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Tweaking of Groundplane (Spider) Antenna

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  • Tweaking of Groundplane (Spider) Antenna

    It is well known that in a Groundplane antenna, the slope of radials affects its impedance, and hence affects SWR.

    1) If slope = 0 degrees (i.e. radials are horizontal), Antenna Impedance is around 30 ohms
    2) If slope = 45 degrees down (i.e. radials are slanting), Antenna Impedance is around 50 ohms
    3) If slope = 90 degrees down (i.e. radials are vertical), Antenna Impedance is around 75 ohms

    Before the advent of ADS-B, the Groundplane/Spider was mainly used by Hams. Since the defacto standard of ham transmitters & receivers is 50 ohms, the 45 degrees slanting Groundplane/Spider, which has an impedance of 50 ohms, well suited ham systems. Generally other antennas were also designed to have 50 ohm impedance, either inherently, or by use of impedance matching components. The coaxial cables used were also of 50 ohms rating.

    When ADS-B receiving started, its equipment (receivers antennas and coax) followed the already established ham standard of 50 ohms impedance. The sites like Flightaware, Flightradar24, Radarbox24, Planefinder also followed the suite and offered 50 ohm equipment.

    With introduction of DVB-T for ADS-B, the scenario changed. The DVB-T is designed for TV reception. The defacto standard for TV & Satellite is 75 ohms, hence the DVB-T and RG6 Coax both have impedance of 75 ohms.

    Unfortunately, the 50 ohms Groundplane/Spider with 45 degree slanting radials was BLINDLY used for 75 ohms DVB-T & RG6. Logically a 75 ohms Groundplane/Spider (i.e. the one with vertical radials) should suite the DVB-T+RG6 system.

    I have done some simulations, which support above arguments. However Antenna making without proper test equipment & technical know how is a "Dark Art". Hence the only way left to know if the simulation results are true, is to make prototypes, and put these on trial run. I intend to do this when I find time, as there are 4 models, and trial runs should be lengthy to be meaningful.

    Last edited by abcd567; 2016-03-13, 09:49.

  • #2
    But if they are pointing down don't you have a dipole antenna? which would then be 300 ohm...
    FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.


    • #3
      See the images and compare with your RTL antenna plug/port:

      mcxa.jpg mcxx.jpg
      Last edited by K5TED; 2016-03-14, 01:55.


      • #4
        Originally posted by F-EGLF1 View Post
        But if they are pointing down don't you have a dipole antenna? which would then be 300 ohm...
        True, it will be a dipole, but impedance of a dipole antenna depends on length of it's limbs.

        (1) If each limb is 1/4 wavelength it is a 1/2 wave dipole (tip to tip length = 2 x 1/4 wl =1/2 wl). The impedance of this dipole is about 90 ohms, and it's swr @ 75 ohm is nearly 1.8 . By trimming few mm, the impedance can be brought down close to 75 ohms, and swr can be brought down close to 1 (end effect).

        (2) If each limb is 1/2 wavelength it is a full-wave dipole (tip to tip length = 2 x 1/2 wl =1 wl). The impedance of this dipole is about 600 ohms, and it's swr @ 75 ohm is about 24.

        Please see simulation results of dipoles of different lengths:

        Last edited by abcd567; 2016-03-15, 15:12.