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Re: [loopantennas] How about phone cable?

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  • Jack Smith
    First choice depends whether your design requires balanced or unbalanced feed line and the required impedance. If high Z balanced, then 450 ohm ladder line,
    Message 1 of 28 , May 13, 2013
      First choice depends whether your design requires balanced or unbalanced feed line and the required impedance.

      If high Z balanced, then 450 ohm ladder line, window type is reasonably low loss, mechanically decent and not terribly expensive. If you need low Z balanced line, there is a 72 or 75 ohm paired balanced line available, made with insulation designed for RF use. Some of these are available with copperweld conductors - that's fine if you need mechanical strength, but your description suggests a standard copper conductor version will be mechanically satisfactory. (Copperweld is miserable stuff to work with so I try to avoid it unless absolutely necessary.)

      If unbalanced then there is a wealth of coaxial cables to consider. For a 30 ft run, it probably doesn't matter much, but  RG-58 for small diameter coax or RG-213 for larger diameter would be it for me. Look for high quality cable, stay away from aluminum braid types unless you know exactly what you are doing. I would not buy cheap import coax cable unless you have the knowledge and experience to separate good from bad because a lot of it is really trash.

      I've written a bit about coax cable:
      http://www.cliftonlaboratories.com/the_curious_case_of.htm
      http://www.cliftonlaboratories.com/Aug_2011.htm skip the first couple entries.

      Jack K8ZOA

      On 5/13/2013 7:42 PM, Brian Burns wrote:
       

      Hello Jack,

       

      Thanks for the info!

       

      ~ I assume you mean the 4 conductor indoor (local premises) wiring cable - red, black, yellow and green.

       

      Yep, that’s it all right, but it happens to be stranded. Each conductor is made up of 7 strands of .006” diameter copper wire—I had to put on my 10X Optivisor to see the stuff!


      ~ On the other hand, there are umpteen miles of that wire still in use, and some of it is used with DSL which operates in the few MHz range at the upper end, so it will pass RF. The question is how well it will pass the RF

       

      ~ Also don't know if the jacket is UV stabilized of waterproof, as it's strictly an indoor use only cable.

      ~ As for impedance, I can't recall any specs on it, but I suspect from the dimensions it's in the 100 ohm range. Given the normal wire and insulation dimensions, it's difficult to make a cable the size of indoor telephone wire much outside the 100 ohm figure.

       

      The antenna will be located right outside my shack wall, and will be a PVC pipe that goes up vertically to support a couple of cross members that in turn support the loop. The phone cable could run up the interior of the PVC, and exit at the lower center of the loop so that it wouldn’t be exposed to UV. The total length of the run would be about 30 feet.

       

      I have a Harvey Wells antenna tuner that has a balanced output for open wire line, so it can match the transmitter to the phone line nicely. If the 100 ohm impedance of the line matches the 100 ohm impedance of the antenna, then “Bob’s my uncle”, right? I do love the idea of matching the impedances everywhere, even if it really doesn’t matter very much (;->)…


      ~ So, yes it can be made to work--wouldn't be my first choice but if that's the only way to transport RF, give it a try.

       

      I could probably sell the spool of phone cable for enough to buy the “first choice”, but there is something appealing about the idea of using the phone cable. BTW what would be your first choice?

       

      Cheers,

       

      Brian




       

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    • Andy
      The JK (or JKT?) indoor phone wire I have seen, is four wires with a gentle twist of the whole bunch. Taking a cross section, the four wires are in a square.
      Message 2 of 28 , May 15, 2013
        The JK (or JKT?) indoor phone wire I have seen, is four wires with a gentle twist of the whole bunch.  Taking a cross section, the four wires are in a square.  The "pairs", if you will, are opposite corners of the square.  Thus, if the signals on both pairs are differential, crosstalk is minimized because even though each pair is fully immersed within the field of the other pair, it is in such a way that the differential components cancel.

        The use of opposite corners to form a pair, raises the characteristic impedance from what it would be if they had twisted the pairs.

        Since this stuff was intended for voice use only, they might have never characterized its impedance Zo.  At audio frequencies, twisted pair copper wires do not have a constant characteristic impedance.  It typically increases by more than an order of magnitude at audio frequencies, compared to higher frequencies where it finally levels off.

        There is some mythology that telco uses Zo = 600 ohms for all its wire pairs.  I have seen references that argue why this is not true.  In the very very old days telco used open wire lines which might have been in that range, or greater.  Twisted wire lines in cable bundles, which make up the majority of telco lines, were closer to 100 ohms Zo, but did not reach that until well above audio frequencies.

        Andy


      • Rick Karlquist
        ... If you wire the cable as the traditional 4-wire line then it turns out that the characteristic impedance is 70.7% of the impedance of an individual side
        Message 3 of 28 , May 16, 2013
          Andy wrote:

          > The use of opposite corners to form a pair, raises the characteristic
          > impedance from what it would be if they had twisted the pairs.

          If you wire the cable as the traditional "4-wire line" then it turns
          out that the characteristic impedance is 70.7% of the impedance of
          an individual side by side pair. 4 wire line ties together opposite
          wires and then drives the tied pairs against each other. This is
          normally described in the context of open wire line with circular
          disk spacers, but the concept should be extensible to quadrifilar
          twisted line.

          Rick N6RK
        • John Popelish
          ... Have you a reference for this 4 wire transmission line configuration and impedance? I can t find one. All I find is references to star-quad audio cable,
          Message 4 of 28 , May 16, 2013



            On Thu, May 16, 2013 at 1:00 PM, Rick Karlquist <richard@...> wrote:
            Andy wrote:

            If you wire the cable as the traditional "4-wire line" then it turns
            out that the characteristic impedance is 70.7% of the impedance of
            an individual side by side pair.  4 wire line ties together opposite
            wires and then drives the tied pairs against each other.  This is
            normally described in the context of open wire line with circular
            disk spacers, but the concept should be extensible to quadrifilar
            twisted line.

            Rick N6RK 

            Have you a reference for this 4 wire transmission line configuration and impedance?
            I can't find one.  All I find is references to star-quad audio cable, that is intended to be used this way, for low impedance mics.  And it is shielded.

            --
            Regards,

            John Popelish
          • Richard (Rick) Karlquist
            ... See Edmund Laport s Radio Antenna Engineering now free on the web at: http://www.lulu.com/product/ebook/ra...shelf/center/2 Rick N6RK
            Message 5 of 28 , May 16, 2013
              On 5/16/2013 10:33 AM, John Popelish wrote:
              >
              >

              >
              >
              > Have you a reference for this 4 wire transmission line configuration and
              > impedance?
              > I can't find one. All I find is references to star-quad audio cable,
              > that is intended to be used this way, for low impedance mics. And it is
              > shielded.


              See Edmund Laport's "Radio Antenna Engineering" now free on the web at:

              http://www.lulu.com/product/ebook/ra...shelf/center/2

              Rick N6RK
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