Analysis of Ultralow Power Radio Frequency Beamforming Using Transmission-Line Transformers and Tunable Passives

Abstract

Traditional methods for radio frequency (RF) beamforming require significant amounts of power, making them difficult to deploy in some low-power applications. A detailed analysis of RF beamforming utilizing transmission-line transformers (TLTs) and balanced impedance phase shifters (BIPSs) is presented. This technique enables ultralow-power RF beamformers by: 1) leveraging the RF feed lines to implement a low-loss TLT for signal combining and 2) using the balanced impedance tuning of shunt passives to implement simple, low-loss phase shifters. We highlight some of the key advantages of this beamforming approach for low-power systems and show how this idea can be expanded to arrays of more than two antenna elements by introducing a crossover switch into the phase shifters to extend the beam-steering range. In addition, a thorough comparison between the theoretical and measured performances of a simplified prototype system is provided for key system metrics, including loss and bandwidth. The prototype system is a fully passive 2-GHz two-element RF beamformer, utilizing TLTs and BIPS, realized with off-the-shelf components and a standard FR4 PCB.