Pre-Phased 1-bit Reflective Passive Metasurface With Optimum Number of Pre-Phasing for Low QLL and SLL at 140 GHz

Abstract

Recent advancements introduced a pre-phased reflective metasurface (MS) using 1-bit control to suppress the quantization lobe (QL) inherent in a typical 1-bit MS under plane wave incidence, particularly in millimeter and submillimeter wave bands. However, the optimal method for pre-phasing using random phases has yet to be fully explored. This study investigates the optimal number of pre-phasing, which denotes the minimum number of discrete random phases for effective suppression of QLs and side lobes while ensuring low design complexity. The results indicate that employing four discrete phases, equally spaced at 45° intervals, is the optimal solution for both normal and oblique incidences. The random phases with a 45° separation are implemented into four pairs of unit cells with passiveon/off switching using multilayer PCBs at 140 GHz. Each unit cell consisted of a metallic patch coupled to a transmission (Tx) line through a slot, with embedded staticon/off switching in the Tx line to enable 1-bit reflection phase control. Finally, three MSs with $30\times 30$ unit cells, each with fixed random pre-phases, are designed and fabricated to reflect normal incidence at reflection angles ( $\theta _{r}$ ) of 15°, 30°, and 45° at 140 GHz. The experiment results demonstrate peak reflected beams at 15°, 29°, and 45° with frequencies of 139.7, 139.1, and 135.9 GHz, respectively. In addition, significant reductions in QL to −20 dB were observed, along with a reflection efficiency of 26%, and a 3 dB gain bandwidth exceeding 10%, on average, across all reflection angles in the 140 GHz band.