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  • .
    Calculation of stub gap for impedance matching:

    Comment


    • @ gregy:
      Why not you try 2.5 mm (or better 1.5 mm) dia copper wire obtained from electrical power cable?
      With wire having smaller dia, and made of soft copper (more malleable & ductile than brass rod), you may make a good Franklin.

      Comment


      • @gregy:

        How about making the stub by soldering pieces instead of bending brass rod in a semi-circle? See sketch below:


        Comment


        • might be possible, however
          - it would have to be an "overlap weld" ... not sure in the fairly tight area how this might effect the characetristics
          of the stubs
          - even with an overlap weld, the actual surface area of contact is quite small in this configuration (even after grinding
          flat spots on rod to increase contact area)
          hence i doubt the soldered joint will have much strength ... and this is a high stress area - given the bottom part
          of antenna is a lever arm acting on the stub ends

          i did however find some "other" "braising rod " - which appears to be an alloy of copper. phospohor and ???
          its a flat bar style - around 3x1.5mm (TBC - im at work and cant check)
          it appears to be easier to bend (Round the smaller dimension ) .. and shows promise
          im assuming the critical antenna dimensions are the gaps and lengths, hence the rectangular section
          can be dealt with accordingly
          eg when i can confirm the "thickness .. i can determine if it matches any of your simulated designs to date for round rods.

          Comment


          • and also .. just checking ... for the stubs, my understanding was its the electrical,length of the stub that
            has to be a certain wavelength...
            so if changing to a welded end per your above diagram, then this would also change the 69mm length?
            (because for circular stub end, it has a longer electrical length than a square end?)

            Comment


            • Originally posted by gregy View Post
              and also .. just checking ... for the stubs, my understanding was its the electrical,length of the stub that
              has to be a certain wavelength...
              so if changing to a welded end per your above diagram, then this would also change the 69mm length?
              (because for circular stub end, it has a longer electrical length than a square end?)
              square end is more accurately 1/4 wavelength than rounded end, but the difference is not so critical to affect the antenna's performance, as adjusting the tap point covers it.
              You are right that mechanically soldered bend is weaker than the continuous round bend, as the bending moment due to weight of lower arm is highest at the stub's shorted end.

              Comment


              • Originally posted by gregy View Post
                might be possible, however
                - it would have to be an "overlap weld" ... not sure in the fairly tight area how this might effect the characetristics
                of the stubs
                - even with an overlap weld, the actual surface area of contact is quite small in this configuration (even after grinding
                flat spots on rod to increase contact area)
                hence i doubt the soldered joint will have much strength ... and this is a high stress area - given the bottom part
                of antenna is a lever arm acting on the stub ends

                i did however find some "other" "braising rod " - which appears to be an alloy of copper. phospohor and ???
                its a flat bar style - around 3x1.5mm (TBC - im at work and cant check)
                it appears to be easier to bend (Round the smaller dimension ) .. and shows promise
                im assuming the critical antenna dimensions are the gaps and lengths, hence the rectangular section
                can be dealt with accordingly
                eg when i can confirm the "thickness .. i can determine if it matches any of your simulated designs to date for round rods.
                I have done the rectangular matching stub & round phasing stubs in my Franklin. I used 18 AWG (1 mm dia) wire from RG6 coaxial cable, and sharp bending was not a problem. The reason was that to insert the terminal block, the stub end was cut open, and after inserting the terminal block, the two stub wire were shorted by means of a small piece of wire soldered to stub wires, as I have shown in the sketch in my post #1725 above.

                Please see below the photograph :

                DSC03055R.jpg
                Last edited by abcd567; 2014-10-12, 07:48.

                Comment


                • ok ... i can see the "pressure"
                  i will have a crack at another franklin using the copper wire.... if nothing else im determined to
                  find a good benchmark for the coco ... so i can determine when im at the best range from antenna
                  performance ... then i fit the LNA.
                  (then the nitrogen cooling ... only joking on the cooling

                  Comment


                  • Another Simulation: 8-Element CoCo made of RG6 cable (FPE insulation, V.F. =0.83)

                    EDITED on October 13, 2014:
                    The screenshots of this simulation have been removed as the results were wrong due to wrong data entry (Dielectric Constant).
                    Revised simulation has been done after correction of input data, and results of simulation have been posted (#1740)
                    Last edited by abcd567; 2014-10-13, 14:11.

                    Comment


                    • very interesting simulation... this helps to understand the VSWR impact from errors in cutting accuracy

                      any thoughts on how the heliax (as noted in your earlier pst) outer shield "longer electrical length" impacts
                      your formula?

                      i will try my next coco at the longer length using your revised formula (VF 0.81) of 123mm
                      (as compared to current version using 113mm from traditional formula)
                      .. worst case i can always desolder and cutdown elements to shortier length!)

                      Comment


                      • hmmm on further reflection
                        the simulation is not matching the empirical experience ....
                        with my "shorter" version ... im getting VSWR around 2.5 ... which using your sim rsukts should be much higher.
                        dilemma! what length to try the next coco....

                        Comment


                        • Originally posted by gregy View Post
                          very interesting simulation... this helps to understand the VSWR impact from errors in cutting accuracy

                          any thoughts on how the heliax (as noted in your earlier pst) outer shield "longer electrical length" impacts
                          your formula?

                          i will try my next coco at the longer length using your revised formula (VF 0.81) of 123mm
                          (as compared to current version using 113mm from traditional formula)
                          .. worst case i can always desolder and cutdown elements to shortier length!)
                          Originally posted by gregy View Post
                          hmmm on further reflection
                          the simulation is not matching the empirical experience ....
                          with my "shorter" version ... im getting VSWR around 2.5 ... which using your sim rsukts should be much higher.
                          dilemma! what length to try the next coco....
                          All above formulas are applicable to SMOOTH shield cables. Since you are using CORRUGATED shield cable, using 113 mm length from traditional formula (V.F. x λ/2) effectively leads to higher electrical length of shield, possibly 120mm, giving swr around 2.5.

                          Heliax 2.PNG

                          If you apply my formula to corrugated Heliax, and use pieces of 123 mm length, their electrical length will be much higher than 123 mm (say around 130 mm), and will knock-off the antenna's performance.

                          Comment


                          • Originally posted by Marsmaensch View Post
                            Hi ab cd,
                            ..................
                            Benjamin
                            Hi Benjamin,
                            Is this you flying the plane? http://www.youtube.com/watch?v=zQN7OK-AeCg

                            Comment


                            • Originally posted by abcd567 View Post
                              All above formulas are applicable to SMOOTH shield cables. Since you are using CORRUGATED shield cable, using 113 mm length from traditional formula (V.F. x λ/2) effectively leads to higher electrical length of shield, possibly 120mm, giving swr around 2.5.

                              If you apply my formula to corrugated Heliax, and use pieces of 123 mm length, their electrical length will be much higher than 123 mm (say around 130 mm), and will knock-off the antenna's performance.
                              understood and this would explain why my result was not as far out, but neverless its far enough away that i know
                              i have a length problem .... the question is ... is it too long or too short.
                              I will VNA sweep it and see if i can find clues .... but there didnt appear to be an obvious dip in SWR...
                              im going to assume that ive made it too long ... eg traditional formula gives a
                              longer outcome because of the helix, so i will go shorter than using traditional formula.
                              ... by my calc ... every 1mm is approx 10MHz .. so i will try about 3mm less
                              Last edited by gregy; 2014-10-12, 20:05.

                              Comment


                              • Hi ab cd,

                                damn, compromised internet-anonymity by using the same rather unique username twice..
                                yes, this was me flying, except for the part with the limited visibility, that must have been someone else

                                cant wait to have a look at your revised files

                                Once i get around to building my next coco i'll make note of the measurements during construction and drop by a friend with an antenna analyzer to validate the calculations.

                                Comment

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