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Re: [loopantennas] Loop Circumference

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  • Andy
    ... Yes, I think that is true. But the way in which it becomes an extension to the loop may be complicated. With normal ladder line, you get
    Message 1 of 11 , Aug 13, 2013
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      > I guess that as the conductors in a ladder line feeder become further
      > apart, there comes a point at which radiation will no longer cancel and
      > it then starts to become an extension to the loop rather than a feeder.

      Yes, I think that is true.  But the way in which it becomes an extension to the loop may be complicated.

      With normal ladder line, you get near-cancellation.  It is not perfect cancellation, but it is so close, that you can pretty much ignore the small amount that does radiate.  As you separate the conductors, the cancellation probably becomes less perfect.  But separating the wires also changes the impedance, and changes the electric and magnetic fields surrounding the wires, so it might be more complicated than just saying that it "radiates more."

      Some powerful radio stations still use open wire feeders to their antennas.  For high power radio stations, the wire separation needs to be moderately large (a foot or more), so that arcing doesn't happen between the wires, on account of the high voltages required for megawatt power levels.  Some stations even use multiple wires arranged in a kind of open coaxial arrangement, with one wire (or a close grouping of wires) as the center conductor, surrounded by a "cage" of a half dozen or more wires acting as the coax shield.  Looking at it, it's amazing to think that it actually works, and it works quite well.

      Andy


    • Hylton Thompson
      Thanks yet again Andy. My horizontal quad loop is used multi-band. It is fed by about 10m of 450 ohm windowline, via a 4:1 balun (at rig) and an ATU but, due
      Message 2 of 11 , Aug 13, 2013
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        Thanks yet again Andy. 
         
        My horizontal quad loop is used multi-band. It is fed by about 10m of 450 ohm windowline, via a 4:1 balun (at rig) and an ATU but, due to limited available space, I cannot get a full 40m loop up in a reasonable shape - 32m is my max and that is all I am usiing right now. It is on 4 fibreglass poles 8.5m above ground at the east end (house) and 6.5m at the west end. It is corner-fed at the east end.
         
        The internal ATU of my IC-7700 copes well from 6 to 20m but "draws the line" at 40m where the antenna is mainly reactive. I use an external ATU for 40m and it can also tune 80m (Palstar AT2K).
         
        I want to add a 12m "tuck" midway down the north leg of the loop above a wooden out building and propose dropping it in a delta shaped loop with the base being supported about a foot above the roof of the out building.
         
        I realise that the "tuck" will perform nothing like a resonant delta loop at any frequency I will use, but it is a convenient shape and I think it will provide useful radiation.
         
        I would welcome any comments before I commence the work.
         
        Hylton Thompson (g6avl)
         
         
        ----- Original message -----
        From: Andy <ai.egrps@...>
        Subject: Re: [loopantennas] Loop Circumference
        Date: Tue, 13 Aug 2013 21:41:05 -0400
         
         

         

        > I guess that as the conductors in a ladder line feeder become further
        > apart, there comes a point at which radiation will no longer cancel and
        > it then starts to become an extension to the loop rather than a feeder.

        Yes, I think that is true.  But the way in which it becomes an extension to the loop may be complicated.
         
        With normal ladder line, you get near-cancellation.  It is not perfect cancellation, but it is so close, that you can pretty much ignore the small amount that does radiate.  As you separate the conductors, the cancellation probably becomes less perfect.  But separating the wires also changes the impedance, and changes the electric and magnetic fields surrounding the wires, so it might be more complicated than just saying that it "radiates more."
         
        Some powerful radio stations still use open wire feeders to their antennas.  For high power radio stations, the wire separation needs to be moderately large (a foot or more), so that arcing doesn't happen between the wires, on account of the high voltages required for megawatt power levels.  Some stations even use multiple wires arranged in a kind of open coaxial arrangement, with one wire (or a close grouping of wires) as the center conductor, surrounded by a "cage" of a half dozen or more wires acting as the coax shield.  Looking at it, it's amazing to think that it actually works, and it works quite well.
         
        Andy
         
         

         

         
      • Andy
        ... As I say, no experience here with that. But according to ON4UN s book, adding inductance at a current null point (1/4 or 3/4 of the way around the loop)
        Message 3 of 11 , Aug 15, 2013
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          > I want to add a 12m "tuck" midway down the north leg of the loop ...

          > I would welcome any comments before I commence the work.

          As I say, no experience here with that.  But according to ON4UN's book, adding inductance at a current null point (1/4 or 3/4 of the way around the loop) doesn't help the SWR.

          I wonder if you can 'meander' the loop wire, to fit more wire in to the available real estate.  They say it works well with dipoles.

          Andy


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