gave up when I could not get the SWR below 3:1 with a large gamma section on a 1/4 circumference loop.

        Andrew
        N3LCW


        --- In loopantennas@yahoogroups.com, "Irv F" <finkirv@...> wrote:
        >
        >
        >
        >
        > 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.
        >
        > 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.
        >
        > I'm in the process of building a loop now. Due to medical reasons
        > I had to sell my house and move into a residence where I could get
        > nursing assistance. I have a nice patio, and that's where the loop
        > is going to sit. I just picked up a Yaesu ND-817 and am looking
        > to get back on the air again. I'll propably build two loops in order to cover 80 thru 10M with a reasonably degree of efficiency.
        >
        > Irv VE6BP
        >
      • Jim Dunstan
        ... 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
        Message 3 of 12 , 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


          [Non-text portions of this message have been removed]
        • toddroberts2001@aol.com
          In a message dated 6/3/2010 1:31:57 P.M. Eastern Daylight Time, ... Hi Irv, It is not really difficult to couple to the loop at the tuning capacitor. The
          Message 4 of 12 , 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
          • John Hoopes
            I m surprised that no one has mentioned that on electricaly small loops (
            Message 5 of 12 , Jun 4, 2010
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              I'm surprised that no one has mentioned that on electricaly small loops (< .10 lamda) there is no low impedance point. So essentially you can put the matching network where ever it is practicle.
               
              John

              --- On Thu, 6/3/10, toddroberts2001@... <toddroberts2001@...> wrote:


              From: toddroberts2001@... <toddroberts2001@...>
              Subject: Re: [loopantennas] Re: Feeding A Magnetic TX Loop
              To: loopantennas@yahoogroups.com
              Date: Thursday, June 3, 2010, 2:40 PM


               



              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










              [Non-text portions of this message have been removed]
            • bm40g
              ... Hi Todd, I am supprised that nobody has mentioned the british ham that has patented this idea. I first learned of him, and his website on this list
              Message 6 of 12 , Jun 5, 2010
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                --- In 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?
                >

                Hi Todd,

                I am supprised that nobody has mentioned the british ham that has patented this idea. I first learned of him, and his website on this list actually. http://www.g0cwt.co.uk/magloops/new_page_5.htm

                Yes, it does simplify the consturction, as the coupling and tuning can all be put in one box, ala the mentioned MFJ product. Also, check out Ka8zty's pictures.

                I did not go that way with my loop, however. I have lots of nearby conducted interferance soures in the vacinity of my antenna. To help reduce electrostatic coupling from these sources, I wanted the shielded loop feed at the magenetic end of the antenna. A dipole here picks up so much of this noise, that its not usable. The Mag loop is much better, can be moved around and oriented for the best signal and signal quality, unlike the Dipole.
              • Irv F
                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
                Message 7 of 12 , 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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