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Re: [loopantennas] Mobius Shielded Loop Antenna

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  • Steve
    Comments all noted Everett...thanks again...BTW.. ... stations were nulled by 20 S units, it does not seem correct, but that is 120 dB. not 20 S-units but ~
    Message 1 of 20 , Aug 19, 2013
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      Comments all noted Everett...thanks again...BTW..

      >The other station was 30/9 and I was able to null it to 10/9. So both
      stations were nulled by 20 S units, it does not seem correct, but that is
      120 dB.

      not 20 S-units but ~ 20db

      Steve / 73



      WEB - "The VE7SL Radio Notebook": http://members.shaw.ca/ve7sl
    • Andrew Ikin
      John wrote on August 19, Your comment that the loop is a current source with a series reactance is interesting. My understanding, is that a small loop can be
      Message 2 of 20 , Aug 20, 2013
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        John wrote on August 19,
         
        Your comment that the loop is a current source with a series reactance is interesting.

        My understanding, is that a small loop can be considered as a voltage source in series with the inductance of the loop, the ohmic losses, and the negligle radiation resistance.  For a given field strength, the voltage output is proportional to frequency.  The equivalent current source has an output which is proportional to field strength only, and has a shunt, not series, inductance.

        What am I missing?

        Tnx and 73

        John  KC0G

         
         

         Hi John,

         

         

        What I believe is happening; the H Field induces a current into loop, this current is not frequency related. However, as the loop has a rising  series impedance with frequency the loops terminal voltage is the product the induce current x the impedance. Hence we see that the Vout is proportional to frequency.

         

        This issue of the loops series reactance impeding the current is discussed in a GB patent for the Liniplex loop manufactured by Phase Track in the early nineties. I can't find my copy of this patent. A search for patents by Charles Edward Forster my find it. The Liniplex loop used multiple loops in parallel to reduce the series reactance from impeding the current to the amplifier.

         

        Getting back to the real issue here, we have a loop with output z that is frequency dependant, therefore optimum power transfer to the amplifier will only happen when the amplifier input z matches the loop.

         

         

        Kind regards

         

        Andrew
      • Andrew Ikin
        Everett wrote on the 19 August, Hi Andrew, Thank you for your comments and you input. I tried several different hook ups with the standard shielded loop
        Message 3 of 20 , Aug 20, 2013
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          Everett wrote on the 19 August,
           
           
          Hi Andrew,
           
          Thank you for your comments and you input. I tried several different hook ups with the standard shielded loop configuration and even tried a matching transformer using one of the 73 mix binocular cores, with several different ratios, but was never able to get it to work just right. I could not get the loop balanced correctly, as my 180 nulls were no where the same. Maybe I just did not come up with the correct combination.
           
          However, with the Mobius configuration it worked from the get go, with the M0YAF amp, of which I built 4 of them. I also tried the LZ1AQ broadband amp, but it did not work very well on any of the shielded loop configurations, but did okay on the standard 14 gage wire loop.  
           
          Your observation on the lower performance above about 16 MHz is very similar to what I am seeing, but as I have moved up from first, from RG58, then to my home made coax and now to the RG11, I am seeing improvement. The RG 58 has a capacitance of around 30 pf per foot, I did not measure my home made coax and the RG11 runs about 20 pf per foot, so again in your remarks on the capacitance maybe what is degrading my top end performance. 
           
          Hello Everett,
           
          The LZ1AQ amp. has a very low input z. That is why it does not work with the higher z Mobius.
           
          73
           
          Andrew
           
           
           
           
        • Hylton Thompson
          In general, any voltage source with series impedance can be converted to an exactly equivalent current source with parallel impedance at the same frequency.
          Message 4 of 20 , Aug 20, 2013
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            In general, any voltage source with series impedance can be converted to an exactly equivalent current source with parallel impedance at the same frequency. The only constraint is that there must be no non-linear devices such as diodes in the the circuit. See Thevenin's and Norton's theorems.
             
            Hylton Thompson (g6avl)
             
            ----- Original message -----
            From: Andrew Ikin <andrew.ikin@...>
            Subject: Re: [loopantennas] Mobius Shielded Loop Antenna
            Date: Tue, 20 Aug 2013 10:57:39 +0100
             
             

             

            John wrote on August 19,
             
            Your comment that the loop is a current source with a series reactance is interesting.

            My understanding, is that a small loop can be considered as a voltage source in series with the inductance of the loop, the ohmic losses, and the negligle radiation resistance.  For a given field strength, the voltage output is proportional to frequency.  The equivalent current source has an output which is proportional to field strength only, and has a shunt, not series, inductance.

            What am I missing?

            Tnx and 73

            John  KC0G
             
             

             Hi John,

             

             

            What I believe is happening; the H Field induces a current into loop, this current is not frequency related. However, as the loop has a rising  series impedance with frequency the loops terminal voltage is the product the induce current x the impedance. Hence we see that the Vout is proportional to frequency.

             

            This issue of the loops series reactance impeding the current is discussed in a GB patent for the Liniplex loop manufactured by Phase Track in the early nineties. I can't find my copy of this patent. A search for patents by Charles Edward Forster my find it. The Liniplex loop used multiple loops in parallel to reduce the series reactance from impeding the current to the amplifier.

             

            Getting back to the real issue here, we have a loop with output z that is frequency dependant, therefore optimum power transfer to the amplifier will only happen when the amplifier input z matches the loop.

             

             

            Kind regards

             

            Andrew

             

             
          • Andrew Ikin
            John wrote on August 19, Your comment that the loop is a current source with a series reactance is interesting. My understanding, is that a small loop can be
            Message 5 of 20 , Aug 20, 2013
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              John wrote on August 19,

               

              Your comment that the loop is a current source with a series reactance is interesting.

              My understanding, is that a small loop can be considered as a voltage source in series with the inductance of the loop, the ohmic losses, and the negligle radiation resistance.  For a given field strength, the voltage output is proportional to frequency.  The equivalent current source has an output which is proportional to field strength only, and has a shunt, not series, inductance.

              What am I missing?

              Tnx and 73

              John  KC0G

               

               

               Hi John,

               

               

              What I believe is happening; the H Field induces a current into loop, this current is not frequency related. However, as the loop has a rising  series impedance with frequency the loops terminal voltage is the product the induce current x the impedance. Hence we see that the Vout is proportional to frequency.

               

              This issue of the loops series reactance impeding the current is discussed in a GB patent for the Liniplex loop manufactured by Phase Track in the early nineties. I can't find my copy of this patent. A search for patents by Charles Edward Forster my find it. The Liniplex loop used multiple loops in parallel to reduce the series reactance from impeding the current to the amplifier.

               

              Getting back to the real issue here, we have a loop with output z that is frequency dependant, therefore optimum power transfer to the amplifier will only happen when the amplifier input z matches the loop.

               

               

              Kind regards

               

              Andrew

            • Jack Smith
              John and Andrew are both correct. If modeled as a voltage source in series with inductance, you will find the output voltage into a resistive load is
              Message 6 of 20 , Aug 20, 2013
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                John and Andrew are both correct.

                If modeled as a voltage source in series with inductance, you will find the output voltage into a resistive load is independent of frequency above a certain threshold, just as with the parallel current source model.

                This is because the loop voltage source EMF increases linearly with frequency at 6 dB/octave (induced voltage is proportional to rate of change of the magnetic flux, so when you differentiate the sin(wt) the flux rate of change becomes -w*cos(wt)) and voltage divider formed by the loop's inductive reactance (which doubles per octave) in series with the amplifier input resistance
                decreases the signal at the amplifier input at the same 6 dB/octave if the frequency is sufficiently high such that the amplifier input impedance is much less than the inductive reactance of the loop.  In other words, the 6 dB/octave rising voltage output is canceled by the -6 dB/octave voltage divider effect and the net voltage input into the loop amplifier is constant with frequency.

                This series model predicts the same behavior as a parallel current source, so either may be used provided you take care to get the maths correct.

                The simplistic -6 dB/octave LR voltage divider assumes, as mentioned earlier,  that the loop's inductive reactance is large compared with the input impedance of the amplifier.


                Jack K8ZOA



                On 8/20/2013 6:49 AM, Andrew Ikin wrote:
                 

                John wrote on August 19,

                 

                Your comment that the loop is a current source with a series reactance is interesting.

                My understanding, is that a small loop can be considered as a voltage source in series with the inductance of the loop, the ohmic losses, and the negligle radiation resistance.  For a given field strength, the voltage output is proportional to frequency.  The equivalent current source has an output which is proportional to field strength only, and has a shunt, not series, inductance.

                What am I missing?

                Tnx and 73

                John  KC0G

                 

                 

                 Hi John,

                 

                 

                What I believe is happening; the H Field induces a current into loop, this current is not frequency related. However, as the loop has a rising  series impedance with frequency the loops terminal voltage is the product the induce current x the impedance. Hence we see that the Vout is proportional to frequency.

                 

                This issue of the loops series reactance impeding the current is discussed in a GB patent for the Liniplex loop manufactured by Phase Track in the early nineties. I can't find my copy of this patent. A search for patents by Charles Edward Forster my find it. The Liniplex loop used multiple loops in parallel to reduce the series reactance from impeding the current to the amplifier.

                 

                Getting back to the real issue here, we have a loop with output z that is frequency dependant, therefore optimum power transfer to the amplifier will only happen when the amplifier input z matches the loop.

                 

                 

                Kind regards

                 

                Andrew

                _

              • crabtreejr
                Hello Chris et al The idea of using a low impedance amplifier for a loop antenna is not new. I have just dug out my copy of H.F. active antenna performance
                Message 7 of 20 , Aug 20, 2013
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                  Hello Chris et al

                  The idea of using a low impedance amplifier for a loop antenna is not new.  I have just dug out my copy of "H.F. active antenna performance requirements and realization",  by B.M. Sosin, published in "Communication and Broadcasting", Summer 1976.  Sosin was at one time Chief Scientist at one of the Marconi companies in the UK. 

                  Sosin discusses how a low-impedance amplifier gives near optimal amplification over the H.F. frequencies.  His assumptions, ie 10dB receiver noise figure, 3 dB noise degradation, low noise areas at quiet times, lead to an antenna factor of -2dB, ie a field strength of 0.8V/m is required for 1 Volt output into 50 ohms.

                  Sosin goes on to say that only the very best receiving systems would be affected by the noise degradation, and then by a small amount.  In those cases the amplifier gain could be increased by 3 to 8 dB, or an additional low noise, low intermodulation products used. 

                  Sosin references a paper by Callendar on wideband loop arrays from 1972. 

                  73 John  KC0G


                  In a message dated 8/19/2013 4:59:23 PM Central Daylight Time, christrask@... writes:
                  >
                  >Your comment that the loop is a current source with a series reactance is
                  >interesting.
                  >
                  >My understanding, is that a small loop can be considered as a voltage
                  >source in series with the inductance of the loop, the ohmic losses, and the
                  >negligle radiation resistance. For a given field strength, the voltage output is
                  >proportional to frequency. The equivalent current source has an output
                  >which is proportional to field strength only, and has a shunt, not series,
                  >inductance.
                  >
                  >What am I missing?
                  >

                  I was looking at loops as being voltage sources with series reactances until I came across a paper that look at them as being current sources with shunt reactances, and it made sense as a loop in a magnetic field generates a current to the load that is proportional to the loop diameter and the signal frequency. That ptomted me to design my wideband loop amplifier with a very low input impedance rather than trying to match the loop reactance over a wide band.

                  Chris Trask
                  N7ZWY / WDX3HLB
                  Senior Member IEEE
                  http://www.home.earthlink.net/~christrask/

                • Chris Trask
                  ... Could you scan that and add it to the files section? Chris
                  Message 8 of 20 , Aug 20, 2013
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                    >
                    >Hello Chris et al
                    >
                    >The idea of using a low impedance amplifier for a loop antenna is not new.
                    >I have just dug out my copy of "H.F. active antenna performance
                    >requirements and realization", by B.M. Sosin, published in "Communication and
                    >Broadcasting", Summer 1976. Sosin was at one time Chief Scientist at one of the
                    >Marconi companies in the UK.
                    >

                    Could you scan that and add it to the files section?


                    Chris
                  • Andrew Ikin
                    John, Ref, to my earlier post concerning the GB Patent for the Liniplex loop. The UK Patent Application no. is GB 2 235 337 A Inventor is Edward Charles
                    Message 9 of 20 , Aug 20, 2013
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                      John,

                       

                      Ref, to my earlier post concerning the GB Patent for the  Liniplex loop. The UK Patent Application no. is GB 2 235 337 A  Inventor is Edward Charles Forster. It discusses the problem of the loops series inductance limiting the current to load. In the case of this patent the load is a common base amplifier.
                       
                      73
                       
                      Andrew
                       
                    • hmaxim2000us
                      Hello Group I just wanted to update you on the Mobius loop as described by Everett N8CNP . I finally got my RG/11 although it turned out to be a cable with
                      Message 10 of 20 , Sep 1, 2013
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                        Hello Group
                        I just wanted to update you on the Mobius loop as described by Everett N8CNP . I finally got my RG/11 although it turned out to be a cable with aluminum shielding and some kind of lubricant between the shield and the jacket…a real mess to work with. I used it anyway.
                        So I find that it performs much better than a single turn loop. In fact it equals or outperforms my reference antenna which is an 88 foot twinlead fed inverted Vee at 45 feet. But only in the evening. During the day the loop is almost useless. Have not figured that out yet.
                        One other thing I was hoping you might be able answer. The loop performance drops off considerably by the time I get to 40 meters. On 30 meters it is nearly deaf.
                        I am thinking there is a problem with my amp. Do you think T1 should be adjusted? Maybe I am not coupling enough high frequency voltage from T1? I was thinking maybe 15 turns is too much? Also, can I use a F T50 core, and if so, how would this effect the number of turns?
                        What do you think about the transistors? I get superb performance from 100 khz up to around 4 or 5 Mhz, but after that it really takes a dive. I have a pair of 2n5109's but have not installed them yet. Do you think it would be better than the 2n3904's I use now?
                        Jim
                        K2TL



                        --- In loopantennas@yahoogroups.com, everettsharp@... wrote:
                        >
                        > Hi Andrew,
                        >
                        > Thank you for your comments and you input. I tried several different hook
                        > ups with the standard shielded loop configuration and even tried a matching
                        > transformer using one of the 73 mix binocular cores, with several different
                        > ratios, but was never able to get it to work just right. I could not get
                        > the loop balanced correctly, as my 180 nulls were no where the same. Maybe
                        > I just did not come up with the correct combination.
                        >
                        > However, with the Mobius configuration it worked from the get go, with the
                        > M0YAF amp, of which I built 4 of them. I also tried the LZ1AQ broadband
                        > amp, but it did not work very well on any of the shielded loop configurations,
                        > but did okay on the standard 14 gage wire loop.
                        >
                        > Your observation on the lower performance above about 16 MHz is very
                        > similar to what I am seeing, but as I have moved up from first, from RG58, then
                        > to my home made coax and now to the RG11, I am seeing improvement. The RG 58
                        > has a capacitance of around 30 pf per foot, I did not measure my home made
                        > coax and the RG11 runs about 20 pf per foot, so again in your remarks on
                        > the capacitance maybe what is degrading my top end performance.
                        >
                        > 73 Everett
                        >
                        >
                        > In a message dated 8/19/2013 2:55:40 P.M. Central Daylight Time,
                        > andrew.ikin@... writes:
                        >
                        >
                        >
                        >
                        >
                        > Dear Everett,
                        > Thanks for sharing your observations. I would suggest that the difference
                        > you are seeing in loop performace comes down to how well the loops are
                        > matched to the ampfiliers and whether you are using a balanced amplifer to
                        > cancel out any near E field noise. All the mobius loop is doing is increasing
                        > the loops impedance. This could be have been achieved with a transformer.
                        > Some time ago I compared a Mobius loop (1m diam) to a 1m 3/4 tube loop
                        > using an ALA1530 Amplifier and a higher impedance low noise FET amplifier.
                        > What I found was; below approx. 16MHz both loop types would provide similar
                        > performance when taking into consideration the differing loop z to amplifier
                        > input z.
                        > Above 16MHz the capacitance of the Mobius loop was too high and thus the
                        > gain drops off. Where both loop types have a resonable match to the
                        > amplifier the signal and s/n are the same.
                        > If one looks at the two loop types intuatively, both have the same area,
                        > so it would be fair to say that they capture the same signal. With the
                        > mobuis the deliverable current to the amplifier is 4 x lower because the
                        > impedance is 4 x higher. Hence this is why Pixel use a 50 ohm input z amplifier
                        > to acheive good results compared to some designs using a near zero input
                        > impedance. The basic fallacy with using a near zero input z amplifier is
                        > that a loop is not a low z current source, but a current source with a series
                        > reactance. It is this rising reactance verses frequency which dominates
                        > the loss in a broadband loop antenna, hence the need to provide a reasonable
                        > loop/amplifier match.
                        > 73
                        > Andrew Ikin
                        >
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