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Re: [loopantennas] correct position of coupling loop

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  • roberts50000
    As I understand it, the end of a transmitting loop with the tuning capacitor is the high-impedance end of the loop. The opposite end is the low-impedance or
    Message 1 of 11 , May 2, 2013
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      As I understand it, the end of a transmitting loop with the tuning
      capacitor
      is the high-impedance end of the loop. The opposite end is the
      low-impedance
      or "cold" end of the loop.

      The coupling loop is normally low impedance around 50 ohms so for best
      match
      it should be located at the low impedance end of the loop and exactly
      opposite
      the tuning capacitor for best loop balance.

      It is too bad the best location for the coupling loop is opposite the
      tuning capacitor as it would be more convenient to have the two close
      together especially on a larger loop and for mechanical reasons.

      Most builders place the coupling loop at the bottom end of the loop
      near the ground, so the tuning capacitor should be placed at the highest
      end of the loop, the hardest place to reach mechanically on a large
      loop.

      There is lots of room for experimenting with best coupling to a
      transmitting loop so someone may come up with a better method
      to couple near the tuning capacitor end and solve several problems at once.

      73 Todd WD4NGG


      In a message dated 4/28/2013 3:00:46 A.M. Eastern Daylight Time,
      ik1wvq@... writes:

      Hi to all,
      there is a particular reason to place the coupling loop in the opposite si
      de of the capacitor?
      This is beacuse I am building a mono frequency mag loop for 28 MHz WSPR,
      and the capacitor (9pF FIXED) is a piece of RG8 coax cable without external
      braid slipped INSIDE the copper tube of the loop, and then is pratically
      invisible.
      I would like (for mechanical reasons) to mount the loop with the capacitor
      at BOTTOM side, and put the coupling loop close to the "capacitor".

      any contraindications?

      thanks in advance for your attention..
      best regards de Mauro IK1WVQ
    • qrpbear
      There is another way to couple into a loop antenna which is done at the tuning capacitor. It s known as the army loop method tuner since it was originally
      Message 2 of 11 , May 3, 2013
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        There is another way to couple into a loop antenna which is done at the tuning capacitor. It's known as the "army loop method tuner" since it was originally designed for use by the US Army. You can see it here...

        http://homepage.ntlworld.com/g4kki.william/army_loop_tuner.htm

        Might be worth a try.

        73,

        'Bear' NH7SR

        --- In loopantennas@yahoogroups.com, toddroberts2001@... wrote:
        >
        > As I understand it, the end of a transmitting loop with the tuning
        > capacitor
        > is the high-impedance end of the loop. The opposite end is the
        > low-impedance
        > or "cold" end of the loop.
        >
        > The coupling loop is normally low impedance around 50 ohms so for best
        > match
        > it should be located at the low impedance end of the loop and exactly
        > opposite
        > the tuning capacitor for best loop balance.
        >
        > It is too bad the best location for the coupling loop is opposite the
        > tuning capacitor as it would be more convenient to have the two close
        > together especially on a larger loop and for mechanical reasons.
        >
        > Most builders place the coupling loop at the bottom end of the loop
        > near the ground, so the tuning capacitor should be placed at the highest
        > end of the loop, the hardest place to reach mechanically on a large
        > loop.
        >
        > There is lots of room for experimenting with best coupling to a
        > transmitting loop so someone may come up with a better method
        > to couple near the tuning capacitor end and solve several problems at once.
        >
        > 73 Todd WD4NGG
        >
        >
        > In a message dated 4/28/2013 3:00:46 A.M. Eastern Daylight Time,
        > ik1wvq@... writes:
        >
        > Hi to all,
        > there is a particular reason to place the coupling loop in the opposite si
        > de of the capacitor?
        > This is beacuse I am building a mono frequency mag loop for 28 MHz WSPR,
        > and the capacitor (9pF FIXED) is a piece of RG8 coax cable without external
        > braid slipped INSIDE the copper tube of the loop, and then is pratically
        > invisible.
        > I would like (for mechanical reasons) to mount the loop with the capacitor
        > at BOTTOM side, and put the coupling loop close to the "capacitor".
        >
        > any contraindications?
        >
        > thanks in advance for your attention..
        > best regards de Mauro IK1WVQ
        >
      • qrp.gaijin
        ... This is one of those ideas I keep meaning to try. But then I keep thinking about the massive electrical field around the capacitor, and I just can t bring
        Message 3 of 11 , May 3, 2013
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          --- In loopantennas@yahoogroups.com, "qrpbear" <qrpbear@...> wrote:
          >
          > There is another way to couple into a loop antenna which is done at the tuning capacitor. It's known as the "army loop method tuner" since it was originally designed for use by the US Army. You can see it here...
          >
          > http://homepage.ntlworld.com/g4kki.william/army_loop_tuner.htm
          >
          > Might be worth a try.

          This is one of those ideas I keep meaning to try. But then I keep thinking about the massive electrical field around the capacitor, and I just can't bring myself to place the loop controls, cables, and mounting structure near the capacitor where they risk upsetting the loop balance.

          Here is a posting describing one user's problems with common mode currents when using the MFJ loop tuner, which is essentially the same as the army loop tuner linked above.

          http://www.eham.net/ehamforum/smf/index.php?topic=87076.0

          Note how WB6BYU states "such circuits are inherently unbalanced." So you can probably get the army loop tuner to work, but you may have to deal with common mode currents.
        • JK De Marco
          Thanks for the info about 4NEC2, I was able to run the file saved according to your directions. A very informative page on voltages and current, electric and
          Message 4 of 11 , May 4, 2013
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            Thanks for the info about 4NEC2, I was able to run the file saved according to your directions.

            A very informative page on voltages and current, electric and magnetic fields near a small loop can be read here:

            http://www.w8ji.com/magnetic_receiving_loops.htm

            regards,

            JK


            --- In loopantennas@yahoogroups.com, "qrp.gaijin" <qrp.gaijin@...> wrote:
            >
            >
            >
            > --- In loopantennas@yahoogroups.com, "JK De Marco" <py2wm@> wrote:
            > > > The loop voltage is highest near the capacitor. For example, in the following image I calculated the electric field around the capacitor for a 2 meter by 2 meter loop antenna resonant at 7 MHz:
            > > >
            > > > http://postimg.org/image/n091ogqbl/
            > >
            > > Very nice and informative image! What software did you use to produce it? Thanks.
            >
            > 4nec2 (available free of charge). It's available here: http://www.qsl.net/4nec2/
            >
            > For the near field, it can calculate and display in 3D the near E-field and near H-field. Note that in the above image, the horizontal purple line (very high voltage), extending far outside of the loop boundaries along the axis of the top side of the loop, may be some numeric calculation error, maybe due to the geometry or position of the loop. It doesn't make sense that the E-field would extend so far beyond the loop boundaries. (Maybe I should ask on the 4nec2 forum about this anomaly.) Apart from that, the results seem reasonable.
            >
            > Here is the input file, if you want to experiment with the model. Save the following text to "something.nec" and load it into 4nec2. Note that the field separator, between items on a single line, is the tab character. The loop is resonant at 7.003 MHz, but has not been matched to 50 ohms.
            >
            > CM
            > CE
            > SY C=6.6e-11
            > GW 1 20 -1 0 0.5 -1 0 2.5 .0127
            > GW 2 20 -1 0 2.5 1 0 2.5 .0127
            > GW 3 20 1 0 2.5 1 0 0.5 .0127
            > GW 4 20 1 0 0.5 -1 0 0.5 .0127
            > GE 1
            > LD 0 2 10 10 0.2 0 C
            > LD 5 1 1 20 58000000
            > LD 5 2 1 20 58000000
            > LD 5 3 1 20 58000000
            > LD 5 4 1 20 58000000
            > GN 2 0 0 0 5 0.001
            > EK
            > EX 6 4 10 0 1 0 0
            >
            > FR 0 0 0 0 7.003 0
            > EN
            >
          • ceo@andygardner.com
            Interesting read. Thanks!
            Message 5 of 11 , May 4, 2013
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              Interesting read. Thanks!

              On 5/05/2013, at 2:29 PM, JK De Marco wrote:

              > A very informative page on voltages and current, electric and magnetic fields near a small loop can be read here:
              >
              > http://www.w8ji.com/magnetic_receiving_loops.htm
              >
              >
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