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

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  • 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 1 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 2 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 3 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 4 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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