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  • Originally posted by Rooster View Post
    This "is this coax better than the other" discussion is confusing me...
    Given that an antenna made from 1m of RG6 costs 7 pence (uk) and one made from RG402 would be 20 pounds... over 25 times the cost!!

    my 7p job covers over 150 miles... So why try for more?
    plus, we are in a network that has cells way smaller than this....

    If its just for kicks, then why not use Andrews heliax? wider bandwidth and costs a load more ;-)

    Sent using Tapatalk from my ZX81
    This has nothing to do with one being better than the other, it is down to designing one that can be made by someone with basic tools to a pre-tested design, it is hard to get repeatable accuracy with RG6, especially if you are soldering the joints.
    Also the price is a LOT less than you quote, RG402 is only around 3 per meter including postage, a small sum to pay for easier construction.
    Ben.
    FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

    Comment


    • Originally posted by F-EGLF1 View Post
      This has nothing to do with one being better than the other, it is down to designing one that can be made by someone with basic tools to a pre-tested design, it is hard to get repeatable accuracy with RG6, especially if you are soldering the joints.
      Also the price is a LOT less than you quote, RG402 is only around 3 per meter including postage, a small sum to pay for easier construction.
      Ben.
      I agree with you. The biggest problem with RG6 is Velocity Factor inaccuracy, which results in inaccuracy in element length and poor gain/swr. Please see my recent post #1116 where intolerance of CoCo to dimensional error is highlighted. Also soldering the joints is a definite advantage in accuracy & mechanical strength. The main advantage with RG6 is that it is readily available almost universally, while RG402 is to be ordered or purchased from special stores.
      Last edited by abcd567; 2014-05-20, 06:56.

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      • Okay got my RG402 cable today from china DSCN1311.jpg...
        can somebody give me the specifications to build antenna? waiting eagerly .!
        RTL SDR : T-VEVZ1

        Comment


        • Originally posted by F-EGLF1 View Post
          RG402 semi-rigid co-ax,
          Velocity factor 69.5, 1/2-wave =95.58mm, 1/4-wave = 47.79mm.
          The above lengths are for free-field antennae, thay will need adjusting to allow for plastic housing but by how much I cannot say.
          I have some on order from china via ebay to do the same.
          Ben.
          okay got my specs but how should i build how many halfwave elements , quarter wave. can please explain. im asking this ques because http://ads-b.ca/00-00.htm <- here there is long element in the base of the antenna what is that? i just want to make a perfect replica ....
          RTL SDR : T-VEVZ1

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          • Hi ab cd,
            In your post #1116 above, what frequencies are the different lengths of coax resonant at? We are looking for 1090Mhz but the dongles would need to be calibrated for optimum performance. Surely you need to match your coax length to your particular dongle by doing a few tests with different lengths of coax? Do all aircraft transmit at exactly 1090Mhz?

            or am I talking rubbish?
            T-EGUB1

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            • Originally posted by charan View Post
              okay got my specs but how should i build how many halfwave elements , quarter wave. can please explain. im asking this ques because http://ads-b.ca/00-00.htm <- here there is long element in the base of the antenna what is that? i just want to make a perfect replica ....
              The bit at the bottom is a 'sleeve balun' and it's function is for impedence matching, this is one of the variables I am trying to get my head around, it is not helped by the fact that there are actually 2 of them, the metal sleeve on the plastic housing forms a second one !
              The lower length of RG402 is 3/4 wavelength (x VF) and the balun 1/4 wavelength (in air), but allowance has to be made for VF and the effect of the housing.
              I cannot give final measurements yet as I am still trying to get it right.
              Ben.
              FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

              Comment


              • Originally posted by F-EGLF1 View Post
                The bit at the bottom is a 'sleeve balun' and it's function is for impedence matching, this is one of the variables I am trying to get my head around, it is not helped by the fact that there are actually 2 of them, the metal sleeve on the plastic housing forms a second one !
                The lower length of RG402 is 3/4 wavelength (x VF) and the balun 1/4 wavelength (in air), but allowance has to be made for VF and the effect of the housing.
                I cannot give final measurements yet as I am still trying to get it right.
                Ben.
                i will wait for your measurements
                RTL SDR : T-VEVZ1

                Comment


                • Originally posted by trigger View Post
                  Hi ab cd,
                  In your post #1116 above, what frequencies are the different lengths of coax resonant at? We are looking for 1090Mhz but the dongles would need to be calibrated for optimum performance. Surely you need to match your coax length to your particular dongle by doing a few tests with different lengths of coax? Do all aircraft transmit at exactly 1090Mhz?

                  or am I talking rubbish?
                  The frequency for all lengths (134mm to 140mm) were same I.e 1090 Mhz. Everything was kept constant and ONLY element length was varied.

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                  • Originally posted by abcd567 View Post
                    The frequency for all lengths (134mm to 140mm) were same I.e 1090 Mhz. Everything was kept constant and ONLY element length was varied.
                    with VF, RG6 quad shield it worked out at 115mm here, but, did you ever try that length?

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                    • Over here in South Africa 2-1/4" Heliax is commonly used on Cellular Base Stations (on the 950mHz systems), as well as by the National Telecoms provider as feedlines for some of its older microwave systems. All the newer microwave links are using much thinner cables (lesser chance of cable theft).

                      Comment


                      • Originally posted by abcd567 View Post
                        The frequency for all lengths (134mm to 140mm) were same I.e 1090 Mhz. Everything was kept constant and ONLY element length was varied.
                        Hi ab cd,
                        I didn't explain myself very well as I don't understand exactly what I'm trying to say. OK try this:-

                        The 134mm length will be resonant at a particular frequency. Similarly the 136mm, 138mm etc will all have their own resonant frequency. Can you tell me the resonant frequency and gain for each of the lengths you plotted above?

                        Thanks for your patience.
                        T-EGUB1

                        Comment


                        • Originally posted by trigger View Post
                          Hi ab cd,
                          I didn't explain myself very well as I don't understand exactly what I'm trying to say. OK try this:-

                          The 134mm length will be resonant at a particular frequency. Similarly the 136mm, 138mm etc will all have their own resonant frequency. Can you tell me the resonant frequency and gain for each of the lengths you plotted above?

                          Thanks for your patience.
                          I already understood your point from your earlier post, and about an hour ago, started frequency sweep for 136mm element length, varying frequency between 1080 MHz and 1100 MHz. Results were astonishing as the gain curve still moving up at 1100 MHz and not reached a peak. I extended range to 1110, and gain curve still going up. Right at this moment I am running a sweep upto 1150 MHz. All sweeps were done in steps of 1 MHz. Since one simulation of 8-element coco takes about 2 minutes, each step of 1 MHz means 2 minutes. From 1080 MHz to 1110MHz, it took more than an hour. Now 1110MHz to 1150MHz will take 80 minutes. Please be patient. To do all 6 lengths (134, 135, 136, 137, 138 & 139mm) will take very long time.

                          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?
                            The antenna simulation software I am using (4nec2), does not have provision of insulated conductors. It is based on air insulation. That is why I have simulated an air-insulated CoCo which requires an element length of 137 mm (VF for air=1). Since my simulations of CoCos were for a comparative study of various configurations, the use of air-insulated models did not make a difference.

                            Comment


                            • Originally posted by trigger View Post
                              Hi ab cd,
                              I didn't explain myself very well as I don't understand exactly what I'm trying to say. OK try this:-

                              The 134mm length will be resonant at a particular frequency. Similarly the 136mm, 138mm etc will all have their own resonant frequency. Can you tell me the resonant frequency and gain for each of the lengths you plotted above?

                              Thanks for your patience.
                              Originally posted by abcd567 View Post
                              I already understood your point from your earlier post, and about an hour ago, started frequency sweep for 136mm element length, varying frequency between 1080 MHz and 1100 MHz. Results were astonishing as the gain curve still moving up at 1100 MHz and not reached a peak. I extended range to 1110, and gain curve still going up. Right at this moment I am running a sweep upto 1150 MHz. All sweeps were done in steps of 1 MHz. Since one simulation of 8-element coco takes about 2 minutes, each step of 1 MHz means 2 minutes. From 1080 MHz to 1110MHz, it took more than an hour. Now 1110MHz to 1150MHz will take 80 minutes. Please be patient. To do all 6 lengths (134, 135, 136, 137, 138 & 139mm) will take very long time.
                              Hi Dave
                              I have now completed the frequency sweep for the 8-element coco with element length 136mm i.e. an error of -1mm.
                              The Gain peaked at 1180 Mhz, i.e.the resonant frequency shifted from 1090Mhz to 1180Mhz, a 90Mhz shift just for 1mm error in length.

                              It looks that although 1mm in one element has caused small loss due to small phase shift, but adding the phase-shifts of all 8 elements caused a big loss. I feel that if I repeat this experiment with 4-element coco, the resonant frequency shift will be much less, presumably about half of that for 8-element.

                              Please see the plot of Gain vs. Frequency below:



                              .

                              Comment


                              • I honestly, hand on heart feel this is getting silly BIG time now, the thread was for MAKING an antenna to get you going wasn't it ?
                                Here we are using a caesium calibrated nuclear powered virtual sledgehammer to crack a non existent nut !!!!!

                                All you need to know is some simple maths, and the velocity factor of the cable you are using (varies from type to type of cable AS WELL as manufacturer) then crack on !

                                My cable was bought from a satellite dish company - even they didn't know the VF, so I dusted off my antenna analyser and ten minutes later we have the number.
                                But I am an anal amateur radio enthusiast... I enjoy the challenge of it "being right" - most folk just want it to work, and a coat hanger will receive something !!!!!!!!! If you don't get the technicals down to 0.00000001% IT DOESN'T MATTER it WILL work in some usable way.
                                If anyone in the UK want me to send you some cable with all the maths, or even a kit ;-)

                                Just crack on lads...

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