In electromagnetics, optimization problems generally require high computational resources and involve a large number of unknowns. They are usually characterized by non-convex functionals and continuous spaces suitable for strategies based on Differential Evolution (DE). In such a framework, this paper is aimed at presenting an overview of Differential Evolution-based approaches used in electromagnetics, pointing out novelties and customizations with respect to other fields of application. Starting from a general description of the evolutionary mechanism of Differential Evolution, Differential Evolution-based techniques for electromagnetic optimization are presented. Some hints on the convergence properties and the sensitivity to control parameters are also given. Finally, a comprehensive coverage of different Differential Evolution formulations in solving optimization problems in the area of computational electromagnetics is presented, focusing on antenna synthesis and inverse scattering.

Differential Evolution as Applied to Electromagnetics

Optical non-linear phenomena are typically observed in natural materials interacting with light at high intensities, and they benefit a diverse range of applications from communication to sensing. However, controlling harmonic conversion with high efficiency and flexibility remains a major issue in modern optical and radio-frequency systems. Here, we introduce a dynamic time-domain digital-coding metasurface that enables efficient manipulation of spectral harmonic distribution. By dynamically modulating the local phase of the surface reflectivity, we achieve accurate control of different harmonics in a highly programmable and dynamic fashion, enabling unusual responses, such as velocity illusion. As a relevant application, we propose and realize a novel architecture for wireless communication systems based on the time-domain digital-coding metasurface, which largely simplifies the architecture of modern communication systems, at the same time yielding excellent performance for real-time signal transmission. The presented work, from new concept to new system, opens new pathways in the application of metamaterials to practical technology.

Programmable time-domain digital-coding metasurface for non-linear harmonic manipulation and new wireless communication systems.

A new approach is proposed to reject certain bands within the passband of an ultra-wideband planar monopole antenna. The proposed approach that utilizes a mushroom-type electromagnetic-bandgap (EBG) structure is proven to be an effective way for band-notched designs. The approach has many advantages, such as notch-frequency tunability, notch-band width controllable capacity, efficient dual-notch design, and stable radiation patterns. Several design examples using conventional mushroom-type EBG and edge-located vias mushroom-type EBG are presented. The examples exhibit good bandstop characteristics to reject the wireless local-area network interference bands (5.2- and 5.8-GHz bands). Besides, the causes that lead to the discrepancies between the simulations and measurements are discussed.

UWB Band-Notched Monopole Antenna Design Using Electromagnetic-Bandgap Structures

A simple, fast and efficient method for designing wideband radar absorbers is proposed. The idea is to modify the circuit analog absorber method without perturbing the bandwidth. This is done by utilizing the asymptotic behavior of such an absorber at low frequency and replacing the band-stop resonating frequency selective surfaces with low-pass capacitive ones, which can be synthesized by square patches. It is shown that higher frequencies are not influenced by these modifications. A thin wideband capacitive circuit absorber (CCA) is presented with 28% reduction of thickness and 57% increase of bandwidth in comparison to the Salisbury screen. It is also explained why some optimized metamaterial designs fail to compete with the CCA method. For high permittivity layers, it is shown that the CCA is a better solution than the Jaumann absorber and improvements both in thickness and bandwidth are possible. A three layered ultra wideband (4-24 GHz) CCA is presented with total thickness of 15.1 mm. Finally, a design capable of handling oblique angles of incidence for both polarizations and fulfilling different mechanical, thermal and fabrication constraints is given. The absorption band covers the entire C, X and Ku radar bands (4-18 GHz), showing significant improvement compared to the published circuit analog absorbers. (Less)

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Capacitive Circuit Method for Fast and Efficient Design of Wideband Radar Absorbers

A novel approach for estimating the direction of arrivals (DOAs) in time modulated linear arrays (TMLAs) with unidirectional phase center motion (UPCM) scheme is proposed in this paper. Based on the fact that the main beams of the patterns at different sidebands can be directed at different directions, the corresponding received signals can be used to compose a received data space. Thus, the spatial locations of the far-field sources can be estimated by using multiple signal classification (MUSIC) algorithm. Simulation results of the DOA estimation in an 8-element TMLA with the UPCM scheme validate the proposed approach, where the performance such as the accuracy and resolution of the DOA estimation is obtained through Monte-Carlo simulations. As compared to the DOA estimation based on conventional uniform linear arrays (ULAs), a much better resolution performance is obtained.

Direction of Arrival Estimation in Time Modulated Linear Arrays With Unidirectional Phase Center Motion

A dual-band dual-polarized base station antenna array is developed for fifth generation (5G) applications. Low-profile characteristic, high isolations, and shared-aperture are realized by introducing a frequency-selective surface (FSS) between radiators operating in the 0.69–0.96 GHz (B1) and 3.5–4.9 GHz (B2) bands. The dual-polarized B1-band antenna has a low-profile height of $0.12~\lambda _{L}$ ( $\lambda _{L}$ is the wavelength at the center frequency of B1). As for the B2 band, a $2 \times 2$ dual-polarized array sharing the same aperture with the B1-band antenna is developed for the sub-6 GHz band. Severe mutual coupling between B1- and B2-band antennas is reduced by the FSS. High port isolation (> 25 dB) between B1- and B2-band antennas is achieved. A prototype consisting of one B1-band antenna and a $2 \times 2$ B2-band antenna array is fabricated. Measured results show that the proposed dual-band dual-polarized array achieves 32.7% and 33.3% bandwidth in the B1 and B2 bands, respectively. Measured results also demonstrate that the proposed array offers stable radiation patterns, high gains, and high cross-polarization discriminations (XPD > 20 dB) across the two frequency bands. These attractive features make this array an ideal candidate for future 5G massive MIMO base station antenna developments.

Decoupling and Low-Profile Design of Dual-Band Dual-Polarized Base Station Antennas Using Frequency-Selective Surface

A low-profile dual-polarized wideband omnidirectional antenna with artificial magnetic conductor (AMC) reflector is proposed. The proposed antenna is operated in the long term evolution band (1.7–2.7 GHz), and has a compact size of 200 mm $\times200$ mm $\times $ 30.6 mm (about $0.25\lambda $ height at 2.7 GHz). The antenna structure consists of a horizontally polarized circular loop antenna, a vertically polarized low-profile monopole antenna, and an AMC reflector. By carefully designing the reflection characteristics of the AMC reflector, the profile height of the proposed antenna is significantly reduced as compared with those of antennas backed by conventional perfect electric conductor (PEC) ground planes. Simulated and measured results show that the proposed antenna is able to achieve over 45% impedance bandwidth (VSWR <1.8) with stable radiation patterns in the band of 1.7–2.7 GHz. Owing to the attractive wide bandwidth, low-profile configuration, and ease of fabrication, the proposed antenna is suitable for microbase station systems, especially for indoor ceiling antenna networking applications.

A Low Profile Dual-Polarized Wideband Omnidirectional Antenna Based on AMC Reflector

A novel stacked antenna configuration is proposed for the designs of shared-aperture base station antennas. An antenna array consisting of a lower-band antenna element working in 690–960 MHz frequency band and four upper-band antenna elements operating in the 3.5–4.9 GHz band is designed to demonstrate the concept. In the proposed configuration, four upper-band antenna elements are placed over the aperture of the lower-band antenna. Four metallic sheets are introduced to provide capacitive loading for the lower-band antenna and simultaneously offer a ground plane for the upper-band antennas. Therefore, low-profile characteristic and high isolations between the dual-band antennas are guaranteed. The overall height of the antenna array is only $0.17\lambda _{\text {L0}}$ ( $\lambda _{\text {L0}}$ is the free-space wavelength at the center frequency of the lower band), which is much lower than those of conventional designs. The measured results demonstrate that the designed antenna array achieves 32.7% and 33.3% impedance bandwidth in the lower and upper bands, respectively. High port isolations (>30 dB), high gains (>8 dBi), and stable radiation patterns are achieved in the two frequency bands. Owing to its simple structure and compact size, the resultant antenna array is easy for fabrication and is a promising candidate for sub-6 GHz band and 690–960 MHz band antennas.

A Novel Stacked Antenna Configuration and its Applications in Dual-Band Shared-Aperture Base Station Antenna Array Designs

An electromagnetic transparent antenna element is proposed for dual-band shared-aperture fifth-generation (5G) MIMO base station antenna array developments. The antenna element that operates in the low-frequency band (LB) of 1.8–2.7 GHz is placed above the aperture of the antenna array that operates in the high-frequency band (HB) of 3.3–3.8 GHz. Because of this antenna array configuration, the antenna elements in the LB have to be transparent to the electromagnetic waves radiated by the HB antenna array. This kind of electromagnetic transparent antenna element allows the HB antenna array working properly in the dual-band shared-aperture antenna array configuration. In this work, the electromagnetic transparent antenna element is inspired by the element of a typical wide-angle bandpass frequency selective surface (FSS). The radiation performance of the electromagnetic transparent antenna element at the LB is realized by properly combining the wide-angle bandpass FSS elements. A dual-band dual-polarized shared-aperture antenna array is then developed to demonstrate the concept and design. The experimental result validates the stable radiation patterns of each antenna element in both LB and HB. It indicates the proposed approach is promising for 5G MIMO base station antenna array developments.

Dual-Band Shared-Aperture Base Station Antenna Array With Electromagnetic Transparent Antenna Elements

Different from conventional-phased arrays providing only angle-dependent beampatterns, frequency diverse arrays (FDAs) produce angle–range-dependent and time-variant transmit beampatterns. Recently, time-invariant range focusing beampatterns have been reported in some works, which could not be achieved by traditional antenna arrays. However, this paper shows that these papers neglected the time–range or frequency–phase relationship and led to erroneous simulation results and conclusions. The formulas of time-invariant FDA are carefully reviewed, and the unnoticed misconceptions are analyzed. By reestablishing the correct signal models, a set of numerical examples are presented to demonstrate that the beampatterns of FDA arrays are actually time variant in free space.

Yikai Chen

Papers

Decoupling and Low-Profile Design of Dual-Band Dual-Polarized Base Station Antennas Using Frequency-Selective Surface

A Low Profile Dual-Polarized Wideband Omnidirectional Antenna Based on AMC Reflector

A Novel Stacked Antenna Configuration and its Applications in Dual-Band Shared-Aperture Base Station Antenna Array Designs

Dual-Band Shared-Aperture Base Station Antenna Array With Electromagnetic Transparent Antenna Elements

Accurate Models of Time-Invariant Beampatterns for Frequency Diverse Arrays