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

S. W. Ellingson, A. J. Yip

TL;DR

The paper addresses satellite interference in radio-astronomy observations by leveraging a reconfigurable intelligent surface mounted on the rim of a subreflector within an offset Gregorian reflector. It introduces a simple, greedy 1-bit phase-only state-setting algorithm to drive a null in the peak of the second sidelobe with negligible main-lobe impact, demonstrated on a 20 m main reflector and 0.3 m-wide subreflector rim using 282 RIS elements. The key contribution is showing that a relatively small RIS on the subreflector can achieve deep sidelobe nulls (null at 1.8 degrees) with only a 0.2 dB loss in main-lobe directivity, making retrofit practical for existing systems. The approach promises practical interference mitigation for radio astronomy by enabling targeted sidelobe suppression without requiring large-scale alterations to the primary reflector.

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.

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

TL;DR

The paper addresses satellite interference in radio-astronomy observations by leveraging a reconfigurable intelligent surface mounted on the rim of a subreflector within an offset Gregorian reflector. It introduces a simple, greedy 1-bit phase-only state-setting algorithm to drive a null in the peak of the second sidelobe with negligible main-lobe impact, demonstrated on a 20 m main reflector and 0.3 m-wide subreflector rim using 282 RIS elements. The key contribution is showing that a relatively small RIS on the subreflector can achieve deep sidelobe nulls (null at 1.8 degrees) with only a 0.2 dB loss in main-lobe directivity, making retrofit practical for existing systems. The approach promises practical interference mitigation for radio astronomy by enabling targeted sidelobe suppression without requiring large-scale alterations to the primary reflector.

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.
Paper Structure (6 sections, 3 figures)

This paper contains 6 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Reference System
  3. Method of Analysis
  4. State-Setting Algorithm
  5. Demonstration of the RIS-Equipped Subreflector
  6. Conclusions

Figures (3)

  • Figure 1: System considered in this paper. Boresight is in the $+z$ direction. Left: Side view (cross-section in the $x=0$ plane). The common focus is at the global origin, and the feed location is the circle marker near $(y,z)=(0,-5)$ m. Right: Front view (projection into the $z=0$ plane). The rim of the main reflector is in black. The original (non-reconfigurable) and RIS portions of the subreflector are in red and blue, respectively.
  • Figure 2: $H$-plane copolarized patterns of (a) the original (reference) system and (b) the RIS-equipped system driving a null at $1.8^{\circ}$ using the algorithm defined in Section \ref{['sSSA']}.
  • Figure 3: Front view (projection into the $z=0$ plane) of the subreflector. Circle markers indicate the locations of the reconfigurable elements comprising the RIS. The red-filled markers indicate the elements for which $c_n=-1$ in order to drive the null at $1.8^{\circ}$ as shown in Figure \ref{['fEx1']}.