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Re: [loopantennas] Re: CAT-5 cable?

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  • Jack Smith
    A few years ago, I measured the loss of typical zip cord, using lab grade equipment, taking due care to treat the line as balanced. Results: The second column,
    Message 1 of 28 , May 13, 2013
      A few years ago, I measured the loss of typical zip cord, using lab grade equipment, taking due care to treat the line as balanced. Results:

      The second column, "measured loss" is for a 17.4 ft length and the loss / 100 ft is extrapolated from that measured length. This is with matched loads - i.e., 109 ohms source and termination impedance (measured Zo for this piece of zip cord as 109 ohms). Operating it into a mis-matched load will, of course, increase the loss.

      Frequency MHzMeasured Loss (dB)Loss (dB)/100 ft
      50.42.3
      100.63.5
      150.84.6
      200.955.5
      251.26.9
      301.48.1

      It's not the worst possible transmission line, I suppose, although it has greater loss than RG-174 (3.3 dB / 100 ft @ 10 MHz versus 3.5 dB/100 ft measured for the twin lead), it has the virtue of being available in an emergency from a hardware store. And, not all zip cord is equal. However, cables constructed to carry signals use rather different dielectric material than those constructed to carry 50/60 Hz power and that difference translates into significant loss.

      At 10 MHz, CAT-5 ethernet cable has a loss of about 2 dB/100 ft, despite using significantly smaller diameter wire than for the measured zip cord.

      So, will zip cord work as a  transmission line? Yes. Does it work well? No, at least if one views the term "well" as including the concept of low loss.

      Jack K8ZOA


      On 5/13/2013 3:25 PM, John wrote:
       

      Another commonly available balanced line in the 70-100 Ohm range is standard #18/#20 Zip cord - once known as lamp cord, but today known as speaker wire! Seriously - the vf was ~.66 back then - modern dielectrics may have changed that. Certainly there must be UV-tolerant variants now. Once upon a time, there was a company - maybe B&W - that made SWL receive-only trap dipoles that came either with the balanced Zip cord or a coax connector. For a receive feedline, it'll work well - if the dielectric is weatherproof. A lot of speaker cable is no longer pure copper - it's 'CCA' - Copper Clad Aluminum wire. Probably wouldn't do well flapping in the breeze - for long (Plastic failure of the Al core.).

      I used it, years ago, to feed a homebrew Tilted Terminated Folded Dipole with, including a 2:1 (n ratio - Z ratio: 4:1) Z transformer (400 Ohm to ~100 Ohm) at the feedpoint insulator. It left the TTFD with no DC path to ground, so I went to 450 Ohm ladder line to a homebrew balanced tuner with a CT ground to drain off the static charge. Received signal strengths from common/stable SW stations, like CFRX, CHU, Radio Australia, WRNO, were similar in strength averaged over a few days. The TTFD is a great low noise antenna that pretty much loses the FD/Dipole bi-directional nature and seems more sensitive than it's signal dissipating resistor would imply. A 'loop' of sorts!

      John


    • Brian Burns
      Hello again All, Thanks very much for all the responses on CAT-5! A week ago a guitar making student of mine, who happens to be an EE, dumped a carload of
      Message 2 of 28 , May 13, 2013

        Hello again All,

         

        Thanks very much for all the responses on CAT-5!

         

        A week ago a guitar making student of mine, who happens to be an EE, dumped a carload of “gems and junque” in my shop. Among the gems was a working ‘scope, so I was happy to take on the disposal of the rest. Amongst what I thought of as junque was a 300 foot spool of standard RJ-11 telephone hook-up wire (several lifetime’s supply). It’s the 4 conductor standard stuff.

         

        Until Jerry mentioned that CAT-5 would probably be useable up to 100 watts of RF, the possibility of using the RJ-11 wire as transmission line hadn’t entered my head. There is a powerful incentive for using what you’ve got, if you have taken the vows of poverty, as I have.

         

        I googled around for quite a while, but couldn’t find any data on the characteristic impedance of the stuff, and was wondering if anyone happens to know anything about it. I would probably just parallel pairs of conductors to make a two conductor parallel line out of it.

         

        Cheers,

         

        Brian

         

         

      • Jack Smith
        I assume you mean the 4 conductor indoor (local premises) wiring cable - red, black, yellow and green. My Bell System friends called this JK cable if I
        Message 3 of 28 , May 13, 2013
          I assume you mean the 4 conductor indoor (local premises) wiring cable - red, black, yellow and green. My Bell System friends called this JK cable if I remember correctly, but don't hold me to that.

          It's also known, subsequently, as Cat-1 cable, and the normal recommendation is not to use it above a few hundred KHz.

          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 as in a normal residential installation, the local premises wire will be only a few dozen feet in length and the overall circuit response is dominated by the distribution plant wiring, not the local premises wire. (Lots of equalization is used in the DSL modems to offset the high loss in telephone distribution plant once you up above audio range.)

          The wire I'm thinking of is solid conductor, and probably would rapidly fail from metal fatigue if flexed by the wind.  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.

          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.

          Jack K8ZOA


          On 5/13/2013 6:46 PM, Brian Burns wrote:
           

          Hello again All,

           

          Thanks very much for all the responses on CAT-5!

           

          A week ago a guitar making student of mine, who happens to be an EE, dumped a carload of “gems and junque” in my shop. Among the gems was a working ‘scope, so I was happy to take on the disposal of the rest. Amongst what I thought of as junque was a 300 foot spool of standard RJ-11 telephone hook-up wire (several lifetime’s supply). It’s the 4 conductor standard stuff.

           

          Until Jerry mentioned that CAT-5 would probably be useable up to 100 watts of RF, the possibility of using the RJ-11 wire as transmission line hadn’t entered my head. There is a powerful incentive for using what you’ve got, if you have taken the vows of poverty, as I have.

           

          I googled around for quite a while, but couldn’t find any data on the characteristic impedance of the stuff, and was wondering if anyone happens to know anything about it. I would probably just parallel pairs of conductors to make a two conductor parallel line out of it.

           

          Cheers,

           

          Brian

           

           


        • Brian Burns
          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
          Message 4 of 28 , May 13, 2013

            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




             

            .

            Image removed by sender.

          • 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 5 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




               

              .

              Image
removed by sender.


            • 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 6 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 7 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 8 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 9 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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