Abstract: ABSTRACT In today's wireless communication landscape, the demand for compact MIMO antennas with low mutual coupling is critical. To meet this requirement, this paper presents two compact MIMO antenna configurations based on microstrip line principles. Initially, a compact dual‐band antenna is designed for 5G N77/N78/N79 sub‐6‐GHz applications. The introduction of an inverted L‐shaped slot into the broadband radiating element enables the generation of two distinct resonances. Without the slot, the antenna covers a broad frequency range from 4.30 to 5.54 GHz. Upon slot insertion, it achieves dual‐band operation across 3.3–3.78 GHz and 4.3–5.15 GHz. Subsequently, by integrating an additional inverted L‐shaped slot on the opposite lateral side, a triple‐band response is realized, covering 3.6–3.85 GHz, 4.75–5.05 GHz, and 5.3–7.5 GHz. Both antenna designs are fabricated on an FR‐4 substrate (εᵣ = 4.4, tanδ = 0.025) with 1.6 mm thickness and a compact footprint of 23.3 × 29.125 mm 2 (0.37λ 0 × 0.46λ 0 ). The radiating structure possesses an inverted T‐shaped geometry occupying (8 × 9.65) + (29.125 × 13.65) mm 2 that meets the requirements for modern space‐limited devices. Furthermore, two compact two‐port MIMO configurations (23.3 × 66.06 mm 2 , or 0.37λ 0 × 1.04λ 0 ) are proposed, utilizing orthogonally placed dual‐band and triple‐band monopole elements. Electromagnetic simulation using Ansys HFSS confirms that the designs achieve excellent MIMO performance, including envelope correlation coefficients (ECC) below 0.0019 and 0.001, channel capacity loss (CCL) under 0.26 and 0.46 bits/s/Hz, peak gains of 4.9 and 5.1 dBi, and radiation efficiencies exceeding 94% and 85%, respectively, for MIMO antennas with dual‐band and triple‐band monopole antenna elements. Notably, mutual coupling remains below −23 dB and −24.5 dB across the operating bands, despite maintaining an edge‐to‐edge spacing of less than 0.125λ.

Abstract: This letter presents an 8 × 8 low-sidelobe waveguide slotted array antenna fed by a microstrip ridge gap waveguide (MRGW) feeding network. Two innovative approaches are adopted, by introducing a novel coupling-based unequal power divider, along with a broadband ±90° waveguide twist, to achieve a wideband low sidelobe performance for the array. The proposed power divider provides a 9.2 dB amplitude difference between its two output ports over a 24.5% bandwidth, with amplitude fluctuation within ±0.2 dB and phase difference maintained within ±5°. In addition, the ±90° waveguide twist not only simplifies the feeding network design but also obtains directional radiation. To validate the proposed approaches, a prototype array is designed, fabricated, and measured. The measured results demonstrate an impedance bandwidth of 27.5% (25 GHz to 33 GHz) for |S11| < –10 dB, a peak gain of 20.4 dBi, and sidelobe levels below –20 dB in both E- and H-plane across the 25 GHz to 32 GHz band.

Abstract: This research presents a Characteristic Mode Analysis (CMA)-guided circular ring microstrip patch antenna integrated with a Defected Ground Structure (DGS) and PIN diodes for dual-band reconfigurability. The proposed antenna operates at 2.4 GHz and 4.6 GHz, with the reconfiguration achieved through selective switching of four PIN diodes embedded within a narrow DGS slot on the ground plane. Characteristic Mode Analysis (CMA) identifies the most significant modes and the optimal feed point, ensuring effective modal excitation. The suggested antenna is constructed on a Rogers RT5880 substrate that is $68 \mathrm{~mm} \times 68 \mathrm{~mm}$ and 1.575 mm in thickness. A circular patch with a radius of 26 mm with a central slot with a radius of 17 mm is introduced. The proposed design incorporates a single narrow DGS slot on the ground plane, controlled by four PIN diodes. The outcome allows the two bands to switch frequencies. The simulation findings indicate that the impedance is well matched (S11 <10 dB), the Voltage Standing Wave Ratio (VSWR) is below 2, and the radiation patterns are satisfactory. This research provides stable frequency reconfigurability with a simplified ground configuration and optimized feed placement.

Abstract: This article proposed a balanced diplexer featuring intrinsic common-mode (CM) suppression and a wide stopband. The proposed design incorporates three balanced hybrid microstrip and slotline transition structures (HMSTSs) and two sets of channel bandpass filters (BPFs). Each HMSTS combines a U-shaped microstrip feedline with a stepped-impedance slotline, enabling inherent full-band CM suppression and simplifying the differential-mode (DM) design process. In the channel filter design, E-shaped stub-loaded resonators (ESLRs) and single-mode C-shaped resonators (SCRs) are employed as nonresonant nodes, acting as virtual sources and loads for the BPFs. Each channel filter is configured as a fourth-order filter using folded U-shaped resonators (FURs), resulting in a compact layout for the diplexer. To achieve a wide stopband, the first spurious harmonic of the ESLR is designed to be well separated from the harmonic of the FURs, effectively disrupting unwanted resonant paths. To validate the proposed design, a prototype balanced diplexer operating at 3.1 and 3.8 GHz was designed, fabricated, and measured. The results demonstrate favorable DM and CM performance, with excellent agreement observed between simulated and measured.

Abstract: This paper presents an automated design methodology to improve the radiation performance of microstrip antenna arrays using boosting-based machine learning (ML) algorithms in the X-band frequency range. The proposed approach replaces computationally expensive full-wave simulations with an ML-driven framework trained on a large dataset of wide-angle impedance matching (WAIM) and microstrip antenna structures. To address various design requirements, two types of microstrip antennas are incorporated into the framework. The behavior of both antenna configurations is predicted with only one network achieved by adding preprocessing and postprocessing modules to the method. This reduces the number of trained networks while maintaining the prediction accuracy. Training networks for WAIM and antennas involve four different boosting algorithms: AdaBoost, Gradient Boosting (GB), Extreme Gradient Boosting (XGB), and Light Gradient Boosting (LightGB). Among the evaluated boosting algorithms, LightGB achieved the highest prediction accuracy for both WAIM and antenna models. Two design examples are investigated to demonstrate the framework’s capability in extending the microstrip array scanning range. The results confirm no grating lobes and improved gain at extreme scanning angles across the frequency range. Compared to traditional full-wave solvers, the ML-based method significantly reduces the order of computation time from several hours to seconds while minimizing hardware resource requirements. This automated method offers an efficient framework for designing wide-angle microstrip arrays and expanding their applications without requiring designer expertise.

Abstract: This study presents the design, simulation, and performance analysis of a rectangular microstrip patch antenna (RMPA) intended for wireless communication systems operating at 3 GHz. The antenna is modelled using Advanced Design System (ADS) software and fabricated on a RO4360 substrate with a dielectric constant of 6.15 and a thickness of 1.524 mm. The RO4360 material is selected due to its low loss, high-frequency stability, and compactness at microwave frequencies. To ensure proper impedance matching, an inset-fed microstrip line technique is employed. Simulation results demonstrate a return loss of -24.735 dB at 3.019 GHz and a fractional bandwidth of 1.2%, satisfying the bandwidth target for narrowband applications. Due to the single patch configuration, the radiation pattern is nearly omnidirectional with modest gain. The paper also reviews various enhancement techniques—such as feeding optimization, substrate modification, use of parasitic elements, and defected ground structures—to overcome limitations in bandwidth and gain. The proposed antenna design is suitable for compact and cost-effective wireless devices.

Abstract: This paper introduces a dual-functional circularly polarized (CP) surface using polarizer integration. It is the first attempt to propose a concept of polarizer fusion design, realizing CP partial reflection function at 7GHz and CP reflection effect at 30GHz, enabling the combination of a CP partially reflective surfaces (PRS) and a CP reflectarray (RA). For excitation, a horn operated at high frequency band for RA and a magnetic dipole operated at high frequency band for FPC are used as feed sources. For validation, a prototype of the proposed surface is designed, fabricated, and measured. The measured results show the 3-dB gain bandwidths of 8.5% and 23.7% with peak gains of 17.3 and 20.5 dBic over two bands are achieved, respectively. The proposed design is a good candidate for future communications system that require high integration, wideband and high-CP-gain.

Abstract: This paper presents a compact two-port microstrip patch antenna system designed to enable passive self-interference suppression through enhanced inter-port isolation. The proposed antenna operates at 3.5 GHz, suitable for both MIMO as well as full-duplex transceiver applications. The isolation is significantly improved, achieving ≈ 36 dB without relying on circulators, hybrids, or complex feed networks. This is accomplished using a unique design strategy that combines: (1) electromagnetic field confinement around individual antenna elements using metallic vias, and (2) a near-field decoupling structure (NFDS) positioned within the limited spacing between ports. The antenna also maintains favorable impedance matching and achieves a broadside gain of 4.86 dBi. The design methodology and performance validation are carried out using full-wave simulations in CST Microwave Studio (CST-MWS).

Abstract: This study presents a multifunctional metasurface capable of achieving polarization conversion across narrow and wide frequency bands. The proposed polarization converter comprises a diagonal strip, a V-shaped resonator, and an Eshaped resonator, fabricated on a 1.6 mm thick FR-4 substrate. The design demonstrates orthogonal polarization conversion in the $8.45-8.55 \text{GHz}$ range with a maximum polarization conversion ratio (PCR) of 97.5%, as well as wideband orthogonal polarization conversion in the $11.5-12 \text{GHz}$ and $16.5-17.73 \text{GHz}$ bands, achieving PCRs exceeding 90 % and 95 %, respectively. Additionally, the metasurface converts linearly polarized waves to left-hand circular polarization within the $9.5-10 \text{GHz}$ frequency range. Beyond polarization conversion, the structure functions as a metareflector, preserving the handedness of circularly polarized waves upon reflection. This metasurface holds significant potential for applications in satellite communications, radar systems, and 5 G networks due to its multifunctional capabilities with compact design.

Abstract: The presented work involves the novel design of a microstrip patch 5x1 antenna array fed by a microstrip quarter-wave transformer that can function in X-Band. The proposed structure of the antenna comprises of five patches attached in an array with two quarter-wave transformers on the side connected to a central feedline. The patch is compact and is ideally suitable for X-band applications. The substrate is made up of Rogers RT/Duroid 5880 with a dielectric permittivity of 2.2 and a standard height of 1.6 m. The design resonates within the X-band range of 9.54 GHz – 10.89 GHz having a reflection coefficient of-62.39 dB and a gain of 10.61 dBi, over an impedance bandwidth of 1.35 GHz, which is in agreement for resonant frequency band applications. Another technique implemented to achieve the desired frequency band is the integration of defected ground structure (DGS) which is also used for impedance matching.

Abstract: The terahertz (THz) band is critical for 6G systems, yet high-gain circularly polarized (CP) antennas remain challenging due to fabrication and performance constraints. This work presents a compact 3D-printed THz lens antenna at 300 GHz, combining a Fresnel lens and triangular polarizer to achieve wideband CP and low profile. The Fresnel lens reduces height and loss, while the polarizer enables 90° phase shifts for CP via anisotropic dielectric unit cells. Fabricated with High Temp V2 material, measurements show 40% impedance bandwidth (220–330 GHz), 26.4 dB peak gain, and 28.7% axial ratio bandwidth. Measurements confirm collimated beams with −20 dB sidelobes, gain alignment with simulations with almost no gain gap. The design demonstrates the viability of 3D-printed CP antennas for 6G, balancing performance and manufacturability.