Loading ...
Sorry, an error occurred while loading the content.

Re: My Magnetic Loop Fascination

Expand Messages
  • Michael Hebert
    Daniel, 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
    Message 1 of 6 , Jun 30, 2004
    • 0 Attachment
      Daniel,

      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.

      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 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.

      73,

      'Bear' NH7SR

      --- In loopantennas@yahoogroups.com, aa0ni@y... wrote:
      > 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.
      >
      > I get a pretty broadband response on 40m (160kHz 2:1, 110kHz
      1.5:1).
      > I'm concerned that maybe I'm doing something wrong. I don't have
      the
      > sharp response expected from a Hi-Q circuit on 40m. Is it possible,
      > like was mentioned, that the distributed capacitance is making the
      > antenna broader in frequency response? Since it is broader in
      > frequency response - does that hint of great inefficiency? I don't
      > have a field strength meter to run a comparison.
      >
      > Unfortunately, it does not seem to be very efficient on 40m. I was
      > running 10-15W SSB on 40m, and my buddipole (at 9-10 ft) did better
      > than both the 20' loop and my 100' wire (both at 5'). This was with
      a
      > station near Dallas (I'm in OKC) near local sunset. He was running
      > 500W to an inverted V up 1/4 wave (35 feet). The buddipole was just
      > above the noise level. Both the loop and longwire were unreadable.
      >
      > Hoping there's something I can do to remedy this loop (other than
      > putting it back on 20m).
      >
      > - Daniel
    • 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 2 of 6 , Jul 1, 2004
      • 0 Attachment
        --- 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 3 of 6 , Jul 19, 2005
        • 0 Attachment
          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
        Your message has been successfully submitted and would be delivered to recipients shortly.