Phase Gradient Metasurface Radome Offering Beam Angle Translation and Wideband Absorption

A novel metasurface radome design is presented which combines the properties of a rasorber as well as a phase gradient metasurface (PGMS). By replacing the traditional frequency selective surface or lossless layer, in a radar absorber (i.e. a rasorber) with a PGMS, a new structure can be realised which provides dual-functionality in terms of both beam pattern control and wideband absorption. In particular, a 60 ∘ phase gradient metasurface is designed which is composed of six different unit cells (with the same periodicity) while being placed a quarter-wavelength below two lossy (or resistive) layers. By this stack-up configuration, the radome structure supports complimentary beam steering translation whilst providing absorption bands from about 1.3 GHz to 5.5 GHz and 6.1 GHz to beyond 10 GHz. This design, to the best knowledge of the authors, is the first example of a phase gradient metasurface rasorber (PGMSR) and has many interesting applications for future multi-functional radomes. It can also help to reduce the requirements for mechanical steering, antenna beamformers as well as array phase shifting networks

Miniaturized-Element Frequency-Selective Rasorber Design Using Characteristic Modes Analysis

A dual-polarization frequency-selective rasorber with two absorptive bands at both sides of a passband is presented. Based on the characteristic mode analysis, a circuit analog absorber is designed using a lossy FSS that consists of miniaturized meander lines and lumped resistors. The positions and values of resistors are determined according to the analysis of modal significances and modal current. After that, the presented rasorber is designed by cascading of the lossy FSS and a lossless bandpass FSS. Equivalent circuits of the frequency-selective rasorber are modelled, and surface current distributions of both FSSs are illustrated to explain the operation mechanism. Measurement results show that, under the normal incidence, a minimum insertion loss of 0.27 dB is achieved at a passband around 6 GHz, and the absorption bands with an absorption rate higher than 80% are 2.5 to 4.6 GHz in the lower band and 7.7 to 12 GHz in the higher band, respectively. Our results exhibit good agreements between measurements and simulations

A Reflecting/Absorbing Dual-Mode Textile Metasurface Design

A textile-based reflecting/absorbing dual-mode metasurface is proposed in this paper. For the reflecting mode of the design, a conventional square patch electromagnetic band gap (EBG) structure is adopted and the zero-degree refection phase center is tuned to 2.4 GHz. For the absorbing mode, a carbon-coated resistive net is applied on top of the EBG patches to redirect the current flow at resonance and hence achieve energy dissipation with the resistance. The underlying reconfigurable logic is analyzed with a dispersion diagram, surface current distribution, and equivalent circuit/impedance matching analysis. By applying a state-of-the-art AI-driven antenna design technique, self-adaptive Bayesian neural network surrogate model-assisted differential evolution for antenna optimization (SB-SADEA) method, the geometry parameters can be accurately determined meanwhile maintaining absorption and reflection band of the design centered at the same frequency. The fabricated prototype of the design can achieve a maximal absorption of 99.8% (-29.2 dB) and maintain an absorption over 90% in the frequency range of 2.39 to 2.42 GHz. To verify the reflection properties, a textile monopole antenna was fabricated and tested along with the reflection metasurface. A 5-dB realized gain enhancement can be achieved at 2.4 GHz with the applied metasurface. Both simulations and measurements verify the effectiveness of the proposed dual-mode metasurface design

Frequency-Selective Surfaces for Microwave and Terahertz Spectra

FREQUENCYselective surfaces (FSSs) made of subwavelength periodic structures have been broadly applied in various electromagnetic applications. Their main function is to tailor the frequency response to incident waves, or to obtain electromagnetic (EM) properties that do not exist in homogeneous natural materials. When increasing the design complexity to enhance performance, however, the computation cost hikes dramatically in analysis and synthesis as additional design variables are introduced. In contrast to such complexity increase, this thesis aims at systematically developing effective and efficient design and optimization approaches for FSS-based structures adopting fundamental unit-cell patterns, such as rectangular patches, rings and grids. Additionally, impedance matching to free space is thoroughly investigated and adapted as a means towards performance improvement in both absorbers and filters. Hereby, multiple designs are demonstrated with realizations from the microwave to the terahertz (THz) frequency spectrum. In spite of their simplicity, the proposed designs outperform the state-of-the-art counterparts in the literature by fully exhausting the potentials of their spatial structures and material attributes. Specifically, Chapter 3 challenges a common belief that adding an impedance matching superstrate to an absorber will broaden its operation bandwidth at the cost of increased total thickness profile. This Chapter proves that it is possible to increase the bandwidth-to-thickness ratio. The concept is firstly demonstrated at the circuit level, and then verified by full-wave simulations. The optimization process can be illustrated with an admittance Smith chart. The distinctive performance of the proposed single- FSS-layer absorber is justified with a figure of merit (FoM) which comprehensively involves the relative bandwidth, the normalized thickness and the level of reflectivity. In Chapter 4, a semi-analytical approach for absorber design is developed by systematically combining analytical, empirical and numerical techniques. The optimization space can be simplified from millions of exhaustive search cases to merely a few hundreds of seed simulations, by exploiting insights into the linearity, scalability and independence regarding the major components of an absorber. For any specified level of absorption and operation bandwidth, the obtained semi-analytical algorithm enables fast synthesis of an absorber with a minimal thickness. Both absorbers proposed in the above chapters have been realized using patterned resistive layers and experimentally validated under oblique angles of incidence for transverse-electric (TE) and transversemagnetic (TM) modes. The design methods can be readily expanded for structures of multiple FSS layers. In the terahertz frequency range, common microfabrication technologies do not accommodate those resistive inks used for silk-printing lossy FSS patterns. As an alternative, a sub-skin-depth metal layer with nanoscale thickness is proposed in Chapter 5 to meet this requirement. The Drude model is adopted to simulate the ultra-thin metallic FSS, as it satisfactorily describes the frequency dependent properties of noble metals. The proposed absorber is robust to dimensional tolerance in fabrication and attains a stable absorption bandwidth under oblique impinging waves. In Chapter 6, a frequency reconfigurable terahertz bandpass filter is proposed and experimentally verified. It includes two identical double-layer FSSs separated by an air spacer which can be mechanically tuned. The filter allows a highly selective transmission sweeping across a wide spectrum. The underlying mechanism can be explained from two perspectives, namely through interpretation as Fabry-Perot resonant cavity and through consideration of the admittance Smith chart. The designed device is insensitive to fabrication tolerances and stable to oblique angle of incidence. The fabricated filter confirms a 40% tuning range and less than 1 dB insertion loss. This design is among the first few practical reconfigurable terahertz bandpass filters reported in the literature. Overall, the research outcomes suggest that developing complicated FSS patterns with a large number of degrees of freedom is unnecessary in many cases if the potential of fundamental geometries is fully exploited through rigorous algorithmic optimization methods. The design approaches illustrated in this thesis are generic to all FSS-based structures and can potentially be extended to multi-FSS layers and impedance surfaces, to satisfy performance requirements in specific applications.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 202

Investigation of Radar Signal Interaction with Crossflow Turbine for Aviation Application

The increased adoption of wind energy is an important part of the push towards a net zero-emission economy. One obstacle that stands in the way of a higher rate of wind energy adoption is the interference that wind turbines cause to nearby radar installations. Wind turbines negatively affect the performance of nearby radar sites in a variety of different ways. Almost all types of radar are affected in at least one of these ways.In order to understand the degree to which an object such as a wind turbine interacts with radar, it is important to have detailed radar cross section (RCS) data for the object. In this work, a novel, low-cost, scale model radar cross section characterization system is presented with various advantages over traditional designs. This system was used to characterize the RCS of the novel Crossflow wind turbine. Additionally, work has been carried out on the characterization of metamaterial absorber coatings that can be applied to new and existing turbines for the purposes of reducing their radar cross section and the degree to which they cause radar inter-ference. The works presented can be leveraged to reduce concerns around radar interference from wind turbines, as well as to iteratively generate ge-ometries with lower radar cross sections for the aviation and infrastructure sectors, ultimately accelerating the pace of wind energy adoption and the move towards a net zero-emission economy

Artificial materials for microwave applications

This thesis has focussed on the properties and manufacturing techniques of artificial RF materials. These artificial materials can be divided into two types depending on the whether their individual unit cell is resonant or non-resonant. Both these types have been discussed. It has been shown that the efficiency and bandwidth of a patch antenna using a flexible 3D printed substrate can be improved by using composite materials as heterogeneous substrates. Composite materials with a large range of relative permittivity values were manufactured by combing 3D printing with commercial laminates. An equation to design such composite materials has been presented. The engineering tolerance and repeatability of 3D printing as a manufacturing process to fabricate ‘on demand’ dielectrics has been presented. [Continues.

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium