Research on unwanted reflections in an OATS for precise omni antenna measurement

Open-area test site (OATS) is a basic facility for measuring omni-directional antennas at VHF/HUF band. Usually, the reflections from the trees nearby, from the edge of the metal ground plane of an OATS are detrimental to precise omni-directional antenna measurements from 20 MHz to 1 GHz; however they are very difficult to quantify accurately, since no rigorous theory on the relationship between the reflections and the configurations of an OATS. In order to study these unwanted reflections, a pair of precise ultra-broadband calculable dipole antennas (CDAs) are developed and verified with a slightly modified near-field method, whose site-insertion-loss deviation (ΔSIL) between measurements and simulation is less than 0.3 dB from 10 MHz to 340 MHz for a single pair of dipoles resonated at 90 MHz; then the effects of ground plane sizes, wire mesh shapes around the edge of the metal ground plane, trees nearby are researched. The research shows that these unwanted reflections from the edge of an optimized ground plane is less than 0.1 dB at 10 m range. Finally, the performance of an optimized OATS is validated: for 10 m separation, ΔSIL is within 0.26 dB at horizontal polarization (HP) and within 0.34 dB at vertical polarization (VP) for the typical 24 frequencies from 30 MHz to 1 GHz; at 20 m separation, ΔSIL is within 0.59 dB (HP) and 0.85 dB (VP) from 20 MHz to 500 MHz. An uncertainty of calibrating tuned dipole antennas are provided, too.