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3813Re: Tranmitting Loop for 6m & 10m

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  • Len Warner
    Jul 31, 2007
      At 7:40 am ((PDT)) Mon Jul 30, 2007, in Digest 881, Richards wrote:

      >[snip] For my experiment, I tuned weak and moderate signals
      >in the 31 m short wave band (around 9.4 MHz).
      >
      >What seemed to work as well as anything, was a mere 16 seat of wire
      >looped twice around a 2 foot by 2 foot square PVC pipe frame I built for
      >another purpose. Using coax cable feed line, it did not seem to matter
      >whether I connected just one end of the wire loop, or both ends - one
      >end to the coax cable center conductor, and the other to the shield.
      >I don't know why it did not matter how I connected it, but it
      >seemed to work the same either way.

      You do not say what the winding pitch is: my bet is that
      your loop is close wound, in which case the loop's self-
      resonance will be at around 2Mhz. (It depends somewhat
      on wire diameter, which you also didn't care to tell us.)

      You would benefit from RJELOOP3.EXE by Reg Edwards.

      This is designed for square receiving loops and will tell you
      the inductance, approximate self-resonance frequency,
      capacitor for resonance (if used) and other interesting
      parameters.

      By using RJELOOP3 you will discover that you need to
      increase the winding pitch to 2 wire diameters or more
      in order to raise the self-resonance above your frequency
      of interest.

      If you are planning to tune the loop I would suggest a bigger
      spacing, since self-capacitance is worse for Q than a good-
      quality external C and will also limit the tuning range attainable.

      >[snip]
      >I wonder whether tuning capacitor would help adjust the wire to various
      >frequencies above

      Not practical: the loop is already capacitive above its
      parallel resonance.

      >or below its resonant frequency?

      Yes, but first you need to raise its self-resonance above
      your frequency of interest.

      >I wonder whether I could locate the tuning capacitor in
      >the shack, rather than out at the loop in the backyard?

      Yes, if you don't mind adding the resistance and capacitance
      of your feeder to the loop resonant circuit (which you should).

      >I am, based on the discussion here, presuming that my
      >16 foot long wire in a

      How does

      >figure-8 double loop

      relate to

      >(i.e. going twice around my 2 feet per side pvc frame)

      ?

      Or are you trying to tell us at this late stage in the discussion
      that you wound it with figure-8 zip cord?

      >is approximately 1/10 wavelength for the 31 m (9.5 MHz)
      >broadcast band.

      300m/9.5Mhz = 104ft approx.; 16ft = 0.154 wavelength;
      a >50% error over 1/10 wavelength but, since you have
      coiled it, you have brought the low-current ends back to
      overlap the maximum-current centre, so the spacial
      distribution of current will be more uniform than a single
      turn of this wire length would have been and I'm guessing
      that means it will give you good "small loop" performance.

      >I also presume that I would have to select a different
      >length of wire, perhaps, or a different band in the spectrum. Or is
      >this mistaken, would any short loop work? Would any short length work,
      >but with a tuner?

      Any loop of wire below its self-resonant frequency behaves
      just as you should expect a loop of wire to behave: it has an
      area which determines how it interacts with the spatial RF field
      and an inductance and capacitance which determines how
      you can couple it to the rest of your circuit.

      When the length is greater the spatial arrangement and
      possible self-resonant modes take over, and you will need
      either experiment or an antenna-modelling package to
      discover what its impedance and radiation pattern are,
      which is one reason for sticking with published designs
      so you can benefit from someone else's experience.



      Regards, LenW
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