Sidelobe Modification for an Offset Gregorian Reflector System using a Reconfigurable Intelligent Surface-Equipped Subreflector

TL;DR

This paper demonstrates that a reconfigurable intelligent surface on the subreflector of an offset Gregorian reflector can effectively nullify sidelobes, reducing interference with minimal impact on main lobe directivity, suitable for radio astronomy applications.

Contribution

It introduces a novel RIS placement on the subreflector of an offset Gregorian system, enabling sidelobe nulling with passive elements and simple optimization, improving upon previous main reflector approaches.

Findings

Null in second sidelobe achieved with 0.2 dB main lobe reduction

Passive 1-bit phase control suffices for effective sidelobe suppression

Subreflector RIS requires smaller size and easier implementation

Abstract

In past work, we described the use of a reconfigurable intelligent surface (RIS) mounted on the rim of an axisymmetric prime focus-fed reflector to create nulls in the close-in sidelobes. In this paper, we show that similar performance is possible in an offset Gregorian reflector system using a RIS on the rim of the subreflector. Applications include radio astronomy, where offset Gregorian reflectors are common and observations are subject to deleterious levels of interference from satellites entering through sidelobes. We show that an efficient RIS replacing the outer one-third of the subreflector surface, employing passive elements with 1-bit phase-only control, can create a null in the peak of the second sidelobe in the quiescent pattern. This is achieved using a simple unconstrained optimization algorithm to set the states of the RIS elements. The algorithm yields a deep null with just 0.2~dB reduction in main lobe directivity, despite lacking any constraints on main lobe pattern. Compared to our previous approach of mounting the RIS on the rim of the main reflector, the subreflector-based approach demonstrated in this paper requires a much smaller RIS and can implemented in existing systems by replacing the subreflector.