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  • We have been using formulae:
    coco element length = 1/2 x wavelength x VF
    where VF = velocity factor of cable.

    I have now built a 4-element coco using same RG6 cable as before, but have used the following formulae:
    Coco element length = wavelength x VF/(1+VF)

    I have put it on trial run an hour ago, and first hour's result show it to be much better than coco built with same cable with element length calculated by using the first formulae above.

    I will wait for 24 hr trial run to complete to be able to have some certenity.

    At this stage, I don't want to give the details of the theory I used to reach the second formulae, but it is based on my yesterday's post #1278 about "the coco which we make using coaxial cable, is a compromise"
    Last edited by abcd567; 2014-07-23, 01:46.

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    • As always, appreciatively and eagerly awaiting the outcome of that experiment!

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      • Originally posted by HermanZA View Post
        As always, appreciatively and eagerly awaiting the outcome of that experiment!
        I will post my findings (positive or negative) tomorrow. It took me 1/2 hours to make a 4 element coco. If you, and any other forum members have about 1/2 meters of RG6 cable to waste, why not make a 4-element coco using new formulae & put it to trial?

        Using VF of 82%, my element length has worked out to be 275 mm x 0.82/(1+0.82) = 124 mm
        The total height of my coco is 4 x 124 = 496 mm

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        • Just in a bit of disorganised chaos regarding my own ADSB equipment, antennas and cabling (did half of the relocation to a new mast)... but will try to make one tomorrow and get it up as high as possible.

          You say 4 elements... with open top end, same length of top element, right?

          You might be on to something here! Looking at that buggy J-pole they sold me, I have about 128mm elements there.
          IMG_20140722_131448.jpg

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          • Originally posted by HermanZA View Post
            ...... but will try to make one tomorrow and get it up as high as possible.

            You say 4 elements... with open top end, same length of top element, right? ............
            Yes, 4 full elements, open top. Since the RG6 cable between antenna and receiver dongle/computer is only 15 feet (indoor installation), 4 element is ok for me. Since you will use long run of feed cable from roof to ground floor, you need a higher gain antenna. You better make 8 element open top, which has 3 db more gain than 4 element one, and this additional 3db will cover for feeder cable losses..

            Later addition: if you are using an inline amplifier no need to waste your efforts at this experimental stage to make 8 element one. The 4 element one should suffice. If the experiment proves successful, you can anytime add 4 more elements.
            Last edited by abcd567; 2014-07-22, 21:46.

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            • With or without amp on this one ? Current in-use one is amp'd.

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              • Originally posted by HermanZA View Post
                With or without amp on this one ? Current in-use one is amp'd.
                Use the same number of elements as the existing one. If you are using amplifier for existing one, use amplifier for new one also. This way the comparision will be fair.
                Last edited by abcd567; 2014-07-22, 22:23.

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                • Originally posted by abcd567 View Post
                  Yes, 4 full elements, open top. Since the RG6 cable between antenna and receiver dongle/computer is only 15 feet (indoor installation), 4 element is ok for me. Since you will use long run of feed cable from roof to ground floor, you need a higher gain antenna. You better make 8 element open top, which has 3 db more gain than 4 element one, and this additional 3db will cover for feeder cable losses..

                  Later addition: if you are using an inline amplifier no need to waste your efforts at this experimental stage to make 8 element one. The 4 element one should suffice. If the experiment proves successful, you can anytime add 4 more elements.
                  Thats pretty much what I have been running since I started, 8x125mm rg6 however not 124mm, and not shorted, inside 20mm grey pvc pipe, I get about 150nm avg, typcial peak of 190nm, have got over 300nm once (though I suspect ducting that day), if you saw where the coco is and what's around it (a forest of much bigger trees in most directions), and huge mountain range to the west, its AMSL, plus being only 2 metres above roof top, you would probably be flabbergasted at how well it actually works

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                  • Originally posted by Ressy View Post
                    Thats pretty much what I have been running since I started, 8x125mm rg6 however not 124mm, and not shorted, inside 20mm grey pvc pipe, I get about 150nm avg, typcial peak of 190nm, have got over 300nm once (though I suspect ducting that day), if you saw where the coco is and what's around it (a forest of much bigger trees in most directions), and huge mountain range to the west, its AMSL, plus being only 2 metres above roof top, you would probably be flabbergasted at how well it actually works
                    Very interesting. Just one question: On what basis you used 125mm?

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                    • ANY pipe colour other than white is to be avoided if at all possible, the black or grey pipe uses carbon as a filler/colourant... which will couple (attenuate) to the antenna within !

                      Always use WHITE !

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                      • If results of my trial run work out to be positive, I will post the details of the theory, based on which I worked out the following formula:

                        Element length = wavelength x VF/(1+VF)

                        This formula gives an element length of 124mm for VF=82%


                        Up till now I have made several CoCos using element length of 110 mm, 113 mm & 116 mm, which were worked out using the formula

                        Element length = 1/2 x wavelength x VF

                        1/2 x 275 x 0.80 = 110 mm
                        1/2 x 275 x 0.82 = 113 mm
                        1/2 x 275 x 0.84 = 116 mm

                        All these CoCos gave me frustratingly poor performance.
                        Last edited by abcd567; 2014-07-23, 07:51.

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                        • Originally posted by abcd567 View Post
                          Very interesting. Just one question: On what basis you used 125mm?
                          from memory was based on VF on my RG6, though since its only reception I doubt that extra 1 mm would make much if any difference

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                          • Originally posted by Rooster View Post
                            ANY pipe colour other than white is to be avoided if at all possible, the black or grey pipe uses carbon as a filler/colourant... which will couple (attenuate) to the antenna within !

                            Always use WHITE !
                            this isnt a pure grey, its a deeper than off-white, light grey (they had no pure white in stock) but I know of what you speak of, it was my first question to the sales attendant, and the distances I get fairly well prove what they sold me was not carbon based (I am aware some sales folk will say whatever to make a buck, I threw a myriad of questions at him, and he knew his stuff, so I trusted him), since its only $AU3.50 or something for 4 metres, if I see the pure white in stock whenever I'm there I might pick a length up anyway for the hell of it, though I do not expect any improvement.

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                            • Question
                              Looking at the Super Antenna built by 1090 MHz ( http://ads-b.ca/antenna-collinear-F-CYYZ2 ) and noting that this tested as nearly perfect electronically, I started thinking ( a dangerous occupation for me) -- Is it not perhaps the type of coax used to make the antenna that makes the difference?
                              The outer conductor could , because of the shape in this case, present a longer path for the signal than for instance a braid as in RG6 and thereby confirming the theory by abcd567 that the outer conductor should be longer than the inner.
                              Knowing the "black art" of antenna making, it could just work out to the correct length.
                              BTW my carefully made LMR 400 coco and tested open works perfectly without an amplifier but with amp only 50 or so miles on SBS-1eR, Radarbox and dongle.

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                              • This "Black Art" of ours is indeed a very technical art!
                                Running a 4 element CoCo with the new 124mm element lenghts on a test setup. Have some test data of a previously made 3 1/2 element Coco.

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