HELP for 2,4 GHz Helix Antenna .... an expert ?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by IW2BSF, Nov 20, 2019.

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  1. IW2BSF

    IW2BSF Ham Member QRZ Page

    a fellow amateur radio asks you for help, who knows how to help?

    "If you want to make a 3-turn propeller antenna at 2.405 Ghz, for the frequency of the Oscar 100, to illuminate a parabola, you should perform this procedure: First you need to find an On-Line calculator for the various dimensions, so you save yourself the time to apply the formulas of the Hanbook Amateur Radio.

    The program says to wrap the coils on an external diameter of a 39.7 mm wide tube and the literature says that this type of antenna has a characteristic impedance of about 140 Ohm, in fact then you have to put an impedance adapter to Lamba Quarters , to be able to combine the 140 Ohms to the 50 Ohms.

    Now I would not like to discuss how to make the adapter with a coaxial line turned like the one I made, or with the various strips or tubes put on the first spire.

    Now that all the premises are over, I can go straight to talking to you about the problem.

    Taking care of the On-Line calculator, I wrapped a quantity of coils of 3 coils each close to the calculation values both in terms of diameter and spacing.

    This is because in this way I was hoping to find a point on the Smith caste, where the resonance intersected the horizontal line, that of pure resistance, on the frequency of my interest.

    Obviously I could not hope to find that the impedance of the helix was 50 pure Ohms without the stub, in fact I found various values where there were three different resonances, but close together with high frequency and high Ohmmic values.

    Suppose you had found a resonance on a frequency of my interest, even with the lathe I would have been able to calculate and return an impedance adapter to Lambda Quarters to return the output to 50 Ohm.

    Now the problem with all this ugly business and that made me stop trying, was because although I had wrapped a considerable amount of reels, I wasn't able to make one at 2.405 Ghz as I wanted.

    Moreover, despite having tried to increase the diameter value a little, I could not understand the progress of this parameter and the values found changed strangely without any rule.

    My hope was to wrap anything that resonated on that frequency and perhaps with its own impedance due to various factors, then once measured, it would have been easy to adapt to it and instead I had to drop everything.

    Then I made one that I gave to a colleague of mine here in Rome, taking the values from a calculator and making him the turned stub and I managed to make him even -42 dB of Return Loss, right at 2.405 Ghz!

    But this thing didn't convince me, because once you make it into a stub, in some way you change the cards on the table and in the end you can always see dell beautiful deep holes, because in any case the stub goes somehow to modify the resonance frequency and in the end intruding in any way you can bring it where you want.

    Paradoxically I could have made an antenna that resonated at 2.6 Ghz and then adapted impedance with a stub, to find a perfect adaptation on the frequency of my interest.

    The fact of having an antenna that resonates on one side and adapted from another, is certainly a malevolent thing in terms of irradiation of the same and maybe the performance of the system can collapse quite a lot, despite the vector gave me an excellent Return Loss .

    I asked various people what loss there could be in making an antenna resonate on one side and adapted from another and obviously I had no answers from anyone!

    Then I did not understand yet another thing: On those plotted if you wade through the three markers you see three different resonances and if they were on a frequency of my liking, which of the three had to be taken into consideration, despite being all on the resistive zone ? "

    thanks from you ... 73
  2. W4OP

    W4OP Ham Member QRZ Page

    There are much better feeds than a helical for parabolic dishes, but unless you give us the dish diameter and its F/D it is impossible to say what feed to use.

    Dale W4OP
  3. AE8W

    AE8W Ham Member QRZ Page

    Back when 2 GHz was a commercial point to point band, I covered the Commenwealth of Pennsyltucky with 2 to 2.5 GHz microwave links all using Anixter-Mark grid dishes. At least 150 links. *All* feedhorns were dipoles stuck out infront of the dish. Dishes ranged from 4' to maybe 12' ... a whole lot of 6' & 10' (or maybe 8') so the only variable was the length of the hardline from the Helix Type N to the dipole.

    Dipole that is a short stubby thing, will work fine in other words.
    IW2BSF likes this.
  4. W4OP

    W4OP Ham Member QRZ Page

    The dipole may work fine but it is not taking advantage of the entire surface and will also spill over and 'see' warm earth- degrading the G/T. Look at what the EME gang is using. And apparently, the OP requires circular polarization- achieved with a patch feed. Google SM6FHZ patch feed. Little blockage and excellent G/T.

    Dale W4OP
    IW2BSF likes this.
  5. IW2BSF

    IW2BSF Ham Member QRZ Page

    thanks for you reply and help ! this the antenna of the collegue :


    and ...... the plotters .....

  6. W4OP

    W4OP Ham Member QRZ Page

    Very nice, but this tells us nothing as to how efficiently it illuminates the dish.

    Dale W4OP
    IW2BSF likes this.
  7. IW2BSF

    IW2BSF Ham Member QRZ Page

    yes great job ! NO here a VERY EXPERT collegue that reply to the questions for this ham-radio for this elix antenna ???

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