Applying the N-Port Network Theory to Match a Symmetrical Four-Port Antenna to a Two-Port Source

A general approach to simultaneously match and decouple a symmetrical two-port, four-element antenna system by developing an appropriate six-port tuning network is presented. The six-port tuning network is passive and reciprocal and consists of four LC matching circuits and four transmission lines. The method was applied to a symmetrical four-port mobile antenna testbed, focusing on the challenging LTE band around 800 MHz. The topology of the tuning network is detailed and the values of the corresponding LC components are derived from the analytical/numerical solution set. The performance of the four-element, two-port system was characterized in terms of total multiport return loss, total efficiency, and diagram correlation factor, as well as multiplexing efficiency at each port. The proposed design strategy is a new, direct analytical approach to the simultaneous matching/decoupling problem, assisted by EM- and circuit-simulation. Antenna system efficiency is constrained only by the losses in the lumped components and the substrate material; using an FR4 substrate, the testbed achieved a total efficiency of 80%.