Plane Wave Generator (PWG) has recently attracted great attention from industry and academia for over-the-air (OTA) testing of base station (BS) antennas in the fifth-generation (5G) wireless communication systems. This paper aims to reduce the scattering from the PWG to the antenna under test (AUT), which might be problematic in the near-field OTA testing. First, we introduce a low monostatic radar cross section (RCS) PWG array element design. The PWG is also loaded with pyramidal microwave absorbing material (MAM) to suppress the multi-reflections between the AUT and the PWG. Furthermore, unlike coplanar PWG design generally reported in the literature, a novel non-coplanar design is proposed to realize the destructive interference of the scattered signals from the PWG, thereby significantly reducing the multiple reflections. PWG elements in the non-coplanar design are placed in the propagation direction according to planar and non-planar field distribution on the PWG radiated from the BS AUT. To validate our proposed design, a 4×4 PWG array with a 108 mm element spacing and a non-coplanar structure is developed and experimentally validated. The measured results show that the second incident wave of the proposed non-coplanar PWG array structure based on non-planar field distribution is 7.9 dB lower than that of the classical coplanar PWG design from 2.3 GHz to 3.8 GHz, and 1.9 dB lower than that of the proposed non-coplanar PWG array structure based on planar field distribution, which demonstrates the effectiveness and robustness of our proposed design. INDEX TERMS 5G, OTA testing, PWG, low scattering design, probe antenna design, non-coplanar PWG structure. I. INTRODUCTION ASSIVE multiple-input multiple-output (MIMO) is one of the key technologies employed at the base stations (BSs) in the sub-6 GHz 5G wireless communication systems [1], [2]. Accurate measurement of the massive MIMO BS antenna systems is essential in the performance and conformance testing. Over-the-air (OTA) testing is a competitive method to evaluate BS antennas due to its advantage of convenient operation and cost savings, compared to conventional cable conducted testing [3], [4]. The plane-wave generator (PWG), which aims to approximate a plane wave in the quiet zone with a nearfield distance, has attracted great interest recently [5]-[10]. Compared with the traditional antenna measurement methods (e.g., near-field to far field transformation, direct far-field and compact antenna testing range (CATR)), the PWG has the advantages of compact size, fast measurement, low-cost, and support for both continuous wave (CW) and modulated signal measurement. The PWG has a good potential to support for massive MIMO BS antenna parameter measurement, array calibration, radio frequency (RF) transmit and receive performance measurements. Extensive works have been reported on the PWG design in the literature, e.g. algorithms to determine the complex excitation coefficients for the PWG element [11]-[13], and guidance to design a good PWG [14]. Multiple refle...
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