Constantinos L. Zekios

This work introduces the first, to our knowledge, PCB-based dual-polarized antenna array covering the Q (33-50 GHZ), V (50-75 GHZ), and part of the W-band (75-110 GHZ), called the dual-polarized fully planar inverted-L element (DP-FILE) array. To enable operation at high mmWave frequencies (> 50 GHz), the DP-FILE array employs an extremely compact unit-cell architecture composed of two orthogonal FILE radiators and a common ground via (CGV), requiring only three vias per unit cell. According to infinite array predictions, the DP-FILE array achieves common mode and scan blindness-free performance from 34 GHz to 94 GHz (2.75:1 bandwidth ratio), while scanning up to ±45° in all principal planes and preserving its VSWR < 3. To experimentally validate the array’s performance, a finite 25×25 DP-FILE prototype is fabricated and measured. Excellent agreement is observed between the measured and simulated results.

In this work, a novel method for constructing low-fidelity models tailored for eigenanalysis-based multi-fidelity surrogate optimization frameworks is proposed. Namely, the proposed approach leverages reference eigenfunction expansions to solve a properly formulated boundary value problem (BVP), enabling the rapid computation of characteristic mode solutions for arbitrary structures. To ensure uniqueness and well-posedness of the BVP, a computationally efficient two-dimensional Method of Moments (MoM) solution is employed to define the necessary boundary conditions. The resulting low-fidelity solutions are incorporated into a multi-fidelity surrogate modeling scheme by combining them with data from high-fidelity simulations. To enable consistent training and improve convergence, the characteristic mode solutions are organized through a robust correlation-based matching scheme. The proposed method is validated through two multi-objective characteristic mode analyses of microstrip patch antennas. Notably, our results demonstrate that the eigenfunction-based low-fidelity model yields solutions over three orders of magnitude faster than state-of-the-art coarse-mesh-based low-fidelity approaches. This acceleration translates to an approximate twofold reduction in total training time for multi-fidelity surrogate model, compared to conventional multi-fidelity strategies.

A novel method is proposed for designing and optimising a 3D compact bioinspired truncated cylindrical reflector antenna, mimicking the horseshoe bat's ear shape. The suggested methodology may be utilised in different antenna designs for performance optimisation at a selected frequency band. The antenna, measuring 225.16 mm in height and 78 mm in diameter, displays a unified–contiguous ultra‐wide band (UWB) behaviour with linear polarisation. It also exhibits a unidirectional radiation pattern, suitable for diverse RF applications, including 5G wireless systems whilst also offering versatility for applications such as sensing and direction‐of‐arrival (DoA). The results show a satisfying performance in terms of impedance matching ( reaching 50 Ω), reflection coefficient parameter ( entirely below −10 dB), gain ( = 17.25 dB) and frequency range from 3 to 24 GHz. This unified operational band is achieved employing a corrugated ground plane as a selective hard–soft surface as well as the reflector dielectric lining. Simulation is validated by fabricating the antenna through additive manufacturing, showing consistency with simulated results.

This article introduces a novel multi-physics β-formulation eigen-analysis simulator designed for shielded multiferroic devices/transmission lines. The Finite Difference Frequency Domain (FDFD) method was selected to simultaneously address both the electromagnetic (EM) and elasto-mechanic (EL) aspects of the problem. Building on previous work, the mathematical formulation has been expanded and implemented using a finite difference scheme to solve the coupled Maxwell and Newton equations through the constitutive relations of piezoelectric and piezomagnetic materials. Following the validation of the simulator, two types of printed planar transmission lines were tested for their potential use as tunable phase shifters or isolators. The ultimate goal is to utilize the proposed method as a initial design tool for electronically tunable microwave devices. Notably, this marks the first instance in the literature where a β-formulation eigen-analysis approach has been applied to arbitrary planar multiferroic structures, offering new insights and contributing to a deeper understanding of this complex multiphysics phenomenon.

The transition from 5G to 6G wireless systems introduces new challenges at the physical layer, including the need for higher frequency operations, massive MIMO deployment, advanced beamforming techniques, and sustainable energy harvesting mechanisms. A plethora of feature articles, review and white papers, and roadmaps elaborate on the perspectives and research challenges of wireless systems, in general, including both unified physical and cyber space. Hence, this paper presents a comprehensive review of the technological challenges and recent advancements in wireless communication hardware that underpin the development of next-generation networks, particularly 6G. Emphasizing the physical layer, the study explores critical enabling technologies including beamforming, massive MIMO, reconfigurable intelligent surfaces (RIS), millimeter-wave (mmWave) and terahertz (THz) communications, wireless power transfer, and energy harvesting. These technologies are analyzed in terms of their functional roles, implementation challenges, and integration into future wireless infrastructure. Beyond traditional physical layer components, the paper also discusses the role of reconfigurable RF front-ends, innovative antenna architectures, and user-end devices that contribute to the adaptability and efficiency of emerging communication systems. In addition, the inclusion of application-driven paradigms such as digital twins highlights how new use cases are shaping design requirements and pushing the boundaries of hardware capabilities. By linking foundational physical-layer technologies with evolving application demands, this work provides a holistic perspective aimed at guiding future research directions and informing the design of scalable, energy-efficient, and resilient wireless communication platforms for the Future Internet. Specifically, we first try to identify the demands and, in turn, explore existing or emerging technologies that have the potential to meet these needs. Especially, there will be an extended reference about the state-of-the-art antennas for massive MIMO terrestrial and non-terrestrial networks.

As we are witnessing the surge of ongoing wireless communication systems, simultaneous transmit and receive (STAR) schemes are proving to be advantageous due to the doubling of spectral efficiency. However, for the successful realization of STAR, we need to overcome a major bottleneck in suppressing the self-interference from the transmitter onto the colocated receiver. Currently, mitigating this interference requires complex hardware and advanced algorithms when employing an array for applications such as multiple input, multiple output (MIMO), and beamforming. This interference can arise from both near-field and far-field coupling in a MIMO beamforming system. Consequently, this paper presents a unique STAR approach that provides an average isolation of 40 dB between Tx and Rx ports across all elements of the MIMO beamforming system. The proposed approach can be extended to large antenna arrays.

This work introduces a novel design methodology to redesign traditional reflectarray antennas (RAs) with bandwidths in the order of 15% and double their bandwidth without increasing their design complexity. A wideband RA unit cell (UC) is designed utilizing multiple connection points between the radiating and phase shifting structure. Notably, to properly set the desired phase shift at each UC of the RA aperture across the entire frequency band, true-time-delay (TTD) lines are connected to each element. To validate the performance of our proposed approach a traditional microstrip patch-based RA is used as an example. By applying our proposed method to this RA, we develop a design that achieves a fractional 1.5 dB gain bandwidth of 31% (8.6 GHz to 11.8 GHz) in the X-band, which is more than two times larger than the 14% fractional bandwidth of the traditional RA design. Our RA is prototyped to validate its performance. The measurements show excellent agreement with simulations thereby validating our proposed methodology.

The demand for fast and accurate electromagnetic solutions to support current and emerging technologies has fueled the rapid development of various machine learning techniques for applications such as antenna design and optimization, microwave imaging, device diagnostics, and more. Multi-fidelity (MF) surrogate modeling methods have shown great promise in significantly reducing computational costs associated with surrogate modeling while maintaining high model accuracy. This work offers a comprehensive review of the available MF surrogate modeling methods in electromagnetics, focusing on specific methodologies, related challenges, and the generation of variable-fidelity datasets. The article is structured around the two main types of electromagnetic problems: forward and inverse. It begins by summarizing key machine learning concepts and limitations. This transitions to discussing multi-fidelity surrogate model architectures and low-fidelity data techniques for the forward problem. Subsequently, the unique challenges of the inverse problem are presented, along with traditional solutions and their limitations. Following this, the review examines MF surrogate modeling approaches tailored to the inverse problem. In conclusion, the review outlines promising future directions in MF modeling for electromagnetics, aiming to provide fundamental insights into understanding these developing methods.