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Re: multi turn loop question

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  • jpopelish
    ... To put some numbers behind the idea that it matters what strands you choose for successive turns, using Cat 5 cable, I ran a few tests of capacitance per
    Message 1 of 10 , Mar 5 8:32 PM
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      John Popelish <jpopelish@...> wrote:

      > If I was using that wire, I would parallel the twisted
      > strands into a single, two strand conductor. That pair has
      > less capacitance to the neighboring pairs than each of those
      > wires has to the other of the pair. That gives you 4 turns
      > per loop of cable.

      To put some numbers behind the idea that it matters what strands you choose for successive turns, using Cat 5 cable, I ran a few tests of capacitance per meter, paralleling two groups of 4 strands, each group (assuming you would make a 2 turn loop out of a piece of cable), using a 3 meter length of cable.

      The results are:

      All striped whites grouped and all solid colors grouped, 188 pF/m

      Green and blue pairs grouped, brown and orange pairs grouped, 107 pF/m

      Green and Brown pairs grouped, blue and orange pairs grouped, 67.3 pF/m

      Green and orange pairs grouped, blue and brown pairs grouped, 67.3 pF/m

      Other cable samples may swap the last three results around (because it depends on the order of colors around the bundle), but I think the contrast between the first result and the other 3 would stand pretty well.

      The result matrix gets more complex when there are 4 groupings (for 4 turns around the loop), and even more complex hen all 8 strands are connected in series for an 8 turn loop. There is also the problem of what to do with the other strands while you measure the capacitance two subsets that do not make up the total. I suspect the most useful way to perform those tests would be to actually make a rigid loop (say, with a hula hoop) and measure the self resonant frequency of each of the different series combinations in effective free space.

      But I am willing to predict that the highest self resonant frequency in the 8 turn case would occur if adjacent turns were made up of the two strands of each twisted pair, so that the highest turn to turn capacitance had the least signal voltage (1/8th of the total loop voltage) across that highest strand ot strand capacitance. The worst case may be connecting all the solid colors in series, then connecting all the striped whites in series, in the same order. But I am not as confident of that result.

      --
      Regards,

      John Popelish
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