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Coupling to the loop?

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  • Jim Miller
    I m interested in building a transmitting loop. As a consequence I m interested in maximizing performance for transmitting as well as receiving. My target is a
    Message 1 of 5 , Dec 9, 2005
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      I'm interested in building a transmitting loop. As a consequence I'm interested in maximizing performance for transmitting as well as receiving. My target is a ~1/4 wave circumference loop constructed of 3/4" or greater soft copper tubing for 20M with some use on 30M as well.

      Many (most) of the loops I've seen on the web have been coupled via a 1/5th diameter loop that's then directly connected to 50ohm coax. I've also seen a variation on that theme which resembles an autotransformer.

      I've looked at a lot(!) of websites but haven't found the mathematical basis for this coupling method. What impedence does it actually yield and since the loop impedence must vary on either side of perfect resonance how is this reflected into the coupling loop? Are there any treatments on the makeup of the coupling coil w.r.t. performance?

      Can anyone point me to a website or reference for study?

      Always enjoy learning something new!

      tnx
      jim ab3cv



      [Non-text portions of this message have been removed]
    • Russell Shaw
      ... It looks like an ideal transformer with some shunt leakage inductance. The transformation ratio is: (turns ratio)*(flux in the larger loop)/(flux in the
      Message 2 of 5 , Dec 9, 2005
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        Jim Miller wrote:
        > I'm interested in building a transmitting loop. As a consequence I'm
        > interested in maximizing performance for transmitting as well as
        > receiving. My target is a ~1/4 wave circumference loop constructed of
        > 3/4" or greater soft copper tubing for 20M with some use on 30M as well.
        >
        > Many (most) of the loops I've seen on the web have been coupled via a
        > 1/5th diameter loop that's then directly connected to 50ohm coax. I've
        > also seen a variation on that theme which resembles an autotransformer.
        >
        > I've looked at a lot(!) of websites but haven't found the mathematical
        > basis for this coupling method. What impedence does it actually yield
        > and since the loop impedence must vary on either side of perfect
        > resonance how is this reflected into the coupling loop? Are there any
        > treatments on the makeup of the coupling coil w.r.t. performance?
        >
        > Can anyone point me to a website or reference for study?
        >
        > Always enjoy learning something new!

        It looks like an ideal transformer with some shunt leakage
        inductance. The transformation ratio is:

        (turns ratio)*(flux in the larger loop)/(flux in the smaller loop).
      • dldorrance
        Hi Jim, I have used both methods of transforming the very low impedance of the big loop to 50 ohms, and I can t say I find any difference; both work well.
        Message 3 of 5 , Dec 9, 2005
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          Hi Jim,

          I have used both methods of transforming the very low impedance of the
          big loop to 50 ohms, and I can't say I find any difference; both work
          well. Both methods yield a 50 ohm non-reactive impedance (SWR=1:1)at
          the transmission line connection to either type transformer as
          measured with an antenna analyzer. In either case, the tuning
          capacitor and the sense loop are exquisitely sensitive to miniscule
          changes, consistent with a high Q LC circuit. Using the 1/5 diameter
          sense loop I generally end up making it too big and adjust the Z to 50
          ohms by deforming the sense loop to something other than a perfect
          circle, unlike most of the photos of perfect circles I have seen on
          the Internet.

          Dave WA6YSO

          --- In loopantennas@yahoogroups.com, "Jim Miller" <jim@j...> wrote:
          >
          > I'm interested in building a transmitting loop. As a consequence I'm
          interested in maximizing performance for transmitting as well as
          receiving. My target is a ~1/4 wave circumference loop constructed of
          3/4" or greater soft copper tubing for 20M with some use on 30M as well.
          >
          > Many (most) of the loops I've seen on the web have been coupled via
          a 1/5th diameter loop that's then directly connected to 50ohm coax.
          I've also seen a variation on that theme which resembles an
          autotransformer.
          >
          > I've looked at a lot(!) of websites but haven't found the
          mathematical basis for this coupling method. What impedence does it
          actually yield and since the loop impedence must vary on either side
          of perfect resonance how is this reflected into the coupling loop? Are
          there any treatments on the makeup of the coupling coil w.r.t.
          performance?
          >
          > Can anyone point me to a website or reference for study?
          >
          > Always enjoy learning something new!
          >
          > tnx
          > jim ab3cv
          >
          >
          >
          > [Non-text portions of this message have been removed]
          >
        • Jim Miller
          dave thanks for the response. are there any implications on the conductor size of the coupling element? do you just use some copper tubing or try to do the
          Message 4 of 5 , Dec 9, 2005
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            dave

            thanks for the response. are there any implications on the conductor size of
            the coupling element? do you just use some copper tubing or try to do the
            shielded thing with a loop of coax?

            i did some modeling with loopcalc for the frequencies i'm interested in and
            to cover the cw band usually only takes 1-2pf change depending on band.
            mechanical stability of the capacitor plates as well as overall loop
            stability would seem to be important as a result especially if the loop is
            mounted outside as i plan.

            73

            jim ab3cv
          • dldorrance
            Jim, the resistance of the sense loop will be 50 ohms. So it does not have to have a large surface area like the main loop, whose resistance due to radiation
            Message 5 of 5 , Dec 9, 2005
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              Jim, the resistance of the sense loop will be 50 ohms. So it does not
              have to have a large surface area like the main loop, whose resistance
              due to radiation is below 0.5 ohms and hence must be capable of
              passing high current. I generally use #8 to #14 bare copper wire. I
              have never run more than 50W, usually about 30W (20 meter psk).
              According to a previous post, the MFJ super loop, rated for (I
              believe) 100W, uses #14 copper wire for its sense loop. What size
              wire would you use for a dipole? The same size will work for the
              sense loop, though it must be sturdy enough to be self supporting. I
              have never attempted to build a shielded sense loop out of coax.

              Regarding the capacitor. From most ideal to least ideal but usable
              (low to higher resistive losses) are vacuum variable, butterfly then
              split stator. For transmitting application you should avoid
              capacitors with a wiper due to resistive losses.

              A commercially built capacitor have sufficient stability. Its
              drive usually includes reduction gears. Being somewhat mechanically
              challenged, I find the remote motor drive setup to be the most
              difficult part of building small transmitting loops. Clearly
              mechanical stability is a strong requirement.

              Try to mount the capacitor close to the ends of the loop with soldered
              connections between the capacitor terminals and the copper loop pipe.

              I agree with your conclusion regarding mechanical stability. These
              small loops are very reminiscent of plumbing.

              Dave WA6YSO

              --- In loopantennas@yahoogroups.com, "Jim Miller" <jim@j...> wrote:
              >
              > dave
              >
              > thanks for the response. are there any implications on the conductor
              size of
              > the coupling element? do you just use some copper tubing or try to
              do the
              > shielded thing with a loop of coax?
              >
              > i did some modeling with loopcalc for the frequencies i'm interested
              in and
              > to cover the cw band usually only takes 1-2pf change depending on band.
              > mechanical stability of the capacitor plates as well as overall loop
              > stability would seem to be important as a result especially if the
              loop is
              > mounted outside as i plan.
              >
              > 73
              >
              > jim ab3cv
              >
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