Tilted beam fabry-perot antenna with enhanced gain and broadband low backscattering

Communication with low radar signature platforms requires antennas with low backscatter, to uphold the low observability attribute of the platforms. In this work, we present the design for a Fabry–Perot (F-P) cavity antenna with low monostatic radar cross section (RCS) and enhanced gain. In addition, peak radiation is tilted inthe elevation plane. This is achieved by incorporating phase gradient metasurface (PGM) with absorptive frequency selective surface (FSS). The periodic surface of metallic square loops with lumped resistors forms the absorptive surface, placed on top of a partially reflecting surface (PRS) with an intervening air gap. The double-sided PRS consists of uniform metallic patches etched in a periodic fashion on its upper side. The bottom surface consists of variable-sized metallic patches, to realize phase gradient. The superstrate assembly is placed at about half free space wavelength above the patch antenna resonating at 6.6 GHz. The antenna’s ground plane and PRS together construct the F-P cavity. A peak gain of 11.5 dBi is achieved at 13◦ tilt of the elevation plane. Wideband RCS reduction is achieved, spanning 5.6–16 GHz, for x-and y-polarizations of normally incident plane wave. The average RCS reduction is 13 dB. Simulation results with experimental verifications are presented

Quarter wavelength fabry–perot cavity antenna with wideband low monostatic radar cross section and off-broadside peak radiation

Since antennas are strong radar targets, their radar cross section (RCS) reduction and radiation enhancement is of utmost necessity, particularly for stealth platforms. This work proposes the design of a Fabry–Perot Cavity (FPC) antenna which has wideband low monostatic RCS. While in the transmission mode, not only is gain enhancement achieved, but radiation beam is also deflected in the elevation plane. Moreover, the design is low-profile, i.e., the cavity height is ~λ/4. A patch antenna designed at 6 GHz serves as the excitation source of the cavity constructed between the metallic ground plane and superstrate. The superstrate structure is formed with absorptive frequency selective surface (AFSS) in conjunction with dual-sided partially reflective surface (PRS). Resistor loaded metallic rings serve as the AFSS, while PRS is constructed from inductive gradated mesh structure on one side to realize phase gradient for beam deflection; the other side has fixed capacitive elements. Results show that wideband RCS reduction was achieved from 4–16 GHz, with average RCS reduction of about 8.5 dB over the reference patch antenna. Off-broadside peak radiation at −38◦ was achieved, with gain approaching ~9.4 dB. Simulation and measurement results are presented

Wideband High-Gain and Low Scattering Antenna Using Shared-Aperture Metamaterial Superstrate

In this paper, a novel wideband high-gain and low scattering antenna using a shared-aperture metamaterial superstrate (SAMS) is designed, fabricated and measured. The superstrate unit cell consisting of two frequency selective surface (FSS) layers with a positive reflection phase gradient is designed to enhance the antenna gain. Then, three different sizes of single units are arranged as a shared-aperture configuration to form the metamaterial superstrate, which is loaded onto the antenna. By utilizing the phase compensation property along different units, the antenna gain enhancement bandwidth is effectively broadened. By adjusting the SAMS loading height, the antenna radar cross section (RCS) is also reduced obviously owing to the different reflective wave phases of PRS and antenna ground. After loading SAMS, the antenna possesses an impedance bandwidth of 44.7% from 7.8GHz to 12.3GHz, covering the whole X band. From 7.9GHz to 12.1GHz, the antenna has an obvious gain enhancement, with a peak of 7dB, meanwhile, the antenna RCS is effectively reduced from 4GHz to 12GHz and the maximum RCS reduction reaches 25.4dB at 8.6GHz for x-polarized incident wave and 15.8dB for y-polarized incident wave. The results are validated by both numerical simulation and experimental measurements. Comparing with traditional fabry-perot (FP) antenna, SAMS can effectively broaden the gain enhancement bandwidth and reduce the antenna RCS, it has great application values in designing high-gain and low scattering antennas

Performance Enhancement of Radiation and Scattering Properties of Circularly Polarized Antennas Using Frequency Selective Surface

At millimetre-wave (MMW) frequencies, losses associated with wireless link and system are critical issues that need to be overcome in designing high-performance wireless systems. To compensate the overall loss in a wireless communication system, a high-gain antenna is required. Circularly polarized (CP) antennas are among preferred choices to design because they offer many advantages due to their good resistance to polarization mismatch, mitigation of multipath effects, and some phasing issues and immunity to Faraday rotation. On the other hand, frequency selective surface (FSS) technology is recently employed to enhance the performance of radiation and scattering properties of antennas used in different sectors such as aerospace, medical, and microwave industry. Therefore, it is appropriate and attractive to propose the use of FSS technology to design practical and efficient CP antennas. CP Fabry-Perot cavity (FPC) antennas based on FSS are investigated in this thesis to fulfil the growing demand for broadband high-gain antennas with low radar cross section (RCS). The thesis investigates both characteristic improvement of CP antennas and RCS reduction issues employing FSS structures. Initially, a high gain CP dielectric resonator (DR) antenna is proposed. Using an FSS superstrate layer, a gain enhancement of 8.5 dB is achieved. A detailed theoretical analysis along with different models are presented and used to optimize the superstrate size and the air gap height between the antenna and superstrate layer. The second research theme focusses on developing an effective approach for mitigating the near-field coupling between four-port CP antennas in a Multiple-Input, Multiple-Output (MIMO) system. This is obtained by incorporating a two-layer transmission-type FSS superstrate based on planar crossed-dipole metal strips. Another technique for suppressing the spatially coupling between DR antennas using a new FSS polarization-rotator wall is studied as well. The coupling reduction is achieved by embedding an FSS wall between two DRAs, which are placed in the H-plane. Utilizing this FSS wall, the TE modes of the antennas become orthogonal, which reduces the spatially coupling between the two DRAs. The third research theme of this thesis is to enhance the purity and bandwidth of CP with the least amount of insertion loss by the use of an LP-to-CP-polarizer which is based on multilayer FSS slab. This polarizer is approximately robust under oblique illuminations. To have a high-gain CP antenna, an 8-element LP array antenna with Chebyshev tapered distribution is designed and integrated with the polarizer. Eventually, in order to enhance the scattering property, the fourth research theme investigates on RCS reduction by the use of two different approaches which are based on FSS. Initially, a wideband FSS metasurface for RCS reduction based on a polarization conversion is proposed. To distribute the scattered EM waves and suppress the maximum bistatic RCS of the metasurface over a broad band of incident angles at both polarizations, the elements are arranged using the binary coding matrix achieved by group search optimization (GSO) algorithm. The reflective two-layer metasurface is designed in such a way to generate reflection phase difference of 180° between two elements “0” and “1” on a broad frequency band. A theoretical analysis is performed on the ratio of the “0” and “1” elements using Least Square Error (LSE) method to find the best ratio value. As the second activity of this research theme, wideband CP antenna with low RCS and high gain properties is presented. The proposed antenna is based on a combination of the FPC and sequential feeding technique

Supervised-learning-enabled EM-driven development of low scattering metasurfaces

The recent advances in the development of coding metasurfaces created new opportunities to elevate the stealthiness of combat aircrafts. Metasurfaces, composed of optimized geometries of meta-atoms arranged as periodic lattices, are devised to obtain desired electromagnetic (EM) scattering characteristics, and have been extensively exploited in stealth applications to reduce radar cross section (RCS). They rely on the manipulation of backward scattering of electromagnetic (EM) waves into various oblique angles. Despite potential benefits, a practical obstacle hindering widespread metasurface utilization is the lack of systematic design procedures. Conventional approaches are largely intuition-inspired and demand heavy designer’s interaction while exploring the parameter space and pursuing optimum unit cell geometries. Another practical obstacle that hampers efficient design of metasurfaces is implicit handling of RCS performance. To achieve essential RCS reduction, the design task is normally formulated in terms of phase reflection characteristics of the unit cells, whereas their reflection amplitudes—although contributing to the overall performance of the structure—is largely ignored. A further practical issue is insufficiency of the existing performance metrics, specifically, monostatic and bistatic evaluation of the reflectivity, especially at the design stage of metasurfaces. Both provide a limited insight into the RCS reduction properties, with the latter being dependent on the selection of the planes over which the evaluation takes place. As a consequence of raised concerns, the existing design methodologies are still insufficient, especially in the context of controlling the EM wavefront through parameter tuning of unit cells. Furthermore, they are unable to determine truly optimum solutions. Therefore, we have introduced a novel machine-learning-based framework for automated and computationally efficient design of metasurfaces realizing broadband RCS reduction. We have employed a three-stage design procedure involving global surrogate-assisted optimization of the unit cells, followed by their local refinement. In its final stage, a direct EM-driven maximization of the RCS reduction bandwidth has been performed, facilitated by appropriate formulation of the objective function involving regularization terms. Moreover, to handle the combinatorial explosion in the design closure of multi-bit coding metasurfaces, a sequential-search strategy has been developed that enabled global search capability at the concurrent unit cell optimization stage. Latterly, the metasurface design task with explicit handling of RCS reduction at the level of unit cells has been introduced that has accounted for both the phase and reflection amplitudes of the unit cells. The design objective has been defined so as to directly optimize the RCS reduction bandwidth at the specified level (e.g., 10 dB) w.r.t. the metallic surface. The appealing feature of the said framework has consisted in its ability to optimize the RCS reduction bandwidth directly at the level of the entire metasurface as opposed to merely optimizing unit cell geometries. Besides, the obtained design has required minimum amount of tuning at the level of the entire metasurface. Lastly, a new performance metric for evaluating scattering characteristics of a metasurface, referred to as Normalized Partial Scattering Cross Section (NPSCS), has been proposed. The metric involved integration of the scattered energy over a specific solid angle, which allows for a comprehensive assessment of the structure performance in a format largely independent of the particular arrangement of the scattering lobes. Our design methodologies have been utilized to design several instances of novel scattering metasurface structures with the focus on RCS reduction bandwidth enhancement and the level of RCS reduction. Experimental validations confirming the numerical findings have been also provided. To the best of the author’s knowledge, the presented study is the first systematic investigation of this kind in the literature and can be considered a step towards the development of efficient, low-cost, and more high performing scattering structures

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

Metamaterials and Metasurfaces

Ye

Antennas and Propagation Aspects for Emerging Wireless Communication Technologies

The increasing demand for high data rate applications and the delivery of zero-latency multimedia content drives technological evolutions towards the design and implementation of next-generation broadband wireless networks. In this context, various novel technologies have been introduced, such as millimeter wave (mmWave) transmission, massive multiple input multiple output (MIMO) systems, and non-orthogonal multiple access (NOMA) schemes in order to support the vision of fifth generation (5G) wireless cellular networks. The introduction of these technologies, however, is inextricably connected with a holistic redesign of the current transceiver structures, as well as the network architecture reconfiguration. To this end, ultra-dense network deployment along with distributed massive MIMO technologies and intermediate relay nodes have been proposed, among others, in order to ensure an improved quality of services to all mobile users. In the same framework, the design and evaluation of novel antenna configurations able to support wideband applications is of utmost importance for 5G context support. Furthermore, in order to design reliable 5G systems, the channel characterization in these frequencies and in the complex propagation environments cannot be ignored because it plays a significant role. In this Special Issue, fourteen papers are published, covering various aspects of novel antenna designs for broadband applications, propagation models at mmWave bands, the deployment of NOMA techniques, radio network planning for 5G networks, and multi-beam antenna technologies for 5G wireless communications