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Re: [loopantennas] Re: My Magnetic Loop Fascination

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  • Daniel Reynolds
    ... I agree... The loop is supposed to be a tuned circuit. With the unusually broad bandwidth, I think it has losses somewhere. ... I will try that key down
    Message 1 of 6 , Jul 1, 2004
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      --- Michael Hebert <qrpbear@...> wrote:
      > With the kind of bandwidth you are getting on 40m I would say the
      > loop's Q is seriously degraded. You should only have about 15-20 KHz
      > between 1.5:1 points. KI6GD's loop calculator predicts an efficiency
      > of about 28% for a loop made of 1/4" copper pipe. I would say that
      > your loop's efficiency is well below that figure especially since the
      > BuddiPole beat it out. The BuddiPole is probably only about 10%
      > efficient on 40m.

      I agree... The loop is supposed to be a tuned circuit. With the unusually broad
      bandwidth, I think it has losses somewhere.

      > Try a CW keydown test for a couple of minutes then feel the tubing in
      > the area around the gap. If it feels warm you have some nasty
      > dielectric losses occuring. What kind of insulation does the wire use
      > and what is it's voltage rating? If it's the common 600 Volt wire the
      > insulation is probably PVC which is OK so long as you use the white
      > wire rather than the black or green wire. PTFE insulated wire would
      > be preferable from a loss standpoint. The ends of the wire should be
      > smoothed off and well insulated. Any sharp points there could arc or
      > develop corona discharges.

      I will try that key down test some time. The wire is #10 stranded THHN with a
      white insulating jacket rated for 600V. I wonder if I would have done better to
      use solid instead of stranded. I tried to use electric tape over the ends of
      the wire to reduce short circuit/corona on the ends of the capacitor-wire.

      > I still think your "wire inside" tuning idea is a good one but you
      > definitely should be getting better results. It may be necessary to
      > bridge the gap with a fixed capacitor and use the wire for fine
      > tuning.

      I may try using a piece of coax for a capacitor and see how that performs. I do
      not have access to high voltage capacitors (fixed or variable).

      Since I'm limited on power (Elecraft K2 @ 15W), the loop would be better used
      on 20m where it should be more efficient for its size. I was lucky to be heard
      last night using 15W SSB into the buddipole at its height. I'm normally a CW
      op, but my mini-paddles lost one of the leads (for the 2nd time).

      FWIW - the buddipole received everything about 10dB stronger than the loop. The
      long wire wasn't any stronger than the loop (although I can't tell if it was
      weaker than the loop). There are overhead utility lines that pass over the last
      30 feet of the wire. Most operators were complaining about all of the
      noise/static crashes they were hearing last night, and I was hearing most
      everyone clearly. I don't know whether to attribute this to the loop antenna,
      antenna height, propagation, K2 receiver, or a combination of these.

      73,
      Daniel / AA0NI
    • progman22000
      Hello Daniel and group, ... After seeing those pics in the group photos area, I definitely want to build one. I really like those detailed shots. You mentioned
      Message 2 of 6 , Jul 19, 2005
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        Hello Daniel and group,

        > I was able to get the antenna to tune up on 40m, but had to switch
        > from magnetic coupling to a direct matching system (shield of coax at
        > center point of loop tubing, and moving end of coax along the length
        > of the loop tubing until properly matched). Before I did this, the
        > best I could get on 40m was 2:1. Now I get 1:1.

        After seeing those pics in the group photos area, I definitely want to build one.
        I really like those detailed shots.

        You mentioned some qrm from the computer and ups which got me
        wondering if some sort of rf-choke on the feedline might help - perhaps
        a few coils of coax in the feedline, or maybe some clamp-on chokes on
        the cable just before it reaches the loop. Maybe there's some common-mode
        current finding it's way to the loop.

        At any rate, you got me itching to grab some tubing and give it a try...

        73 de WN6F
        Brian
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