This research presents the design and development of a microwave sensor capable of detecting and distinguishing hydroxylated organic compounds (HOCs), namely water, methanol, ethanol, and propanol. The work analyzes the physical mechanisms that govern the sensitivity and detectability of these liquids in the microwave range. Differences in sensor response are linked to variations in molecular characteristics such as dipole moment, microwave absorption, and refractive index. Unlike approaches that rely solely on experimentation, this study connects microwave behavior to fundamental molecular properties, enabling a predictive, physics-based understanding of HOC detection. Molecular polarization and relaxation models were combined with experimental observations to explain how these compounds interact with microwave fields. A metamaterial-based sensing cell was designed, simulated, and experimentally validated. Results demonstrate that the sensor effectively identifies hydroxyl compounds with high sensitivity. Water produced the highest resonance-frequency shift (0.76 GHz), followed by methanol (0.7 GHz), while ethanol and propanol showed similar shifts around 0.35–0.37 GHz. Propanol achieved a quality factor of 10.82 in the 12–17 GHz range. The sensor also reached a frequency detection resolution of 6.75 MHz and showed strong amplitude sensitivity, highest for water at 10.66 dB.

The design of filters used in waveguides, which are crucial components of high-frequency communication systems, plays a significant role in improving system performance. In this study, the usage of metamaterials is first proposed, the SLA 3D printing method is used to design and fabricate CSRR meta-resonators-based bandpass waveguide filters (WGFs) with different filter orders for C-band (4-7.5 GHz), and simulated and measured filter performances are compared. Since the proposed novel WG structure is modular, it allows the design of C-band WGFs using different thicknesses of substrate materials. Also, the number of unit elements can be increased and any number of meta-resonators can be inserted to design filters of different orders ranging from 1 to 5. The electrical length of the WGF/WG structure can be changed according to the needs of the applications. The resulting WGFs demonstrated superior RF performance, being 50% lighter than comparable models found in the literature. Over the relevant frequency range, the filter exhibited return losses between 31-43 dB, insertion losses from 0.1 to 0.35 dB, FBW ranging from 12% to 16%, and quality factors between 6.23 and 8.28, depending on the filter order. The obtained experimental results align closely with the simulation predictions, confirming the effectiveness of the design.

This paper proposes a highly selective dual-band bandpass filter (BPF) utilizing hemispherical resonators and an in-band transmission zeros (TZs) method. For obtaining high isolation between two passbands, a compound resonator topology is realized by arranging six hemispherical cavity resonators (HCRs) in a two-by-two vertical configuration, and then three TZs between the passbands are introduced. The filter selectivity is further optimized through elaborately placing three metal pillars in the cavity, and then one TZ in the lower stopband and two in the upper stopband are generated. Notably, the size of the HCR is reduced by about 50% compared to the conventional spherical resonator. To validate the design, a dual-band filter is 3D printed, which operates at 9.1 and 9.77 GHz with bandwidths of 210 and 200 MHz, respectively. The measured results show good agreement with the simulated ones.

This paper presents a dual-band reflectarray antenna based on a 1-bit hybrid active/passive metasurface, achieving independent four-beam radiation at frequencies of 5.8 and 9.7 GHz. The proposed unit cell integrates an active double-split square ring with PIN diodes for 180° phase switching at 5.8 GHz, and a passive cross-shaped patch for 180° phase control at 9.7 GHz. A chessboard-like coding arrangement enables independent beam steering at both frequencies. Experimental results from a fabricated 15 × 15 metasurface prototype show stable four-beam operation, with measured steering angles of 19° and 12.1°, and 3-dB beamwidths of 10.2° and 11.5° at 5.8 and 9.7 GHz, respectively, validating good agreement with simulations. The proposed metasurface demonstrates significant promise for applications in multiband radar and communication systems requiring compact, low-profile, reconfigurable antennas.

In this paper, an ultra-wideband, low-scattering, and stable-gain Fabry–Perot antenna is proposed based on a novel hybrid metasurface. The radar cross-section (RCS) reduction is achieved by employing a 1-bit checkerboard polarization conversion metasurface (PCM) with a high polarization conversion ratio. Moreover, to enhance the antenna gain, broaden the 3-dB gain bandwidth, and maintain stable gain performance within the passband, a nonuniform reflective metasurface with a positively sloped reflection phase is strategically introduced. This metasurface, combined with the tessellated PCM layer, forms a hybrid structure featuring high transmission efficiency. Benefiting from this hybrid metasurface design, the antenna demonstrates a maximum gain enhancement of 4.7 dBi, an average gain improvement of 2.7 dBi, and a 39.8% increase in the 3-dB gain bandwidth. To validate the proposed design methodology, a prototype antenna was fabricated and experimentally measured. The measured results show good agreement with the simulated predictions. Specifically, the fabricated antenna exhibits a –10 dB impedance bandwidth of 22.47% (7.23–9.06 GHz), a 3-dB gain bandwidth of 18.2% (7–8.4 GHz), and a maximum gain of 17.25 dBi at 7.2 GHz. Additionally, the antenna achieves an RCS reduction bandwidth of 102.3%, with a maximum RCS reduction of 35.3 dB at 13.03 GHz.

In this paper, a dual concentric square-loop dual-polarized reconfigurable frequency selective surface with a high tuning ratio of 2.02 operating in the 1.53–3.10 GHz band is proposed. This high tuning ratio has been achieved by using four SMV1430-LF varactor diodes, which have been actuated using a simple biasing arrangement made of stubs and four metallic vias per unit-cell. The unit-cell has been rigorously analyzed, and an equivalent circuit (EC) model has been developed for the physical insight. The proposed EC model also demonstrates the reconfigurability of the unit-cell and thus predicts its behavior. The effect of the angular incidence of the impinging electromagnetic wave on the structure up to a 45º has been experimentally verified, which demonstrates its angular stability and polarization insensitive behavior. The structure may find applications in the electromagnetic spectrum’s L, S, and 2.45 GHz ISM bands.

This paper presents a multiple input multiple output (MIMO) system for sub-6 GHz using a two-port cylindrical dielectric resonator (CDR). This band is ideal for 5G applications, providing a balanced combination of coverage and capacity, as well as offering high data rates and low latency. The proposed MIMO is excited by an aperture-coupled feed structure composed of driven elements consisting of an elliptical-shaped step impedance transform line to enhance CDR coupling. Aperture coupling is used to excite the HEM11δ mode without the involvement of any hybrid mode. The purpose of the aperture coupling in the CDR antenna is to launch the HEM11δ mode, which is a magnetic dipole that provides a stable radiation pattern in the 3.2–3.7 GHz band, allowing for efficient transmission and reception of the data. Symmetrically oriented CDR and the presence of orthogonal fields provide good isolation, which can be further improved by the insertion of an asymmetric cross slot. The designed CDR-based MIMO antenna achieves excellent isolation of over 20 dB and demonstrates improved envelope correlation coefficient (ECC) values, resulting from low field correlation, across the entire frequency band of operation. Almost all the major diversity analysis was carried out for the desired sub-6 GHz band while maintaining a relatively compact size. Based on the measured results, it is confirmed that the proposed CDR-based MIMO antenna is appropriate for sub-6 GHz applications.

Proximity feeding is the simplest technique to achieve wideband response in a microstrip antenna on a thicker substrate. However, the bandwidth is limited on a thinner substrate due to the capacitive impedance offered by the feeding strip. This paper presents wideband designs of proximity fed rectangular microstrip antennas loaded with printed open Ring shape or C-shape resonators, while using a thinner substrate. On substrate thickness of 0.053λg, the proposed design yields a bandwidth of 214 MHz (23.62%) with a broadside radiation pattern and peak gain of 8.0 dBi. Against the thicker substrate proximity fed design, a reduction of ∼ 0.03λg in thickness is obtained. With the obtained antenna characteristics, the proposed design is useful in 800–1000 MHz, GSM band applications.

With the widespread application of smart antennas in 5G communication and radar detection, adaptive beamforming technology based on deep learning has become a research focus for improving the anti-interference performance of antenna arrays due to its powerful nonlinear modeling capability. It can transform the beamforming problem into a neural network regression problem, enabling the model to rapidly output an approximately optimal beamforming weight vector without prior information. Aiming at the issues of poor adaptability to dynamic interference and high computational complexity of traditional algorithms, this paper proposes IRDSNet, a novel adaptive beamforming algorithm based on Inception-ResNet-dual-pool Squeeze-and-Excitation Network (DP-SENet), to optimize the performance of uniform circular array antennas. IRDSNet integrates the Inception structure, depthwise separable convolution, and Ghost convolution to construct a multi-scale feature extraction module, enhancing the model’s feature extraction capabilities while maintaining a low parameter count. By introducing an improved DP-SENet, the model’s ability to focus on key features is enhanced, while the incorporation of residual modules optimizes feature transmission efficiency. Simulation results demonstrate that the IRDSNet algorithm achieves a null depth exceeding −90 dB at various interference angles, with an output Signal-to-Interference-plus-Noise Ratio (SINR) consistently above 23 dB and a short inference time, demonstrating excellent interference suppression performance.

A reinforcement learning (RL)-based automated antenna topology optimization method is proposed. The proposed framework can be divided into three phases, which are high-quality dataset construction, electromagnetic (EM) simulation acceleration, and RL-driven automated antenna topology optimization. Based on the high-quality dataset, a fully trained enhanced hybrid multilayer perceptron is proposed to replace time-consuming EM simulations. This approach allows the RL to acquire knowledge from the interaction between antenna topology and the environment quickly, reducing the optimization time cost caused by the large number of EM simulations. Additionally, two crucial components, topology bidirectional mapping strategy (TBM) and topology hierarchical analyzation strategy (THA), are introduced in this work to address the compatibility problems between ML and high-dimension antenna topology data. To verify the effectiveness of the proposed method, a microstrip patch antenna operating at 2.45 GHz is optimized. According to the measurement results, the antenna performance of gain and impedance bandwidth is improved greatly at the same time through the proposed method.

This paper presents a circularly polarized (CP) antenna employing both uniform and non-uniform metasurfaces to achieve wideband characteristics. The proposed antenna consists of a dual-feed CP antenna as the source element, a 4$\times$4 metasurface lattice as the superstrate, and a sequential feed network. Initially, the performance of the antenna is analyzed using a uniform metasurface. Subsequently, by adjusting the size of the inner unit cells, the uniform metasurface is transformed into a non-uniform configuration. The proposed design with a uniform metasurface achieves a $-$10 dB impedance bandwidth ranging from 5.21 to 6.47 GHz (21.57%) and a 3-dB axial ratio bandwidth from 3.96 to 6.67 GHz (50.9%). The use of a non-uniform metasurface significantly improves the overall antenna performance, achieving a $-$10 dB impedance bandwidth and 3-dB AR bandwidth of 4.98 to 7.45 GHz (40%) and 2 to 7.26 GHz (113%), respectively. The optimized design with a non-uniform metasurface is fabricated and tested. Overall size of the antenna is 40$\times$40$\times$3.4 mm$^3$ (0.72$\times$0.72$\times$0.062 $\lambda_{0}^3$), where $\lambda_{0}$ represents effective wavelength at an operating frequency of 5.5 GHz. The simulated and measured results of the proposed antenna are found to be in good agreement.

A low-profile dual-circularly polarized (CP) antenna array using spiral sequential rotation (SSR) technique is proposed. The array element consists of two stacked CP patches and a double-layer ceramic substrate with high dielectric constant. Dual CP radiations are accomplished by the slender rectangular structures on the radiators, which can excite two orthogonal characteristic modes. An extremely small size of 0.005λ3 (λ is the wavelength in free space for the low bands) and a wide 3-dB axial ratio beamwidth (ARBW) of 206° are achieved. Furthermore, the SSR technique is employed to achieve low cross-polarization level (XPL) within a large scanning angle. Prototype of a 6 × 6 array was fabricated and measured. Experimental results demonstrate that a wide 3-dB AR scanning angle of ± 55° is realized for LHCP as well as RHCP radiation. Additionally, low XPL less than −18.1 dB with gain fluctuation less than 4 dB are achieved over −55° ∼ + 55°. Meanwhile, the array has successfully passed the mechanical test and the thermal vacuum test (−90° ∼ + 90°). All the merits of dual-CP radiation, extremely low profile of 0.06λ, wide 3-dB AR scanning capability, and low XPL make our proposed dual-CP SSR antenna array be attractive candidate for satellite applications.

In this study, a metasurface (MS) polarization converter combined with a two-port dielectric antenna is constructed and studied. The feeding configuration, which consists of a printed line connected to an aperture, offers built-in filtering capabilities. In addition to converting linear to circular polarization between 2.49 and 3.25 GHz, the suspended MS layer enhances port isolation to less than −20 dB. In addition, the suggested radiator’s |S11| is projected using the Random Forest and XGBoost machine learning (ML) models, which demonstrate satisfactory agreement with simulation data. The antenna effectively functions over 2.33–3.35 GHz, demonstrating that it is a leading contender for sub-6 GHz 5G communication systems. Fabricated measurements support both simulation and ML predictions.

A novel compact Ku-band waveguide antenna is proposed that offers left-hand circular polarization (LHCP). The proposed design has Nabla slot on aperture of circular waveguide to attain LHCP. Side fed design of the proposed antenna eliminates the need for rectangular to circular waveguide transition. The antenna has the reflection coefficient (S11) below −10 dB from 12 to 15.2 GHz, corresponds to the impedance bandwidth of 23.5% and axial ratio (AR) below 3 dB from 13.2 to 15.2 GHz, attained circular polarization bandwidth as 14%. Measurement results indicate that the proposed antenna achieves a 12 dB half beamwidth of around 65° across the 13.4–15.2 GHz frequency band, resulting in an attained pattern bandwidth (stable radiation pattern range) of 12.5%. This performance fulfills the illumination requirements for an offset parabolic reflector with an f/RD ratio of 0.25.

In this article, a circularly polarized dielectric resonator antenna (DRA) array with conformal characteristics and improved specific absorption rate (SAR) has been proposed for X-band applications. The proposed structure has been fed through the corporate feed network which excites a radiating mode inside DRA, i.e., $TE_{1\delta1}$. This mode has been utilized to enhance the impedance bandwidth which is below −10 dB for both the E- and H-plane so as to meet the requirements of next-generation defense communication and low-cost satellite systems. To generate the axial ratio (AR), the extended off-set feed has been employed to provide the required 90$^{\circ}$ phase shift. Further, in order to enhance the gain and reduce the SAR, an electromagnetic band gap structure has been used as a reflector. Furthermore, multiple arrays have been introduced to extend the coverage area through beam-forming. The proposed design has been fabricated for the experimental validation. The measured IBW and ARBW is 34.74% and 12.2%, respectively. The gain is 10.1 dBic throughout the band of operation along with the radiation efficiency above 85% in various bending conditions. The SAR is much below the permissible limit of 1.6 W/kg. Thus, the proposed array is compact, and it clearly achieves a smaller footprint, better IBW, ARBW and a low SAR with potential prospect for X-band purposes.