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

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: Mar 13, 2016