Design of angle-selective surfaces

• Main Author: Chen, Zhenting
• Other Authors: Tan Eng Leong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2025
• Subjects: Engineering; Angle-selective surface; Frequency-selective surface; Metasurface
• Online Access: https://hdl.handle.net/10356/183060
• Institution: Nanyang Technological University

Angle-selective surface (ASS) is a structure that function as a spatial filter to select or manipulate the electromagnetic (EM) waves in the angular domain. For instance, an ASS with low-transmission & high-reflection (T-R) angular performance can allow EM waves under the normal incidence to transmit fully while reflecting them at large incident angles. The ASS with such properties makes it a potential device for secure communications, radome design, energy harvesting, spatial multiplexing, and anti-interference at a specific incidence angle. In particular, for antenna or radar systems, a dual-polarized ASS can effectively minimize or eliminate the side lobes, potentially improving detection accuracy. Various methods have been developed to achieve the angular selectivity. Traditional methods, such as utilizing optics, stacking multiple dielectric layers, and exploring Brewster modes, have faced limitations, including high profile, fabrication difficulty and using of unnatural materials. The investigation into the angular sensitivity of the frequency-selective surface (FSS) has seen a growing trend in the ASS design. This method shows promise for the development of ASS with low-profile, high angular selectivity, broad frequency bandwidth and polarization insensitivity. The current design approaches for achieving flexible manipulation of angular performance are quite limited. Firstly, while existing design methods for achieving angular selectivity with transmission and reflection can be applied to both TE- and TM-polarizations, creating a dual-polarized T-R ASS remains a significant challenge. Secondly, the existing ASS design with absorption and reflection (A-R type) is based on the loss of substrate, resulting in an extremely narrow frequency bandwidth and inadequate angular selectivity. Lastly, the methodology for designing ASS that combine transmission and absorption (T-A type) remains unexplored and undeveloped. In this thesis, the operating principle, design methodology, and implementation feasibility of the ASS based on multiple FSS layers have been thoroughly investigated. Firstly, we have successfully realized a dual-polarized ASS with T-R responses in the angular domain. The equivalent circuit analysis was applied to model the entire ASS structure, which was extended to the angular domain. From this analysis, the corresponding analytical formulas for the angular passband and stopband conditions are derived in terms of the susceptance of each FSS layer. Subsequently, a general design methodology for the T-R type ASS was developed to enable flexible manipulation of the angular response. Following this design guideline, two examples of the dual-polarized ASS with T-R and bandpass & out-of-band reflection (R-T-R) angular responses were designed. The T-R ASS was then mounted to a horn antenna for sidelobe suppression. It was observed that the sidelobes of the ASS-covered horn antenna were suppressed effectively, while the back lobe also increased simultaneously. One straightforward solution is attaching an absorber to absorb the energy of the back lobe directly. Alternatively, an ASS with transmission and absorption (T-A type) angular responses can be designed, which allows EM waves to pass at normal incidence while absorbing them at oblique incidence. Inspired by the need for T-R type angular responses, we achieved absorptive angular selectivity by incorporating chip resistors and proposed the ASS with absorption and reflection (A-R type) for simplicity. The A-R ASS consists of multiple lossy FSSs and a metallic ground. An equivalent circuit is constructed to model the entire structure, and analytical derivations for the reflection and absorption conditions of the two-layer ASS are performed, with an extension to ASS configurations comprising multiple layers. The analytical impedance formulas for the FSS based on strips and chip resistors are proposed and then extended to the angular domain. Subsequently, a general design methodology for the absorptive ASS is proposed, which enables the realization of flexible filtering performance within the angular domain. To validate the proposed design methodology, two examples of a low-reflection & high-absorption (R-A) ASS and a band-absorption & out-of-band reflection (R-A-R) ASS are provided. Next, inspired by the design method of traditional frequency-selective rasorbers, we investigate the design method of transmission-absorption (T-A) ASS based on a 2.5-dimensional (2.5-D) structure. Starting from the basic concept of EM wave behavior under TM-polarization at oblique incidence, it is observed that the electric field (E-field) component along the z-axis naturally increases with the incident angle. Leveraging this characteristic, a lossy layer based on a 2.5-D structure can be designed, which exhibits perfect transparency under normal incidence and some absorption when the incident angle is large. To achieve impedance matching for this lossy layer, a T-R ASS with a gradual transition from the angular passband to the stopband is proposed. This T-R ASS can serve as a “ground” layer, also showing high transmission under normal incidence but high reflection at large incident angles. The T-A angular response can be realized by stacking the lossy layer and the ground layer. In summary, we have studied the design methodology of ASS with flexible combinations of transmission, reflection, and absorption in the angular domain. We have constructed equivalent circuits for the ASS structures and extended them into the angular domain. Furthermore, we have derived the corresponding angular transmission band, reflection band, and absorption band conditions analytically, which can guide the design of each frequency selective surface (FSS) component in the ASS design. The proposed ASS design methodologies can be highly beneficial for modern radar systems.