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  • @gregy:
    The simulation results for "Varying the Length of Elements" for CoCos made of:

    RG6 FPE 0.83 VF;
    RG58 PE 0.66 VF;
    Air insulated tube;

    show that the minimum SWR occurs 5 to 10 millimeters away from the length obtained by standard formula "Length = 1/2 wavelength x VF".

    Please see many tables of simulations of different CoCos I have posted yesterday.

    This may be due to:

    the standard formula is not precise but a close approximation only.

    the simulation software is not accurate & gives approximate results.
    Last edited by abcd567; 2014-10-14, 17:21.


    • yes i looked at the sim results to try to get a starting point for my next coco,
      however with the heliax cable, it adds the additional sheild E length aspect.
      either way ive decided im going to go shorter by around 4mm (all other factors identical)
      and both sweep it and live benchmark it against current coco,
      assumption being that even with std formula, the heliax effect still makes it too long ....
      i have plenty of the heliax ... its just the time!


      • @ab cd
        Hi Dave!
        Did you solve rain water/radome problem for your Franklin?
        See photos below how I solved it

        I like your thinking. No haven't resolved it yet. We've had a lot of rain over the last few days so not much incentive to go out to play.


        • Just thinking about CoCo tuning by adding a capacitor in series

          First see the impedance vs length curve for coco made with RG58 polyethylene insulation, 0.66 VF:

          If the length is 99 mm, the resistance (blue curve) is about 75 ohm, and reactance (red curve) is about 100 ohms inductive hence positive. If we add a 1.5 pF capacitor in series in core at feed point, Xc = 1/(2x pi x1090 Mhz x 1.5 pf) = 100 ohms, the capacitive reactance being negative, the net reactance will be zero, and impedance will become equal to resistance = 75 ohms. Just thinking loud. Have to get a trimmer capacitor to try it out.
          Last edited by abcd567; 2014-10-14, 21:01.


          • Well, a trimmer capacitor of 1.5 pF may not be available as most of trimmers have minimum value 3 to 5pF.

            Why not use a small piece of coax as capacitor, core making one plate of capacitor, shield making the second plate.

            Since RG6 with FPE insulation has core-to-sheild capacitance of 53pF per meter length, to make a 1.5pF capacitor, a piece of coax of length = 1000x1.5pF/53pF = 28mm will be required.

            Last edited by abcd567; 2014-10-15, 02:28.


            • the challenge is that the impedance of the antenna will "transform" as you move along the transmission line ;in all cases execpt where a perfect match occurs of Zl=Zo (tranmission line and load have same impedance)
              hence depending on the location chosen to attach the capacitor, indeed at the given point it may need inductance.
              this is explained if you look up "single stub matching"
              im guessing (only) that if the simulation provides an indication of inductive reactance at the feed point,
              then fitting a capacitor at this same feed point would indeed enable the reactance to be cancelled, although this
              would not match the resistive part ... hence there could still be a mismatch between the tranmission line and antenna impedance. (im avoiding getting into smith chart theory and practise here)
              the objective of correctly matching an antenna to feedline, is to not only have reactive component zero but also resistive component equal to tranmission line. (at which point maximum power transfer occurs)
              ... so your suggestion would certainly enable tuning the reactive component to zero (assuming it was attached at an
              inductive point on feedline/feedpoint) but would not help any resistive component mismatch...


              • Thinking loud...DIY situation mostly does not have test equipment.

                If coco element length is less than optimum, gain will be lower & swr will be higher, both bad.

                If coco element length is higher than optimum, gain will be higher & swr will be higher. Both are not bad, only one is bad.

                So if length error is positive, it is better than negetive length error.

                Lets make a coco with elements deliberately made higher than stsndard formula. We are better off for gain, but poor in swr. Now we add a LC Pi, L or T with a variable capacitor. Let DIY hobbyist turn the spindle of variable capacitor to reduce SWR like we tune an FM or AM station, without any test equipment. We judge from output: in FM/AM by sound we listen, here by Range we get.


                • Originally posted by gregy View Post
                  ... so your suggestion would certainly enable tuning the reactive component to zero (assuming it was attached at an
                  inductive point on feedline/feedpoint) but would not help any resistive component mismatch...
                  Both antenna reactance & antenna resistance vary with element length. That is why my first step was to choose (from simulation impedance graph) an element length which gives resistance = 75 ohm.

                  Second step was to read the value of reactance for this element length, and add a capacitor if antenna reactance is +ve (inductive) or inductor if it is -ve (capacitive).

                  The reactance of added capacitance or inductance should be equal & opposite to antenna reactance. This way we can make reactance=0 & resistance=75 ohm, a perfect match.

                  EXAMPLE: 4-Element CoCo made of RG6 coax, FPE insulation, 0.83 VF

                  Last edited by abcd567; 2014-10-16, 05:55.


                  • CoCo Impedance Matching by Simulation:

                    The 4nec2 Simulation software provides:

                    (1) Values of inductor or capacitor (as the case may be) for series & parallel compensation.

                    (2) Values of Inductors & Capacitors in L, Pi or T matching networks.

                    Note: above screenshots are for 4-element, RG6 FPE insulated VF 0.83 - element length = 1/2 x 275 x 0.83 = 114mm.



                    • Rein the Wild DIY CoCo

                      Since the SWR of CoCo is very sensitive to element length, in DIY situation which lacks proper test equipment, it rarely happens that a CoCo with reasonably low SWR is made. As a result the maximum range in most cases is moderate to poor.

                      Changing the length of elements in small steps is one option to improve SWR/Range, but very tedious, and will make most DIY enthusiast to quit. Further it is not known, in any particular situation, weather to increase or to decrease the length of elements.

                      The other option is to use an impedance matching network with VARIABLE capacitor or VARIABLE Inductor, and try to achieve matching by adjusting the variable capacitor/inductor. This is similar to tuning an AM/FM Radio by turning its tuning knob. This is far more easy than trimming the elements, and suits DIY enthusiasts.
                      Last edited by abcd567; 2014-10-17, 18:35.


                      • (1) These antenna tuners are NOT for ADS-B. These are for Amateur Radio, but give a fairly good idea to those who are not familiar with antenna tuner.
                        (2) These tuners are adjustable to a wide range of frequencies. The CoCo tuner we want is for a narrow band of frequencies and therefore should be much simpler than these ones.

                        The $20 eBay Antenna Tuner Kit Assembly and Review - Part 1

                        DIY VHF ANTENNA TUNER made from junk

                        How Antenna Tuners Work - KK4WW & N4USA


                        • 4-Element CoCo made of coax RG6 FPE insulation 0.83 VF element length 114mm

                          (1) Without Impedance Mtching
                          SWR=5.03, impedance = 181-j181 ohms - heavily reactive and far from 75 ohms

                          (2) After inserting an Antenna Tuner - a matching L-network at the point where feed cable meets the CoCo
                          SWR=1.01, impedance = 75.7-j0.75 ohms - almost 75 ohms pure resistive


                          • Dead End
                            After struggling to make an antenna tuner, I realized that due to GHz frequency, the Variable Capacitors & Inductors required are so tiny (fraction of pF & uH), that these are not manufactures and not avaialble.
                            The new ray of hope to use an antenna tuner to match coco impedance has died.


                            • Nice try ab cd. I await your next brain wave.


                              • Yesterday's quest for making an antenna tuner for 1090 MHz, and search for smallest sized Variable & fixed Capacitors & Inductors resulted in purchase of items in the photo below. All for $10

                                Smallest VARIABLE Capacitor (Trimmer) found is 2.5 - 7.5 pF (top left of photo). Smallest inductor found is 0.1 uH (bottom left of photo written .1uH). Fixed ceramic capacitor smallest 1.8pF. All are much higher than required 0.3-1.5pF & 0.025 uH.