This paper presents a 4 × 4 multi-input multi-output antenna array for ultra-wideband applications. The single element of the array is comprised of a modified co-planar waveguide-fed double overt-leaf shaped patch radiator. The co-planar ground is optimized to achieve maximum impedance matching in the operating frequency band. The results show that the single antenna element offers an impedance bandwidth of 13.2 GHz starting from 3.2 GHz to 16.7 GHz. It is also observed from the results that the antenna offers good radiation characteristics and acceptable gain for the frequency band of interest. Furthermore, a 4 × 4 MIMO array is designed by utilizing the polarization diversity technique. To improve the isolation performance among antenna elements, a fan-shaped decoupler is introduced on the other side of the substrate, which ensures minimum isolation of 20 dB. Moreover, the proposed MIMO array operates in the frequency range of 2.75–16.05 GHz. The proposed MIMO array is fabricated and measured for the validation of simulation results, and it is observed that both the results are well in agreement.

In view of the need for communication with distributed sensors/items, this paper presents the design of a single-port antenna with dual-mode operation, representing the front-end of a future generation tag acting as a position sensor, with identification and energy harvesting capabilities. An Archimedean spiral covers the lower European Ultra-Wideband (UWB) frequency range for communication/localization purposes, whereas a non-standard dipole operates in the Ultra High Frequency (UHF) band to wirelessly receive the energy. The versatility of the antenna is guaranteed by the inclusion of a High Impedance Surface (HIS) back layer, which is responsible for the low-profile stack-up and the insensitivity to the background material. A conformal design, supported by 3D-printing technology, is pursued to check the versatility of the proposed architecture in view of any application involving its deformation and tracking/powering operations.

A modified design of a single-layer ultra-wideband (UWB) Frequency Selective Surface (FSS) reflector is presented for microwave applications. The proposed FSS unit cell consists of a square-loop and a ring printed on opposite sides of a low-cost FR-4 substrate. With the optimized dimensions of the elements, the proposed FSS design provides a stop-bandwidth of 11.94 GHz (5.24-17.18 GHz). It is observed from simulation results that the proposed FSS design has a linear decreasing phase response in the desired band. Furthermore, the presented design provides a stable frequency response up to 45° oblique angle of incidence for both perpendicular (TE) and parallel (TM) polarizations.

A new microstrip patch antenna that resonates on more than one frequencies and yields sufficiently high bandwidth is presented. Several variations of the proposed design are simulated using practically used dielectric materials. Detailed input return loss plots, VSWR plots, and radiation pattern plots are presented. A comparative study of all simulated results is presented. The design that yields the best performance is chosen for further optimization using deliberately-created defects in the ground plane, called Defected Ground Structures (DGS). It is observed that improvement in bandwidth is achievable through using the DGS.

This study presents a reflective linear-to-linear (L2L) cross-polarization converter (CPC) for terahertz (THz) applications. The unit cell features a modified wheel-shaped geometry, achieved by merging concentric circular rings of varying radii, placed on a polyimide substrate optimized for THz operation. The proposed design exhibits an ultra-wide bandwidth of 0.370 THz (0.345–0.715 THz) and achieves a polarization conversion ratio (PCR) exceeding 75% for most of the operational band. The polarization conversion mechanism is dynamically controlled via vanadium dioxide (VO ₂ ), a thermally responsive phase-change material. Below its threshold temperature (68°), VO ₂ remains in its insulating state, enabling efficient polarization conversion, while transitioning to a metallic state above this temperature suppresses the conversion entirely. Furthermore, angular stability analysis confirms robust performance under oblique incidence angles up to 50°, demonstrating its practicality for real-world applications.