This paper proposes a planar wideband microstrip feedline to a slot-line-based patch antenna for high-frequency pattern diversity applications. The antenna consists of two adjacent rectangular patches separated by the ground slots, with a directive patch slot along the edge of the substrate. A compact common feedline-to-slot configuration is used to miniaturize the antenna. The antenna is designed in stages, starting with an in-phase feedline, followed by a slot line structure, with two radiating patches on top and a director on the side. This creates radiation diversity with directive radiation patterns. The antenna was fabricated and analyzed inside a far-field anechoic chamber. The experimental results validate the return loss, gain, and radiation performance. The measured results of the antenna within a frequency range of 8.5 to 11 GHz show good agreement with the simulation. The antenna has a maximum gain of 9.2 dBi and has the potential to be beneficial for beam steering and X-band applications due to its low profile, broad bandwidth, high gain, and good directivity.

In this letter, a compact, via-assisted, circularly polarized antenna is presented for body-centric health monitoring, enabling frequency-dependent dual-mode radiation behavior without requiring reconfiguration or periodic structures. It covers the 2.45 and 5.8 GHz ISM bands with a size of $\bf {0.31\lambda _{0} \times 0.31\lambda _{0} \times 0.043\lambda _{0}}$. The stacked radiator is excited by a single CPW feed, while shorting vias introduce controlled inductive loading and mode perturbation to support miniaturization and stable CP. The proposed antenna produces a quasi-omnidirectional pattern at 2.45 GHz for on-body links and a more directive broadside pattern at 5.8 GHz for off-body communication. Measured $\bf {-10}$ dB bandwidths are 5.3% and 27.2%, with AR bandwidths of 15.1% and 21.2%. It achieves up to 80% radiation efficiency, peak gains of 2.94 dBi and 5.1 dBi, and low SAR of 0.049 W/kg and 0.074 W/kg, confirming a compact and versatile solution for wearable healthcare systems.

A compact fork-shaped four-element multiple-input multiple-output (MIMO) antenna system with wide bandwidth for 5G millimeter-wave (mmWave) applications is presented. The antenna elements are arranged orthogonally to achieve a compact footprint of $20\times 26{\mathrm{mm}}^2$. To enhance the gain, a frequency selective surface (FSS) is placed above the MIMO system, providing an average gain improvement of 1.5 dB across the entire operating band and achieving a peak gain of 7.5 dB at 41 GHz. The proposed design operates in the Ka-band (22–46 GHz), making it well suited for 5G communications. The antenna exhibits an isolation greater than 20 dB and radiation efficiency exceeding 80% across the band. Moreover, key MIMO performance metrics, including diversity gain (DG ≈ 10) and envelope correlation coefficient (ECC < 0.05), meet the required standards. A prototype of the proposed system was fabricated and measured, with the experimental results showing good agreement with simulations.

The growing complexity of modern wireless systems, especially for 5G, 6G, and IoT applications, demands efficient and intelligent design approaches for both radio frequency (RF) circuits and antenna systems. Artificial intelligence (AI) has emerged as a powerful enabler that offers data-driven techniques for modeling, optimization, and synthesis of communication hardware. In this paper, we discuss a unified perspective of AI-aided techniques used for RF circuits and antenna design. These techniques mainly encompass a broad range of learning methods, such as supervised learning, deep neural networks, evolutionary algorithms, reinforcement learning, and generative models. The survey highlights their roles in performance prediction, layout optimization, inverse design, and co-design strategies. Unlike existing works that specifically address the RF or antenna domains in terms of isolation, this paper integrates both domains and emphasizes on their joint optimization to meet system-level performance requirements. Comparative analysis, categorized summaries, and a focused discussion on current challenges are discussed that include limited datasets, lack of interpretability, and the gap between simulation and hardware implementation. Furthermore, this paper aims to provide a guide for researchers working on the development of intelligent, scalable, and physically aware AI frameworks for next-generation communication systems.

Holographic MIMO (HMIMO) antennas are emerging as a transformative technology for next-generation wireless networks, offering unprecedented control over EM wave propagation through continuous aperture modulation and intelligent surface integration. Based on the principles of EM holography, HMIMO systems enable dynamic beamforming, high spatial multiplexing, and real-time reconfiguration, surpassing the capabilities of conventional and massive MIMO systems. This review provides a comprehensive overview of HMIMO technologies, with an emphasis on their physical realizations, including metasurface-based designs, holographic leaky-wave antennas (HLWAs), dynamic metasurface antennas (DMAs), and liquid crystal-based reconfigurable architectures. The paper also explores advanced system-level integrations, such as hybrid beamforming, near-field (NF) propagation models, and photonic-assisted processing. By synthesizing recent advancements, we highlight the role of HMIMO in the realization of programmable, energy efficient, and scalable communication environments for 6G and beyond, including applications in smart cities, space-air-ground-sea (SAGS) networks, and secure defense systems. Finally, this review identifies critical challenges in hardware scalability, material limitations, environmental robustness, and system calibration, offering strategic insights and research directions toward the practical deployment of HMIMO-enabled intelligent wireless infrastructures.

A low-profile tri-band metamaterial absorber is developed for microwave attenuation and electromagnetic shielding applications within the S-, C-, and X-band regions. The absorber employs a compact resonant topology comprising a square metallic ring and two nested decagonal resonators, fabricated on an FR-4 dielectric layer with a metallic backing. Numerical optimization results in three highly efficient absorption bands located at 3.6 GHz, 7.4 GHz, and 11 GHz, where the absorptivity exceeds 99%. The physical origin of the absorption response is examined through field localization, induced current distributions, constitutive parameter extraction, and impedance characteristics. The analysis demonstrates that the resonant modes generated by the coupled metallic elements promote strong confinement of electromagnetic energy within the structure, leading to dissipation of the incident power. The geometrical unit-cell symmetry further enables a nearly identical response for different polarization states, while maintaining stable operation for incoming angles up to 60° under both TE and TM excitations. To verify the simulation results, an array prototype was manufactured and tested using a free-space characterization technique. The measured absorption characteristics closely follow the simulated response, showing the effectiveness of the design methodology. Owing to its compact dimensions, near-unity absorption, angular stability, and strong shielding capability, the developed absorber offers significant potential for electromagnetic compatibility enhancement, microwave shielding, and radar-related applications.