Wideband Feeding Network Design For Dual-Polarized Connected Arrays

Abstract

Phased arrays have emerged as a key solution for 5G base stations, to provide higher capacity by means of directive and electronically steerable beams. However, the implementation of workable low-cost antenna arrays for base stations is very challenging, because of the requirements on the total frequency and angular coverage of 5G systems. When many frequency bands are used for different services, having a single narrowband antenna for each sub-band becomes unfeasible for cost and space occupation. For this reason wideband arrays that can cover simultaneously multiple bands are gaining interest. Moreover, a large field of view is required, i.e. scanning at least from -60° to +60°, so that only a few array panels can cover the entire azimuthal angle of 360°. This thesis work has been performed in the framework of a collaboration between Terahertz Sensing Group and HUAWEI, aiming at the development of a wideband phased array for base station. Two array designs are targeted, one covering 6-8 GHz and another for 2-8 GHz. Both designs are required to achieve wide scanning capability up to 60° in all azimuth planes. This thesis focuses on the design of the feeding structure of the array unit cell and the corporate feeding networks. The unit cell feeding structure is based on integrated coaxial lines connected to microstrips or striplines. The performance is analysed first for the stand-alone transition and then for the same feed together with the connected slot element. The entire unit cell including the feed has comparable matching performance with the ideal one without feeding structure. An alternative feed design is also presented, where the input is realized with a coaxial SMP connector. The design of a corporate feeding network for the array is introduced. This consists of two designs of 1-to-32 power dividers, one implemented with microstrip transmission lines and the other with striplines. Such dividers are meant to feed in phase an entire row or column of the array. The highlights of this design is wide bandwidth, which covers two octaves by means of wideband multi-section and tapered impedance transformers, and the compactness, since the entire divider has to fit in an area of 480 mm × 15 mm. Moreover a novel feeding strategy is proposed to simplify the complexity and the costs of the unit cell. The new approach is based on replacing the coaxial lines with integrated parallel plate waveguides (PPW). Two examples of unit cell design are presented: one consists in a single-polarized array of connected slot, covering the band from 70 GHz to 140 GHz, for automotive radars; another example is referring to a dual-polarized connected slot array covering the same 2-8 GHz range, for wideband base stations. The design procedure of a PPW and cavity is easier than that of the integrated coaxial, as the feed and the radiating slot design are better decoupled. The number of layers of the ADL is also reduced with respect to the integrate coaxial, because part of the impedance transformation is implemented in the PPW.