Antenna Pattern Reconstruction Based on Mid-field Phaseless Measured Data Using Simplified Extrapolation Technique

With the rapid development and commercialization of 5G technology, calibration and RF testing using over-the-air (OTA) is becoming increasingly important. Due to the large size of the massive MIMO base station, it is very challenging to fulfill direct far-field (DFF) conditions in OTA tests. Conventional near-field (NF) testing methods suffer from long measurement times due to the stringent sampling requirements (e.g. NF to far-field (FF) transformation) or the requirement for expensive test equipment (e.g. compact antenna test range). In this paper, we propose a simplified extrapolation technique (ET) strategy to reconstruct FF radiation pattern based on mid-field (MF) phaseless measurements, with high measurement efficiency. The traditional ET uses polynomials with many terms to fit the radiated power of an arbitrary antenna at the radial direction, while the simplified ET only use few terms to fit the power variation in the MF, taking into account the structure and element excitation characteristics of common arrays such as base stations. The relationship between the element excitation mode and the coefficients of each fitted term is determined by detailed theoretical derivations. Then, the best-fitting polynomials are determined for different excitation modes. The antenna pattern of the base station is successfully reconstructed in an anechoic chamber by power measurements at two or three different distances of less than 1/4 of the FF distance. The robustness and effectiveness of the proposed method is experimentally verified by comparing the accuracy of the antenna pattern reconstruction with different fitting polynomials in the anechoic chamber for a practical base station antenna.