MmWave Yagi-Uda Element and Array on Liquid Crystal Polymer for 5G

Abstract

Arrays on personal terminals are feasible using mmWave frequencies, but are limited by mutual coupling. A standard specification for MIMO is −15 dB coupling which can be readily attained over a modest bandwidth, and decoupling modifications can reduce this to less than −20 dB. Such a change has relatively little impact on narrowband MIMO performance but is significant for wideband phased arrays, demonstrated here using simulation and measurement of a new design. For the element, we present a Yagi-Uda structure on PCB with a bandwidth from 22 to 44 GHz to cover a set of standard mmWave bands for 5G. Our prototype uses the RFIC packaging material Liquid Crystal Polymer which is suitable for Antenna-in-Package technology. For the array, the element spacing is set small enough to suppress grating lobes at the highest frequency. At the lowest frequencies, the lowest electrical spacing means the highest mutual coupling, and we apply a decoupling modification for this. The associated improvements in the impedance bandwidth, scan range, gain and radiation efficiency are presented for edge- and corner-mounted arrays designed for a mobile terminal such as a cellphone. Our array combining comprises simulation-based signal phasing and the simpler beam switching using element selection. The edge-mounted array has a 59% TARC bandwidth (24–44 GHz) over which the scan range is between ±60° and ±35°, the gain is 9.5 to 13 dBi with a radiation efficiency better than −0.2 dB. The corner-mounted switched array has peak gains of 6 to 9 dBi, showing the gain penalty associated with the simpler switched beam architecture. Finally, the impact of a cellphone chassis on the antenna performance is assessed by simulation.