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Baseline loop testing?

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  • Jim Miller
    As I m scheming the implementation of my ultimate loop I thought it would be smart to think about testing so as to know when I ve screwed something up. ;-)
    Message 1 of 1 , Mar 2, 2008
      As I'm scheming the implementation of my ultimate loop I thought it would be
      smart to think about testing so as to know when I've screwed something up.
      ;-)

      Since my loop is nearly 1/4 wave at 20M I thought that before hooking up any
      fancy capacitor I'd try just overlapping the loop ends a bit to achieve
      resonance. With just air for dielectric and no solder joints or mechanical
      complexity this should represent "as good as it gets."

      Since the loop will be made of 2" copper strap with a copper tube stiffening
      rib I thought I'd start by suspending the loop in free air about 8ft off the
      ground and having the suspension line pass through to the bottom to support
      the whole thing to maintain its circular shape. I'll need to make a matching
      loop and that will get suspended in the same way. Obviously this rube
      goldberg mess will need to be tested on a calm day.

      I've got an AA908 to measure the impedence with but I'm not too confident in
      its ability at this frequency. If can get to 2:1 SWR I can use a few feet
      RG-8X to get far enough away and there shouldn't be too much loss to confuse
      the measurements. As long as I can get the loop to resonate somewhere in the
      CW portion of 20M I should be able to evaluate how good it is by measuring
      the Q.

      I can use my test generator to produce a sweepable test signal and measure
      the -3db points. I thought I might use my PIC powermeter to measure the
      signal strength and drive the test generator into an open 50ohm load several
      feet away. Any other ideas?

      Once I'm satisfied this measurement is repeatable it should give me
      something to compare my various capacitor implementations against as I
      attempt to make caps which can allow the loop to tune 20, 30 and 40M. It
      will also be interesting to see how the Q compares to the values estimated
      for this design by the various loop design programs.

      I'll also repeat the measurements with the overlapped-end approach with a
      PVC central support in place and again once it is mounted on something. I'm
      currently planning to mount it on a metal post with a saddle and a clamp
      with a base height of 8ft.

      I'm going to order the copper strap from Wireman tomorrow and get the 1/4"
      copper tube from HD. Still need to design a plywood fixture to seam the two
      together.

      Comments and recommendations welcomed.

      tnx
      jim ab3cv
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