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Re: [loopantennas] Re: 100Ω -> 50Ω balun?

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  • ceo@andygardner.com
    I m using http://www.mingtek.tw/pdf/B-4.pdf 100Ω balanced fed into pins 14 and 16 50Ω output to R8 from pin 1 pins 4 and 13 grounded Feel free to tell me I
    Message 1 of 10 , Mar 7, 2013
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      I'm using http://www.mingtek.tw/pdf/B-4.pdf

      100Ω balanced fed into pins 14 and 16

      50Ω output to R8 from pin 1

      pins 4 and 13 grounded

      Feel free to tell me I got it all wrong, I would not be at all surprised.


      On 8/03/2013, at 8:34 PM, hejsek2005 wrote:

      >
      >
      > --- In loopantennas@yahoogroups.com, "ceo@..." wrote:
      > >I... decided to use an ethernet isolation transformer...
      > >Ethernet is 100Ω balanced... one side has a centre tap earth.
      >
      > I can see the bal-un bit, but how did you get the 100R:50R?
    • John Popelish
      ... Transformer (and balun) impedance ratios are proportional to the square of the turns ratios. Using only half of a winding reduces the impedance by a
      Message 2 of 10 , Mar 8, 2013
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        On 03/08/2013 02:45 AM, ceo@... wrote:
        >
        > I'm using http://www.mingtek.tw/pdf/B-4.pdf
        >
        > 100Ω balanced fed into pins 14 and 16
        >
        > 50Ω output to R8 from pin 1
        >
        > pins 4 and 13 grounded
        >
        > Feel free to tell me I got it all wrong, I would not be
        > at all surprised.

        Transformer (and balun) impedance ratios are proportional to
        the square of the turns ratios. Using only half of a
        winding reduces the impedance by a factor of 1/4.

        In concept, since a 1:2 impedance ratio requires a turns
        ratio of 1:sqrt(2), that impedance ratio can be only
        approximated.

        A turns ratio of 3:2 gives an impedance ratio of 9:4 or
        about 100:44.44.

        A turns ratio of 4:3 gives an impedance ratio of 16:9 or
        about 100:56.25.

        A turns ratio of 5:3 gives an impedance ratio of 25:9 or
        about 100:36.

        A turns ratio of 5:4 gives an impedance ratio of 25:16 or
        about 100:64.

        etc.

        --
        Regards,

        John Popelish
      • Phil
        That coupled with the fact that the receiver probably doesn t have an exact 50 Ohm input Z, and of course the loop antenna Z is very likely to change over a
        Message 3 of 10 , Mar 8, 2013
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          That coupled with the fact that the receiver probably doesn't have an
          exact 50 Ohm input Z, and of course the loop antenna Z is very likely to
          change over a fairly wide range anyway. So I sure wouldn't worry about
          some mismatch.

          I have a similar setup here where I have an 88 Foot long dipole fed with
          ladder line (the best I can do here). It feeds an oversized 2KW 4:1
          balun (oversized to minimize heating losses from changing the balun
          characteristics) before hitting the transmatch (MFJ949E). The balun is
          primarily there for balanced feed as I realize the impedance varies a
          great deal over the 3.5-29MHz range I use it. But it DOES perform very
          well on all HF bands (and I've even used it on 6M too). I've tried MANY
          different HF antennas in this Mobile home park (NO real estate space to
          speak of) and this one outperforms everything else I've tried.

          Yes, some verticals were a bit better on DX (especially a Butternut HF2V
          I tried on 80/40), but their noise was WAY too high here.

          So, I'd suggest just trying whatever matching ratio you have and see
          what works best for you overall.

          73 de Phil, KO6BB
          http://www.qsl.net/ko6bb/ (Web Page)

          RADIOS:
          Grundigs: S-350 (2006)& G6 (2011).
          Icom: R-75 with Cascaded 250Hz CW Filters.
          Kenwood: TS130S HF Transceiver (circa 1980).
          Radio Shack: DX-380 digital portable (circa 1990).
          Zenith: Royal-7000 Transoceanic (circa 1969).

          Audio Interface: Two Mark One Earholes.
          Decoder Software: Gray-matter between two ears.

          ACCESSORIES: Homebrew LF-MF Pre-Amp, MFJ-949E HF Tuner
          Homebrew 6 Hz Audio Filter.

          ANTENNAS: 88' Long Ladder-line fed dipole at 35 feet AGL.
          Ratzlaff 3 Foot Active Whip at 36 Feet AGL for LF/MW
          4 Foot/side Tuned Rotatable Loop at 15 Feet AGL for LF

          Merced, Central California, 37, 18, 37N 120, 30, 6W CM97rh

          On 3/8/2013 5:02 PM, John Popelish wrote:
          > On 03/08/2013 02:45 AM, ceo@... wrote:
          >> I'm using http://www.mingtek.tw/pdf/B-4.pdf
          >>
          >> 100Ω balanced fed into pins 14 and 16
          >>
          >> 50Ω output to R8 from pin 1
          >>
          >> pins 4 and 13 grounded
          >>
          >> Feel free to tell me I got it all wrong, I would not be
          >> at all surprised.
          > Transformer (and balun) impedance ratios are proportional to
          > the square of the turns ratios. Using only half of a
          > winding reduces the impedance by a factor of 1/4.
          >
          > In concept, since a 1:2 impedance ratio requires a turns
          > ratio of 1:sqrt(2), that impedance ratio can be only
          > approximated.
          >
          > A turns ratio of 3:2 gives an impedance ratio of 9:4 or
          > about 100:44.44.
          >
          > A turns ratio of 4:3 gives an impedance ratio of 16:9 or
          > about 100:56.25.
          >
          > A turns ratio of 5:3 gives an impedance ratio of 25:9 or
          > about 100:36.
          >
          > A turns ratio of 5:4 gives an impedance ratio of 25:16 or
          > about 100:64.
          >
          > etc.
          >
        • hejsek2005
          ... After installing a 11 MB Chinese font file :-), I find there are _five_ transfomer diagrams there - you don t tell us which you use. ... Your pinouts only
          Message 4 of 10 , Mar 8, 2013
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            --- In loopantennas@yahoogroups.com, John Popelish <jpopelish@...> wrote:
            > On 03/08/2013 02:45 AM, ceo@... wrote:
            > > I'm using http://www.mingtek.tw/pdf/B-4.pdf

            After installing a 11 MB Chinese font file :-),
            I find there are _five_ transfomer diagrams there
            - you don't tell us which you use.

            > > 100Ω balanced fed into pins 14 and 16
            > > 50Ω output to R8 from pin 1
            > > pins 4 and 13 grounded

            Your pinouts only make any sense for two of them:
            HN1664CG & HN1674CG. Let's hope it's the former.

            > Transformer (and balun) impedance ratios are proportional to
            > the square of the turns ratios. Using only half of a
            > winding reduces the impedance by a factor of 1/4.

            John has explained it well. A centre-tapped (usually
            bi-filar) balun transformer is the way to connect an
            FM dipole made of 300R twin feeder to the 75R coax
            antenna input on the typical home hi-fi.

            4:1 impedance ratio: half the voltage at 2x the current,
            or 100R:25R. Also, if you are unlucky enough to have a
            HN1674CG, you have the inductance of a half-connected
            and un-specified current balun to add into the mix.

            If you had access to all the transformer terminals
            you could achieve a 3:2 or 4:3 turns ratio for one
            of John's 'near miss' matches, but I imagine these
            commodity transformers are inscrutably potted in
            resin for their Ethernet isolation task.

            However, does it matter for a receiver? There are
            no damaging consequences of mismatch as in a Tx,
            the coupling loop probably isn't 100R either, and
            the only result might be transmission line
            transformer effects when the cable gets to be
            near an odd number of quarter wavelengths.

            Better an antenna that works (mostly) than none.

            Regards. LenW
          • John Popelish
            ... I think you should compare the result you get with one of these Ethernet baluns with a home made unit made of 5 strands of something like 30 ga wire wrap
            Message 5 of 10 , Mar 8, 2013
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              On 03/08/2013 04:30 PM, hejsek2005 wrote:

              > John has explained it well. A centre-tapped (usually
              > bi-filar) balun transformer is the way to connect an
              > FM dipole made of 300R twin feeder to the 75R coax
              > antenna input on the typical home hi-fi.
              >
              > 4:1 impedance ratio: half the voltage at 2x the current,
              > or 100R:25R. Also, if you are unlucky enough to have a
              > HN1674CG, you have the inductance of a half-connected
              > and un-specified current balun to add into the mix.
              >
              > If you had access to all the transformer terminals
              > you could achieve a 3:2 or 4:3 turns ratio for one
              > of John's 'near miss' matches, but I imagine these
              > commodity transformers are inscrutably potted in
              > resin for their Ethernet isolation task.
              >
              > However, does it matter for a receiver? There are
              > no damaging consequences of mismatch as in a Tx,
              > the coupling loop probably isn't 100R either, and
              > the only result might be transmission line
              > transformer effects when the cable gets to be
              > near an odd number of quarter wavelengths.
              >
              > Better an antenna that works (mostly) than none.

              I think you should compare the result you get with one of
              these Ethernet baluns with a home made unit made of 5
              strands of something like 30 ga wire wrap wire, twisted
              together and wrapped a few times through a ferrite core.

              Then you connect all those strands in series. The first 4
              of them form the 100 ohm primary and the last 3 form the
              secondary. Two of those strands are part of both the
              primary and also part of the secondary. The end of strand 2
              (which is the beginning of strand 3) is the ground point.

              This arrangement gives you a 16:9 = 100:56 ohm balun.

              --
              Regards,

              John Popelish
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