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  • GOOD NEWS
    Couple of months ago I have made 4 CoCos (3.5-elements each) from same reel of coaxial cable. The reason was that I did not know the VF of cable. The element lengths were 11.0cm (VF=80%), 11.3cm (VF=82%), 11.6cm (VF=84%) and 11.9cm (VF=86%). I put them on trial run one by one, each for 48 hours. I found the best result was given by 11.6 cm, indicating that the cable I used has a VF of 84%.

    Although there was difference in maximum range for these 4 cocos, the difference was never so crazy as my yesterday's simulation of 8-element coco showed. This sharp conflict between simulation & field test kept pinching me, till I decided to re check my simulation thoroughly for any bugs. I checked all geometry & variable entries one by one (like debugging a computer programming code), and finally found a bug. After removing the bug, I ran simulation, and found that the CoCo is not at all so intolerant to errors. I am now posting the screen shots of debugged simulations which show that varying the element length from 134mm to 139mm has moderate effect on both the Gain & SWR.

    CoCo 8 elements - Dimension Tolerance 1B.jpg . CoCo 8 elements - Dimension Tolerance 2B.png
    Last edited by abcd567; 2014-05-21, 07:05.

    Comment


    • Hi ab cd, Good work! Glad you sorted the "bug".
      T-EGUB1

      Comment


      • Originally posted by F-EGLF1 View Post
        Any chance I can ask a favour from you?
        I am planning on giving a Franklin a try for FLARM glider detection on 868.3MHz and I was hoping you could have a quick look at my sketch, I think the dimensions are right, but I am not entirely sure about the tapping point on the stub.
        [ATTACH=CONFIG]4133[/ATTACH]
        I will be using it with a satellite amplifier similar to yours, however I have modified it by removing the DC choke on the antennae end so there is no DC pass-through, and I removed the guts of the F-connector at the same end and replaced it with an SMA connector, I have photos of the mod if you want to see them, but they are still on the camera card at the moment.
        Ben.
        Hi Ben
        The 65mm tap position shown on your drawing is OK. Simulation gives a figure of 66mm for lowest SWR.
        Below are screenshots of the simulation.
        Regards
        abcd

        Image 1 of 3 - Simulation output: Pattern, Gain, SWR
        Ben's Franklin-pattern.png

        Image 2 of 3 - Simulation output: Tap position sweep
        Ben's Franklin-tap position.png

        Image 3 of 3 - Simulation input: wire geometry, wire dia, frequency
        Ben's Franklin-geometry input.jpg

        Later addition:
        Forgot to tell your wire lengths are ok
        1/2 wavelength limbs = 1/2 x 299/868.3 = 0.172 m =172mm
        Phasing stubs total length
        required = 1/2 wavelength = 172mm
        provided = 84mm+4mm+84mm = 172mm

        Impedance matching stub:
        required horizontal length = 1/4 wavelength = 172/2 = 86mm
        provided = 86mm
        required vertical gap between stub wires (center to center) = 3mm (practically difficult to construct and connect cable with such small gap between stub wires)
        provided = 5mm is OK, still gives a decent SWR of less than 1.5
        Last edited by abcd567; 2014-05-21, 13:03.

        Comment


        • Originally posted by Ressy View Post
          with VF, RG6 quad shield it worked out at 115mm here, but, did you ever try that length?
          I did not try simulation of CoCos with Insulation as my software (4nec2) handles only air insulated wires.
          However couple of months ago I have made 4 CoCos (3.5-elements each) from same reel of coaxial cable, but each CoCo having a different element length. The reason was that I did not know the VF of cable. The element lengths were CoCo1: 11.0cm (VF=80%), CoCo2: 11.3cm (VF=82%), CoCo3: 11.6cm (VF=84%) and CoCo4: 11.9cm (VF=86%). I put them on trial run one by one, each for 48 hours. I found the best result was given by 11.6 cm, indicating that the cable I used has a VF of 84%, but this result is not universally applicable to all RG6 models & manufacturers. It only applies to the cable reel I used for this experiment.
          Last edited by abcd567; 2014-05-21, 13:44.

          Comment


          • adbc567, is 7element with 3/4 wave length element on the bottom odd num of elements of 1/2 wave and 1/4 wave length element on the top (shorted) is better or odd number of 1/2 elements with shorted 1/4 element on the top better? temp.jpg
            RTL SDR : T-VEVZ1

            Comment


            • The picture below shows the incident which triggered my entry into the world of DIY antennas.
              Otherwise I might have continued using stock antenna for ever.
              DSC02173R.jpg

              Comment


              • Originally posted by charan View Post
                adbc567, is 7element with 3/4 wave length element on the bottom odd num of elements of 1/2 wave and 1/4 wave length element on the top (shorted) is better or odd number of 1/2 elements with shorted 1/4 element on the top better? [ATTACH=CONFIG]4144[/ATTACH]
                Can not say anything about this issue as never tried or even thought of a bottom 3/4 wavelength piece. Other experienced forum members may shed some light on this.
                I feel all jugglery at the bottom of Radarcape antenna is for:
                (1) Coin size disc+4 down legs, matching a Balanced antenna with unbalanced coaxial (Balun) .
                (2) A metallic sleeve on pipe, electrically connected to braid, to prevent the feedline braid from re-radiating the received signal which results in a negative feedback to the system.

                Well all this is only my guess. Unless I study the subject in more detail, I cannot say anything with surety.

                Comment


                • Originally posted by abcd567 View Post
                  Can not say anything about this issue as never tried or even thought of a bottom 3/4 wavelength piece. Other experienced forum members may shed some light on this.
                  I feel all jugglery at the bottom of Radarcape antenna is for:
                  (1) Coin size disc+4 down legs, matching a Balanced antenna with unbalanced coaxial (Balun) .
                  (2) A metallic sleeve on pipe, electrically connected to braid, to prevent the feedline braid from re-radiating the received signal which results in a negative feedback to the system.

                  Well all this is only my guess. Unless I study the subject in more detail, I cannot say anything with surety.
                  hmm, but i tested with 3/4th element yesterday, and today i removed it and now testing, can say i have better performance without it.. have to check again.
                  RTL SDR : T-VEVZ1

                  Comment


                  • Originally posted by charan View Post
                    hmm, but i tested with 3/4th element yesterday, and today i removed it and now testing, can say i have better performance without it.. have to check again.
                    Did you test bottom 3/4 with or without sleeve & balun?
                    If you don't provide sleeve or balun, then the bottom 3/4 element is just a length of coax feedline.

                    The 1/2 element (1/4 wavelength) at top has a different function. It is for impedance matching/improving SWR.

                    Comment


                    • Originally posted by abcd567 View Post
                      Hi Ben
                      The 65mm tap position shown on your drawing is OK. Simulation gives a figure of 66mm for lowest SWR.
                      Below are screenshots of the simulation.
                      Regards
                      abcd

                      Image 1 of 3 - Simulation output: Pattern, Gain, SWR
                      [ATTACH=CONFIG]4143[/ATTACH]

                      Image 2 of 3 - Simulation output: Tap position sweep
                      [ATTACH=CONFIG]4141[/ATTACH]

                      Image 3 of 3 - Simulation input: wire geometry, wire dia, frequency
                      [ATTACH=CONFIG]4142[/ATTACH]

                      Later addition:
                      Forgot to tell your wire lengths are ok
                      1/2 wavelength limbs = 1/2 x 299/868.3 = 0.172 m =172mm
                      Phasing stubs total length
                      required = 1/2 wavelength = 172mm
                      provided = 84mm+4mm+84mm = 172mm

                      Impedance matching stub:
                      required horizontal length = 1/4 wavelength = 172/2 = 86mm
                      provided = 86mm
                      required vertical gap between stub wires (center to center) = 3mm (practically difficult to construct and connect cable with such small gap between stub wires)
                      provided = 5mm is OK, still gives a decent SWR of less than 1.5
                      Many many thanks for this, I will keep the stub gap as small as reasonably possible, I just estimated the tap position at 75% of the length, good to have a more accurate measurement.
                      Ben.
                      FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

                      Comment


                      • @F-EGLF1
                        Hi Ben
                        See pictures below showing various methods people have used to connect coax cable to Franklin.
                        What method you have used?





                        Comment


                        • Originally posted by F-EGLF1 View Post
                          Many many thanks for this, I will keep the stub gap as small as reasonably possible, I just estimated the tap position at 75% of the length, good to have a more accurate measurement.
                          Ben.
                          5mm is reasonable & practical. I am using 5mm with good results.
                          If you try to reduce it, the stub wires may touch each other & the optimum tap position may shift few mm left or right.
                          Best keep it as in your diagram - 5mm.

                          Comment


                          • Originally posted by abcd567 View Post
                            Did you test bottom 3/4 with or without sleeve & balun?
                            If you don't provide sleeve or balun, then the bottom 3/4 element is just a length of coax feedline.

                            The 1/2 element (1/4 wavelength) at top has a different function. It is for impedance matching/improving SWR.
                            I have just spent my lunch hour getting my head around the sleeve baluns, now I know where I was going wrong! I did not fully realise the importance of the upper one and ended up going back to basics to model the impedance / reactance of the antennae, I simplified it down to an end-feed Franklin type with quarter wave ground plane stubs so I could visualise what was happening, (actually to tell the truth, I treated it as a TX antennae as it was easier to visualise voltage / current).
                            The lower balun provides a consistent low impedance feed point, without it then the feed cable could affect the antennae, the upper one does the same for the 3/4 wavelength section, without it you have a high impedance and it will not see the rest of the segments!!! So BOTH baluns are important.
                            I don't have any nice diagrams as this was done the old-fashioned way on the back of some scrap paper.
                            I also put my MK1 coco back on the analyser, and now realise why it did not perform as well as hoped, basically I made it too well and the bandwidth is too narrow, I will re-build it next week with the elements at 101%, 100% and 99% of the correct length, this should open out the dip to +-10 to 15MHz.
                            Picture attached showing MK1 with very sharp dip
                            Also I can confirm that the segments using RG402 need to be 95MM long with approx 1/2MM gap, I am away from work till next week, but I will post the results of the tweaked lengths as soon as I can.
                            Ben.image.jpg
                            FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

                            Comment


                            • Originally posted by charan View Post
                              adbc567, is 7element with 3/4 wave length element on the bottom odd num of elements of 1/2 wave and 1/4 wave length element on the top (shorted) is better or odd number of 1/2 elements with shorted 1/4 element on the top better? [ATTACH=CONFIG]4144[/ATTACH]
                              From the calculations I was doing today it is clear that the termination serves very different functions for odd or even numbers of elements, odd numbered segments end with high impedance and need a shorted stub, even numbered segments end with high reactance so need an open ended stub, (I think I have this the right way round).
                              If you have 3/4 wavelength at the bottom then you MUST have both sleeve baluns for it to work (or at the very least the upper one), you only count the elements above it, otherwise you would probably get better results without it, but at the risk of greater mis-match between antennae and feeder.
                              Ben.
                              Last edited by F-EGLF1; 2014-05-21, 17:32.
                              FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

                              Comment


                              • Originally posted by abcd567 View Post
                                Did you test bottom 3/4 with or without sleeve & balun?
                                If you don't provide sleeve or balun, then the bottom 3/4 element is just a length of coax feedline.

                                The 1/2 element (1/4 wavelength) at top has a different function. It is for impedance matching/improving SWR.
                                i did not provided any sleeve or balun. can you give me the specs to make balun?
                                RTL SDR : T-VEVZ1

                                Comment

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