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  • Originally posted by abcd567 View Post
    CoCo is a balanced antenna. It is a dipole, two equal legs, physically merged, but electrically separate. Since it is end fed, it gives an impression of a whip antenna which is a mono-pole and hence is unbalanced.

    Example: 8-element coco.
    Leg 1: Radiating part comprises of shields of odd elements (1, 3, 5, & 7), phasing part comprises of central wire of even elements (2, 4, 6, & 8)
    Leg 2: Radiating part comprises of shields of even elements (2, 4, 6, & 8), phasing part comprises of central wire of odd elements (1, 3, 5, & 7).

    Please see sketch below.



    .
    Great description, I don't think I hav ever seen a clearer description of how a CoCo works, it would be interesting to try and build something along the lines of No.3 to see if it worked, it could be done with coax by removing the braid on alternate sections and then linking them together to ground, presumably you would need to allow for VF in the sleeved sections.
    I have always visualised CoCo's as like a folded Franklin where the stubs are folded parallel to the elements, but your drawing is even clearer.
    Ben.

    Nb it might be slightly less ambiguous if you had the voltages as V*4 or 4*V otherwise some people might question how do you get the figure of 4 Volts? (4V)
    Last edited by F-EGLF1; 2014-05-23, 09:48.
    FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

    Comment


    • Originally posted by abcd567 View Post
      CoCo is a balanced antenna. It is a dipole, two equal legs, physically merged, but electrically separate. Since it is end fed, it gives an impression of a whip antenna which is a mono-pole and hence is unbalanced.

      .
      Taken from a paper produced by several USA Universities, discussing the pros and cons of the coco... I draw your attention to the last paragraph.

      In order to answer a simple question—why an antenna made of a cable would radiate at
      all—we first realize that it consists, in fact, of two antennas, one within the other (the
      inner and outer coaxial-line conductors), connected to the same feed point and source.
      The currents in the two antennas are in oppositedirections. Transposition of the coaxialline conductors at certain intervals does not change the propagation along the line, and is
      intended to produce proper voltages across the gaps between adjacent line segments. It is
      a simple matter to conclude that if the characteristic impedance of the coaxial-line
      sections were made to approach zero (i.e., the radius of the inner conductor to approach
      that of the outer conductor), the CoCo antennawould not radiate any more. This indicates
      that the CoCo antenna radiates because the two parallel antennas which make it have
      different current magnitudes at the feed, i.e., the feed-port currents are unbalanced.

      Comment


      • Originally posted by F-EGLF1 View Post
        Great description, I don't think I hav ever seen a clearer description of how a CoCo works, it would be interesting to try and build something along the lines of No.3 to see if it worked, it could be done with coax by removing the braid on alternate sections and then linking them together to ground, presumably you would need to allow for VF in the sleeved sections.
        I have always visualised CoCo's as like a folded Franklin where the stubs are folded parallel to the elements, but your drawing is even clearer.
        Ben.

        Nb it might be slightly less ambiguous if you had the voltages as V*4 or 4*V otherwise some people might question how do you get the figure of 4 Volts? (4V)
        Thanks for the compliments. You are right, V being used as abbreviation for "Volts", it was better if 4V was written as 4*V or 4xV (or another alphabet altogether was used to denote voltage induced in each element). Grounding shields in 2 & 3 will make them more effective, but not shown in the drawing for clarity purposes, as it would make understanding the principle somewhat complicated. Grounding shields in 4 is not required/not possible as after merger, the shields have dual role: (1) radiate for one limb & (2) shield the phasing wire for other limb.

        Comment


        • Originally posted by Rooster View Post
          Taken from a paper produced by several USA Universities, discussing the pros and cons of the coco... I draw your attention to the last paragraph.

          In order to answer a simple question—why an antenna made of a cable would radiate at
          all—we first realize that it consists, in fact, of two antennas, one within the other (the
          inner and outer coaxial-line conductors), connected to the same feed point and source.
          The currents in the two antennas are in oppositedirections. Transposition of the coaxialline conductors at certain intervals does not change the propagation along the line, and is
          intended to produce proper voltages across the gaps between adjacent line segments. It is
          a simple matter to conclude that if the characteristic impedance of the coaxial-line
          sections were made to approach zero (i.e., the radius of the inner conductor to approach
          that of the outer conductor), the CoCo antennawould not radiate any more. This indicates
          that the CoCo antenna radiates because the two parallel antennas which make it have
          different current magnitudes at the feed, i.e., the feed-port currents are unbalanced.
          Thanks for the information. Can you please provide the link to the paper you have quoted? I want to study it in detail. Thanks. I feel last few sentences, if at all true, may only apply to a plain length of coaxial cable (where shield radiates, central conductor does not) or maybe to CoCo if it consisted of ODD number of elements.
          Later addition:
          I mentioned ODD above because in a coco consisting of odd number of elements two limbs have different number of radiating elements, and hence cause unbalance. On the other hand, in the coco consisting of even number of elements, the number of radiating elements is same for both limbs, making it balanced.
          Example:
          7-element coco - one limb has 4 radiating elements, the other one has 3 radiating elements.
          8-element coco - both limbs have 4 radiating elements.
          Last edited by abcd567; 2014-05-23, 18:01.

          Comment


          • Originally posted by abcd567 View Post
            Thanks for the information. Can you please provide the link to the paper you have quoted? I want to study it in detail. Thanks. I feel last few sentences, if at all true, may only apply to a plain length of coaxial cable (where shield radiates, central conductor does not) or maybe to CoCo if it consisted of ODD number of elements.
            Later addition:
            I mentioned ODD above because in a coco consisting of odd number of elements two limbs have different number of radiating elements, and hence cause unbalance. On the other hand, in the coco consisting of even number of elements, the number of radiating elements is same for both limbs, making it balanced.
            Example:
            7-element coco - one limb has 4 radiating elements, the other one has 3 radiating elements.
            8-element coco - both limbs have 4 radiating elements.
            I wonder if that's why the shorted 7 1/2 has better performance than the open 8

            Comment


            • Originally posted by abcd567 View Post
              Thanks for the information. Can you please provide the link to the paper you have quoted? ...
              .
              Sure...
              https://www.engr.colostate.edu/~nota...20Antennas.pdf

              Comment


              • Thanks Rooster. Will go through it and post what I conclude from the study. Thanks again

                Comment


                • Is it worth buying this RF signal generator 35mhz - 4400mhz at price $190?
                  Review: inexpensive RF generator | EDN http://www.edn.com/electronics-blogs...e-RF-generator
                  Last edited by abcd567; 2014-05-24, 00:29.

                  Comment


                  • Originally posted by abcd567 View Post
                    Is it worth buying this RF signal generator 35mhz - 4400mhz at price $190?
                    Review: inexpensive RF generator | EDN http://www.edn.com/electronics-blogs...e-RF-generator
                    Only you can answer that!
                    What are you planning to use it for?

                    Sent using Tapatalk from my ZX81

                    Comment


                    • Originally posted by Rooster View Post
                      Only you can answer that!
                      What are you planning to use it for?

                      Sent using Tapatalk from my ZX81
                      I want instruments to test performance of antennas, and measure SWR, impedance etc. What instruments do you recommend in a limited budget?

                      Comment


                      • I'd say go for an antenna analyser... But at these frequencies you can't use the word "budget"...
                        You could use an rf source and add ons, but over 500mhz the connections are so critical due to standing waves etc you'd probably end up with poor accuracy.
                        The beauty of an analyser is that all the info is there in one shot... But they are silly expensive - £1000 for a 2ghz unit.
                        Option C might be to find an amateur radio club, where they may have a loaner unit.

                        Sent from my ZX81

                        Comment


                        • essentially i agree with rooster i've seen a network analyser for about 700-800 euro's and that the cheapest i have seen. Best you can hope for is to check places like ebay for a function generator that goes to the frequency you want and a spectrum analyser with those two you are part way to what you want. You wont get a network vector analyser cheap.

                          best you could hope for with a RF generator is to stand it next to your DVB-T tuner and see where you antenna is resonating.
                          T-EGLF8

                          Comment


                          • Originally posted by Rooster View Post
                            I'd say go for an antenna analyser... But at these frequencies you can't use the word "budget"...
                            You could use an rf source and add ons, but over 500mhz the connections are so critical due to standing waves etc you'd probably end up with poor accuracy.
                            The beauty of an analyser is that all the info is there in one shot... But they are silly expensive - £1000 for a 2ghz unit.
                            Option C might be to find an amateur radio club, where they may have a loaner unit.

                            Sent from my ZX81
                            Thanks for the advise. I will check local amateur radio club.

                            Comment


                            • Originally posted by SpaxmoidJAm View Post
                              essentially i agree with rooster i've seen a network analyser for about 700-800 euro's and that the cheapest i have seen. Best you can hope for is to check places like ebay for a function generator that goes to the frequency you want and a spectrum analyser with those two you are part way to what you want. You wont get a network vector analyser cheap.

                              best you could hope for with a RF generator is to stand it next to your DVB-T tuner and see where you antenna is resonating.
                              Thanks for your advise. Seems my hopes to make a test setup in a limited budget were not realistic.

                              Comment


                              • Originally posted by Rooster View Post
                                I'd say go for an antenna analyser... But at these frequencies you can't use the word "budget"...
                                You could use an rf source and add ons, but over 500mhz the connections are so critical due to standing waves etc you'd probably end up with poor accuracy.
                                The beauty of an analyser is that all the info is there in one shot... But they are silly expensive - £1000 for a 2ghz unit.
                                Option C might be to find an amateur radio club, where they may have a loaner unit.

                                Sent from my ZX81
                                £1000 would only get you one of the cheap hand held Chinese ones that they sell on e-bay, I don't think you would be very pleased with the results you got for the money, 'proper' antennae analysers will set you back £5000+ even second hand, however they would give you the results you wanted.
                                Ben.
                                FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

                                Comment

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