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Re: [loopantennas] Loop Circumference

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  • Hylton Thompson
    Thank you Andy .... I greatly appreciate your considered response. What you say certainly makes sense to my ageing brain. Another query that I would like to
    Message 1 of 11 , Jul 20, 2013
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      Thank you Andy .... I greatly appreciate your considered response. What you say certainly makes sense to my ageing brain.
       
      Another query that I would like to raise in here is the effect of balanced feeder on a loop's resonant frequency.
       
      If 20 metres of of well-balanced window line is used as the feeder, 40 metres of conductor has effectively been added into the loop.
       
      To my present understanding, the standing wave pattern around the loop must continue down each leg of the feeder which is therefore an integral part of the loop (but with radiation cancelled out).
       
      If the loop and feeder each contain 40 metres of wire, is the combined resonant frequency based upon 80 metres of wire, even though 50% of it is non-radiating?
       
      Taking this further, how reasonable is it to drop a few metres of window line, with its ends shorted, from some point in the loop and expect the loop to perform as if its length had been increased by double the length of the drop?
       
      The reason I ask these questions is that I am in the process of putting up a small horizontal loop where the maximum circumference is restricted to about 32 metres and I want to achieve resonance at the bottom of the 40m band. Experiments show that my Palstar AT2K + 4:1 external balun can tune 6m thru 80m with a 32m loop so I don't know whether there will be a great advantage in trying to achieve 40m resonance by adding a non-radiating length to the loop.
       
      Again, I will really appreciate any comments / advice.
    • Andy
      ... That s the ideal formula for 1 wavelength in free space. The 1005/MHz feet loop formula is about 2% longer than 1 wavelength. The metric equivalent
      Message 2 of 11 , Aug 12 1:29 PM
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        > What formula for Metric ?
        >
        > 300/MHZ  no ?

        That's the ideal formula for 1 wavelength in free space.

        The "1005/MHz feet" loop formula is about 2% longer than 1 wavelength.  The metric equivalent would be about 306/MHz meters.

        Andy


      • Andy
        ... Hmm. Not really; not added INTO the loop. ... Yeah, I don t think that s true. There really is a boundary between the loop and what feeds it. Remember
        Message 3 of 11 , Aug 12 2:09 PM
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          > If 20 metres of of well-balanced window line is used as the feeder,
          > 40 metres of conductor has effectively been added into the loop.

          Hmm.  Not really; not added INTO the loop.

          > To my present understanding, the standing wave pattern around the loop
          > must continue down each leg of the feeder which is therefore an integral
          > part of the loop (but with radiation cancelled out).

          Yeah, I don't think that's true.  There really is a boundary between the loop and what feeds it.

          Remember that if a transmission line is matched, then its length cannot be measured, or "felt", by "looking into" the end.  It appears identical to an infinitely long line ... or to a resistor equal to the characteristic impedance Zo.  So connecting a 20 meter long, 450 ohm balanced line is identical to connecting a 450 ohm resistor, to the same point.  Also identical to connecting a 10000 meter balanced line.

          The standing wave pattern on the antenna generally does not continue down the feedline.  The presence of standing waves on the feedline depends on the impedance match at the feedline/antenna boundary.  There could be no standing waves on the feedline ... even though they are there on the antenna.

          It is the same as the feeder line connected to a dipole antenna, or a vertical.  Neither one makes the antenna electrically longer.

          > If the loop and feeder each contain 40 metres of wire, is the combined
          > resonant frequency based upon 80 metres of wire, even though 50% of it
          > is non-radiating?

          No, sorry.

          > Taking this further, how reasonable is it to drop a few metres of window
          > line, with its ends shorted, from some point in the loop and expect the
          > loop to perform as if its length had been increased by double the length
          > of the drop?

          Now this is something that actually does do something.  The shorted stub behaves like an inductor (if the stub is less than 1/4 of a wavelength long at the transmitted frequency), and adding an inductor to a loop can make it appear electrically longer.  It's just like a loading coil in a too-short vertical.  You have to be a little careful with a loop antenna, because it does matter where in the loop you insert the stub (e.g., not at the current null points in the loop).

          I am not sure if it behaves longer by twice the length of the stub.  I suspect it goes by some other formula.

          All I can say is, it's a concept I have read about, but don't know very much about.  If I remember correctly, ON4UN talks about it in his book in the chapter on loops.

          And it might confound things for using the loop on higher frequency harmonics.

          > The reason I ask these questions is that I am in the process of putting
          > up a small horizontal loop where the maximum circumference is restricted
          > to about 32 metres and I want to achieve resonance at the bottom of the
          > 40m band. Experiments show that my Palstar AT2K + 4:1 external balun
          > can tune 6m thru 80m with a 32m loop so I don't know whether there will
          > be a great advantage in trying to achieve 40m resonance by adding a
          > non-radiating length to the loop.

          Sometimes it helps to achieve resonance as near to the antenna itself, so that you don't have a poor SWR on the feeder line, adding to loss.

          Andy


        • Hylton Thompson
          Thank you for taking the trouble to explain this so clearly Andy. Your answers certainly make sense. I guess that as the conductors in a ladder line feeder
          Message 4 of 11 , Aug 12 4:04 PM
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            Thank you for taking the trouble to explain this so clearly Andy. Your answers certainly make sense.
             
            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.
             
            Hylton Thompson (g6avl)
             
            ----- Original message -----
            From: Andy <ai.egrps@...>
            Subject: Re: [loopantennas] Loop Circumference
            Date: Mon, 12 Aug 2013 17:09:05 -0400
             
             

             

            > If 20 metres of of well-balanced window line is used as the feeder,
            > 40 metres of conductor has effectively been added into the loop.
             
            Hmm.  Not really; not added INTO the loop.

            > To my present understanding, the standing wave pattern around the loop
            > must continue down each leg of the feeder which is therefore an integral
            > part of the loop (but with radiation cancelled out).
             
            Yeah, I don't think that's true.  There really is a boundary between the loop and what feeds it.
             
            Remember that if a transmission line is matched, then its length cannot be measured, or "felt", by "looking into" the end.  It appears identical to an infinitely long line ... or to a resistor equal to the characteristic impedance Zo.  So connecting a 20 meter long, 450 ohm balanced line is identical to connecting a 450 ohm resistor, to the same point.  Also identical to connecting a 10000 meter balanced line.
             
            The standing wave pattern on the antenna generally does not continue down the feedline.  The presence of standing waves on the feedline depends on the impedance match at the feedline/antenna boundary.  There could be no standing waves on the feedline ... even though they are there on the antenna.
             
            It is the same as the feeder line connected to a dipole antenna, or a vertical.  Neither one makes the antenna electrically longer.
             
            > If the loop and feeder each contain 40 metres of wire, is the combined
            > resonant frequency based upon 80 metres of wire, even though 50% of it
            > is non-radiating?
             
            No, sorry.
             
            > Taking this further, how reasonable is it to drop a few metres of window
            > line, with its ends shorted, from some point in the loop and expect the
            > loop to perform as if its length had been increased by double the length
            > of the drop?
             
            Now this is something that actually does do something.  The shorted stub behaves like an inductor (if the stub is less than 1/4 of a wavelength long at the transmitted frequency), and adding an inductor to a loop can make it appear electrically longer.  It's just like a loading coil in a too-short vertical.  You have to be a little careful with a loop antenna, because it does matter where in the loop you insert the stub (e.g., not at the current null points in the loop).
             
            I am not sure if it behaves longer by twice the length of the stub.  I suspect it goes by some other formula.
             
            All I can say is, it's a concept I have read about, but don't know very much about.  If I remember correctly, ON4UN talks about it in his book in the chapter on loops.
             
            And it might confound things for using the loop on higher frequency harmonics.
             
            > The reason I ask these questions is that I am in the process of putting
            > up a small horizontal loop where the maximum circumference is restricted
            > to about 32 metres and I want to achieve resonance at the bottom of the
            > 40m band. Experiments show that my Palstar AT2K + 4:1 external balun
            > can tune 6m thru 80m with a 32m loop so I don't know whether there will
            > be a great advantage in trying to achieve 40m resonance by adding a
            > non-radiating length to the loop.
             
            Sometimes it helps to achieve resonance as near to the antenna itself, so that you don't have a poor SWR on the feeder line, adding to loss.
             
            Andy
             
             

             

             
          • Maxwell Mclellan
            Greetings all, Just thought re 450ohm line extending the output termination of a loop. If you consider the line as separate from the loop, the line possesses
            Message 5 of 11 , Aug 12 8:22 PM
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              Greetings all, Just thought re  450ohm line extending the output termination of a loop. If you consider the line as separate from the loop, the line possesses both inductance and most of all capacitance the parallel wires acting as the plates of a capacitor in parallel with, and tuning the loop hence a lower frequency. Theory is one thing, practice another. I hope this confuses the issue even more Hi Hi. Regards to all Max vk2tti.

              Sent from my iPad

              On 13/08/2013, at 9:04, Hylton Thompson <g6avl@...> wrote:

               

              Thank you for taking the trouble to explain this so clearly Andy. Your answers certainly make sense.
               
              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.
               
              Hylton Thompson (g6avl)
               
              ----- Original message -----
              From: Andy <ai.egrps@...>
              Subject: Re: [loopantennas] Loop Circumference
              Date: Mon, 12 Aug 2013 17:09:05 -0400
               
               

               

              > If 20 metres of of well-balanced window line is used as the feeder,
              > 40 metres of conductor has effectively been added into the loop.
               
              Hmm.  Not really; not added INTO the loop.

              > To my present understanding, the standing wave pattern around the loop
              > must continue down each leg of the feeder which is therefore an integral
              > part of the loop (but with radiation cancelled out).
               
              Yeah, I don't think that's true.  There really is a boundary between the loop and what feeds it.
               
              Remember that if a transmission line is matched, then its length cannot be measured, or "felt", by "looking into" the end.  It appears identical to an infinitely long line ... or to a resistor equal to the characteristic impedance Zo.  So connecting a 20 meter long, 450 ohm balanced line is identical to connecting a 450 ohm resistor, to the same point.  Also identical to connecting a 10000 meter balanced line.
               
              The standing wave pattern on the antenna generally does not continue down the feedline.  The presence of standing waves on the feedline depends on the impedance match at the feedline/antenna boundary.  There could be no standing waves on the feedline ... even though they are there on the antenna.
               
              It is the same as the feeder line connected to a dipole antenna, or a vertical.  Neither one makes the antenna electrically longer.
               
              > If the loop and feeder each contain 40 metres of wire, is the combined
              > resonant frequency based upon 80 metres of wire, even though 50% of it
              > is non-radiating?
               
              No, sorry.
               
              > Taking this further, how reasonable is it to drop a few metres of window
              > line, with its ends shorted, from some point in the loop and expect the
              > loop to perform as if its length had been increased by double the length
              > of the drop?
               
              Now this is something that actually does do something.  The shorted stub behaves like an inductor (if the stub is less than 1/4 of a wavelength long at the transmitted frequency), and adding an inductor to a loop can make it appear electrically longer.  It's just like a loading coil in a too-short vertical.  You have to be a little careful with a loop antenna, because it does matter where in the loop you insert the stub (e.g., not at the current null points in the loop).
               
              I am not sure if it behaves longer by twice the length of the stub.  I suspect it goes by some other formula.
               
              All I can say is, it's a concept I have read about, but don't know very much about.  If I remember correctly, ON4UN talks about it in his book in the chapter on loops.
               
              And it might confound things for using the loop on higher frequency harmonics.
               
              > The reason I ask these questions is that I am in the process of putting
              > up a small horizontal loop where the maximum circumference is restricted
              > to about 32 metres and I want to achieve resonance at the bottom of the
              > 40m band. Experiments show that my Palstar AT2K + 4:1 external balun
              > can tune 6m thru 80m with a 32m loop so I don't know whether there will
              > be a great advantage in trying to achieve 40m resonance by adding a
              > non-radiating length to the loop.
               
              Sometimes it helps to achieve resonance as near to the antenna itself, so that you don't have a poor SWR on the feeder line, adding to loss.
               
              Andy
               
               

               

               

            • 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 6 of 11 , Aug 13 6:41 PM
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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 7 of 11 , Aug 13 8:30 PM
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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 8 of 11 , Aug 15 5:56 PM
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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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