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Quarter wave ground plane

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  • Quarter wave ground plane

    This is my first try with a quarter wave ground plane for ADS-B. I already use a colinear antenna but wanted to go back to basics.

    I used the m0ukd 1/4 wave ground plane antenna calculator with follwing parameters : centered on 1090 MHZ by taking a 99% Velocity factor. probably some margin on VF, but an ability to cut down the length if adjustemnt are needed.

    68 mm for radiating element
    76 mm for radials

    I built the antenna around a BNC connector and a 3D printed support with threads for M4 Brass rods.


    After mesurements with my NanoVNA, the best VSWR is located @1 136 Mhz. By calculation, the VF is probably more around 96% than 99%.
    S11 VSWR.png

    So i ajusted the length by filing down the radiating element to 64 mm, but the resonant frequency seems to be stuck @1 134 MHz
    Even after filing the radials to 74mm, the VSWR curve does not change.
    S11 3.png

    Shortening the elements should have a direct impact on the resonant frequency... Am I missing something ?

  • #2
    The radials have little influence on the resonant frequency, the whip is the main factor for that.


    • #3
      Originally posted by wiedehopf View Post
      The radials have little influence on the resonant frequency, the whip is the main factor for that.
      Many thanks for your feedback !

      I tried to cut down further the radiating element without any good result...

      But after investigation, i seems that the low VSWR @1130 MHz is generated by the BNC connector. Indeed, i obtain the same curve with the bnc connector only...
      It seems that my radiating element is ignored and have no effect...

      The brass rod is only pushed on the needle of the BNC connector with the thread in my 3D printed support. I checked with an ohmmeter, and the contact is good (4.3 Ohms from top of the rod to the center plot of BNC connector).
      Dou you think it needs any solder or BNC needle flattering to endure a better contact ?


      • #4
        4.3 ohm is NOT good.

        Also DC resistance isn't the same thing as impedance.
        File the brass rod down so it fits inside the connector.

        But anything but soldering is gonna suffer from exposure to elements.
        So soldering all connections is the best way to go.

        If you're having issues with this thing, consider just making something like this:

        Really i'd just spend some money on a good antenna and be done with if it's for permanent outdoor use.
        adsbexchange antenna in the US, vinnant antenna in the EU is the way i'd go.
        Last edited by wiedehopf; 2022-09-24, 10:00.