Microwave doi: 10.3390/microwave2020010

Authors: Xiang Pu Zhongqi He Kai Song Liping Yan Changjun Liu

In this work, a Fabry–Perot (F–P) antenna based on metal integrated suspended lines (MISLs) at the K-band for microwave wireless power transmission (MWPT) is proposed. The antenna’s contribution lies in its adaptation of the MISL structure and its all-metal design, which achieves low loss, high gain, and high-power capability. The entire antenna structure is dielectric-free, further reducing apparent dielectric loss at high frequencies. Meanwhile, the radiation structure is surrounded by a metallic wall to minimize radiation loss. A metal partially reflective surface (PRS) on the top of the antenna, together with a metal ground plane, constitutes an air-filled resonant cavity. The reflection and transmission of electromagnetic waves in the PRS are effectively controlled to be in phase, thereby enhancing its gain by optimizing the PRS and resonant cavity dimensions. A simple slot antenna is employed as the primary source for the F–P resonant cavity. The antenna is processed layer by layer and then assembled to lower machining costs and complexity. Experimental results indicate that the proposed F–P antenna achieves an aperture efficiency over 60% and a measured peak gain of 18.4 dBi at 23.85 GHz with an aperture size of 2.86 λ0 × 2.86 λ0.

Microwave doi: 10.3390/microwave2020009

Authors: Giovanni Leone Giovanni Volpicelli Rocco Pierri

The electromagnetic scattering problem for dielectric objects involves a linear integral equation that relates the total field to the scattered field inside the object. Numerical solution methods require discretization by choosing a suitable set of basis functions. An optimal choice of the expansion functions not only achieves numerical efficiency but also allows the correct subspaces to be spanned. We resort to the spectral decomposition of the pertinent operator to investigate such optimal functions. A one-dimensional (1D) geometry (i.e., scattering by a homogenous dielectric slab) is considered because it allows us to derive some analytical results and simple closed-form solutions to be used in the numerical verifications. Then, the spectral decomposition is performed numerically. The analysis of the eigenvalues and the eigenfunctions allows for predicting the required number once the maximum slab permittivity is given. In turn, the corresponding maximum number of Fourier harmonics can also be established, as they provide the basis to expand the eigenfunctions.

Microwave doi: 10.3390/microwave2020008

Authors: Jun Zhou Heng Luo Haoran Jia Yujie Zhang Huanwei Duan Huaizhong Chen Jian Dong Meng Wang Chenwang Xiao

High gain and low sidelobe level remain challenges for 5G millimeter-wave antenna systems. This paper presents a low-sidelobe, high-gain microstrip array antenna based on non-uniformly slotted identical-sized radiating patch, designed to simultaneously enhance gain and suppress sidelobe levels for 5G millimeter-wave (mmWave) communication systems. The key innovation lies in the use of an intermediate-deep, edge-shallow non-uniform slotting technique to precisely control the surface current distribution of the radiating elements, thereby achieving significant sidelobe level (SLL) suppression and antenna isolation enhancement without increasing the physical footprint of each element. The final design operates at a center frequency of 78.5 GHz, achieving a maximum gain of 15 dB and suppressing the first sidelobe below −20 dB, outperforming conventional linear arrays. It is noteworthy that, compared with a Chebyshev-distributed array, the patch width is reduced to only 1 mm, thereby enabling a compact array layout. The unit width dimension is reduced by over 40%, while in a densely packed array configuration, the inter-antenna isolation is increased by more than 18 dB. This current-distribution engineering approach offers a novel, structure-efficient pathway for designing high-performance, densely packed mmWave antenna arrays, circumventing the need for additional decoupling structures or enlarging the antenna spacing. Simulation results show that the average isolation has increased by more than 5 dB from 76 GHz to 79 GHz. Finally, the same design method was used to design a 24 GHz antenna, which was then fabricated and tested. The antenna achieved a sidelobe suppression of −17 dB.

Microwave doi: 10.3390/microwave2020007

Authors: Haleh Jahanbakhsh Basherlou Naser Ojaroudi Parchin Chan Hwang See

This paper introduces a compact sub-6 GHz multiple-input multiple-output (MIMO) antenna array developed for 5G smartphone applications. The design employs eight planar inverted-F antenna (PIFA) elements arranged to realize dual-band and dual-polarized operation. The antenna achieves impedance bandwidths of 3.3–3.7 GHz (11.4%) and 5.3–5.8 GHz (10%), covering key sub-6 GHz fifth-generation (5G) bands. To enhance diversity performance, the elements are distributed along the edges of the smartphone mainboard, enabling excitation of orthogonal polarization modes while maintaining an overall board size of 75 mm × 150 mm on an FR4 substrate. Even without the use of dedicated decoupling structures, the closely spaced antenna elements exhibit satisfactory isolation levels, varying between −12 dB and −22 dB across the operating bands. The antenna array achieves wide impedance bandwidths of approximately 400 MHz at 3.5 GHz and more than 500 MHz at 5.5 GHz, supporting high data-rate communication. In addition, the proposed system demonstrates very low correlation and active reflection, with envelope correlation coefficient (ECC) values below 0.002 and total active reflection coefficient (TARC) levels better than −20 dB. User interaction effects are also investigated, and the results confirm acceptable SAR levels and stable radiation behavior in the presence of the human body. Owing to its planar, dual-band/dual-polarization capability and compliance with safety requirements, the proposed antenna represents a promising practical solution for contemporary 5G handheld devices and future multi-band mobile platforms.

Microwave doi: 10.3390/microwave2020006

Authors: Jiayao Luo Hao Wen Liping Yan Xiang Zhao Changjun Liu

To overcome the intrinsic trade-off among miniaturization, ultrawideband (UWB) performance, and structural simplicity in conventional frequency-selective rasorber (FSR) design, this paper proposes a miniaturized UWB absorption–transmission–absorption (A-T-A) FSR based on an inter-cell current-interaction mechanism. The structure comprises a dielectric matching layer (DML), a lossy frequency-selective surface (FSS), a lossless FSS layer, and air/dielectric spacers. Both FSS layers are fabricated on Rogers 4350B substrates without any metallized via or multiple lossy/lossless FSS stacking. The proposed FSR achieves a miniaturized structure with dimensions of 0.085 λL × 0.085 λL × 0.118 λL (where λL corresponds to the wavelength at the lowest absorption frequency). A fractional operational bandwidth around 144% is obtained, covering 2.88–12.87 GHz and 14.98–17.61 GHz with absorptivity over 80%, together with a low-loss transmission band of 13.57–14.56 GHz exhibiting a minimum insertion loss of 0.41 dB. As the incident angle increases up to 40°, the FSR retains more than 134% bandwidth for both TE and TM polarizations. A prototype was fabricated and measured, and the results agree well with the simulations.

Microwave doi: 10.3390/microwave2010005

Authors: Abdullah Alothman Andrew DeVries Amir Mortazawi

Wireless power transfer (WPT) systems are generally sensitive to variations in separation distance and coil alignment, which result in reduced power transfer efficiency and delivered power. Various approaches based on control system and active matching circuits have resulted in more complex implementations. This work, by contrast, presents a full DC–DC inductively coupled WPT system employing coupled nonlinear resonators to automatically adapt the system for variations in transfer coil separation and orientation, maintaining high transfer efficiency at a constant output power level. With entirely passive circuit components, the nonlinear resonators suppress the frequency-splitting phenomenon typical of WPT systems that leads to efficiency degradation. A class-EF power amplifier used in the transmitter experiences an approximately constant impedance, providing a constant output power while maintaining high efficiency. On the receive side, a class-E rectifier operates at a constant input power, achieving high overall efficiency without active control. An experimental demonstration delivers 5 W with a 6.12% power variation over a 1 to 9 cm distance variation and achieves a peak DC–DC efficiency of 71.6%. The response of the system to changes in coil separation is compared with a conventional linear WPT circuit, showing a constant-power and high-efficiency operation.

Microwave doi: 10.3390/microwave2010004

Authors: Maurizio Vignolo

Microwave-assisted heating (MWH) has established itself as a transformative and energy-efficient paradigm for advanced materials processing. This review provides a comprehensive overview of the advances achieved at the CNR-SCITEC laboratories in Genoa. In this context, a customized microwave platform has been strategically employed for the synthesis, sintering, foaming, and melting of diverse inorganic, organic, and hybrid systems. The spectrum of materials investigated includes superconducting magnesium diboride (MgB2), hydroxyapatite-based scaffolds, polyethylene components obtained via microwave-assisted rotational molding, cork-based sound-adsorbing composites, recycled expanded polystyrene (rEPS) panels, and polyvinylidene fluoride (PVDF) piezoelectric films. Across the case studies, MWH demonstrated a superior capacity for reducing energy consumption and processing times while maintaining—or even enhancing—the target functional properties. Furthermore, this work evaluates the technological maturity and emerging market opportunities of microwave-based processing, positioning it as a key and sustainable platform for next-generation materials development.

Microwave doi: 10.3390/microwave2010003

Authors: Heping Huang Bo Yang Naoki Shinohara

Continuous-wave magnetrons continue to offer the highest efficiency, lowest cost per watt, and greatest compactness among high-power microwave sources, making them attractive for industrial, scientific, and defense applications. Emerging missions, particularly space solar power systems, industrial microwave heating, and accelerators, demand significantly enhanced performance metrics, including high DC-to-RF efficiency, thermal stability, ultra-low phase noise, and precise phase controllability for coherent operation. To satisfy the critical requirement for high power, low-cost microwave sources with high spectral purity, extensive research has focused on injection-locking techniques, external phase/frequency modulation methods, and large-scale coherent power combining. This paper reviews the fundamental characteristics of CW magnetrons, recent advances in injection-locked magnetron transmitters, power-combining systems employing multiple injection-locked magnetrons, magnetron-based phased-array systems, and emerging applications. Finally, the challenges and promising development directions for next-generation CW magnetrons are discussed.

Microwave doi: 10.3390/microwave2010002

Authors: Dimitrios Georgakopoulos Vasileios Manouras Ioannis Papananos

This work presents a fully integrated, two-stage, deep class-AB power amplifier (PA) operating at a center frequency of 60 GHz. High efficiency and suppression of third-order intermodulation products are targeted, achieving improved linearity compared to reported state-of-the-art designs. A current combining architecture is also employed to enhance the output power capability. The PA is designed in a 22 nm FD-SOI CMOS technology and is optimized through a complete schematic-to-layout design flow. Post-layout simulations indicate that the PA achieves a peak power-added efficiency (PAE) of 28%, a saturated output power (Psat) of 20.2 dBm, and a maximum large-signal gain (Gmax) of 19.6 dB at 60 GHz, evaluated at an operating temperature of 60 °C. The design maintains high linearity across the targeted output power range, exhibiting effective suppression of third-order intermodulation distortion (IMD3), which enhances its suitability for spectrally efficient modulation schemes.

Microwave doi: 10.3390/microwave2010001

Authors: Ioannis A. Bartsiokas Maria-Lamprini A. Bartsioka Anastasios K. Papazafeiropoulos Dimitra I. Kaklamani Iakovos S. Venieris

Accurate user positioning in 6G networks is essential for next-generation mobile services. However, classical approaches such as time-difference-of-arrival (TDoA) remain vulnerable to dense multipath and NLoS conditions commonly found in indoor and industrial environments. This paper proposes an AI-driven RF fingerprinting framework that leverages uplink channel state information (CSI) to achieve robust and privacy-preserving 2D localization. A lightweight convolutional neural network (CNN) extracts location-specific spectral–spatial fingerprints from CSI tensors, while a federated learning (FL) scheme enables distributed training across multiple gNBs without sharing raw channel data. The proposed integration of CSI tensor processing with FL and structured pruning is introduced as a novel solution for practical 6G edge positioning. To further reduce latency and communication costs, a structured pruning mechanism compresses the model by 40–60%, lowering the memory footprint with negligible accuracy loss. A performance evaluation in 3GPP-compliant indoor factory scenarios indicates a median positioning error below 1 m for over 90% of cases, significantly outperforming TDoA. Moreover, the compressed FL model reduces the FL communication load by ~38% and accelerates local training, establishing an efficient, secure, and deployment-ready positioning solution for 6G networks.

Microwave doi: 10.3390/microwave1030012

Authors: Liping Yan Chengrong Wang Xingrui Yin

Microwave heating technology has gained extensive application in various fields, including food processing and drying, material synthesis, and waste treatment, due to its advantages of high efficiency, selectivity, and rapid response. However, the inherent non-uniform electromagnetic field distribution within metallic microwave cavities leads to uneven heating. This issue severely constrains the large-scale industrial application of microwave heating, particularly its extension into high-precision, high-value-added industrial sectors like 3D printing, curing, and microwave plasma micro/nano-fabrication. To address this challenge, extensive research efforts have been undertaken in both academia and industry, focusing on improving microwave heating uniformity, resulting in the proposal of various methods. This review summarizes these methods for enhancing microwave heating uniformity and specifically outlines recent research progress on novel techniques based on artificial electromagnetic surfaces. It aims to offer valuable references and guidance for the broader application of microwave heating technology.

Microwave doi: 10.3390/microwave1030011

Authors: Weiyu He Kaida Xu

This paper provides a comprehensive review of recent advancements in radio-frequency (RF) multifunctional components with integrated filtering characteristics, including tunable filtering attenuators, filtering power dividers, filtering couplers, and filtering Butler matrices, all of which play critical roles in wireless communication systems. With the increasing demand for miniaturization, integration, and low-loss performance in RF front-ends, multifunctional components with filtering characteristics have become essential. This review first introduces tunable attenuators and filtering attenuators based on various technologies such as PIN diodes, graphene-based structures, and RF-MEMS switches, and also analyzes their advantages, limitations, and performance. Then, we discuss filtering power dividers developed from Wilkinson structures, three-line coupled structures, resonator-based coupling matrix methods, and SSPP-waveguide hybrids. Furthermore, filtering couplers and filtering Butler matrices are reviewed, highlighting their capability to simultaneously achieve amplitude and phase control, making them suitable for multi-beam antenna feeding networks. Finally, a brief conclusion is summarized. Future research directions, such as hybrid technologies, novel materials, broadband and multi-band designs, and antenna-matrix co-design, are suggested to further enhance the performance and practicality of multifunctional RF components for next-generation wireless communication systems.

Microwave doi: 10.3390/microwave1030010

Authors: Yusuke Asakuma Ryohei Yakata Anita Hyde Chi Phan Son A. Hoang

In this study, the microwave heating efficiency of a water body is investigated with different shape factors. In particular, the same water volume was deposited in cylindrical containers with different diameters. Here, “shape factor” refers to the ratio between the surface fluid layer, which strongly absorbs microwave energy, and the inner layer, which is heated largely via conduction. For a liquid in a cylindrical container, the shape factor is characterised as the ratio between the depth and diameter of the air/water surface area. The heating efficiency is characterised by relating the energy absorbed in the outer fluid layer with the energy gained in the bulk and monitoring the temperature in the fluid bulk at the point that the outer layer commences boiling. A correlation equation for the uniformity of the sample heating (with stirring) provided a simple link between the physical factors and microwave (MW) parameters. It was found that a depth/diameter ratio approaching 1:1 provided the most uniform heating. The correlations between the fitting parameters and physical conditions provide a simple yet effective method to characterise the thermal homogeneity of microwave heating that can assist with practical parameterisation of the design of microwave reactors.

Microwave doi: 10.3390/microwave1030009

Authors: Changjun Liu

The editors of Microwave extend their sincere thanks to every reviewer who assessed manuscripts for the journal in 2025 [...]

Microwave doi: 10.3390/microwave1020008

Authors: Rajeev Kumar Harish Kumar Choudhary Shital P. Pawar Manjunatha Mushtagatte Balaram Sahoo

In this study, we investigate the dominant electromagnetic wave absorption mechanism–ferromagnetic resonance (FMR) loss versus quarter-wave cancellation in a novel PVDF-based polymer composite embedded with carbonaceous nanostructures incorporating FeCoCr ternary alloy. The majority of the nanoparticles are embedded at the terminal ends of the carbon nanotubes, while a small fraction exists as isolated core–shell, carbon-coated spherical particles. Overall, the synthesized material predominantly exhibits a nanotubular carbon morphology. High-resolution transmission electron microscopy (HRTEM) confirms that the encapsulated nanoparticles are quasi-spherical in shape, with an average size ranging from approximately 25 to 40 nm. The polymeric composite was synthesized via solution casting, ensuring homogenous dispersion of filler constituent. Electromagnetic interference (EMI) shielding performance and reflection loss characteristics were evaluated in the X-band frequency range. Experimental results reveal a significant reflection loss exceeding −20 dB at a matching thickness of 2.5 mm, with peak absorption shifting across frequencies with thickness variation. The comparative analysis, supported by quarter-wave theory and FMR resonance conditions, indicates that the absorption mechanism transitions between magnetic resonance and interference-based cancellation depending on the material configuration and thickness. This work provides experimental validation of loss mechanism dominance in magnetic alloy/polymer composites and proposes design principles for tailoring broadband microwave absorbers.

Microwave doi: 10.3390/microwave1020007

Authors: Farhad Ghorbani Amir Dayan Jiafeng Zhou Yi Huang

This paper presents a highly linear one-bit loaded-line phase shifter that leverages PIN diodes in combination with a coupler-based impedance transformer. The proposed phase shifter adopts a loaded-line topology, where PIN diodes are configured in a parallel-to-ground arrangement to improve linearity performance. To further enhance linearity, a coupler-based impedance transformer is employed to reduce the impedance seen by each PIN diode, thereby minimizing nonlinear behavior. To demonstrate the effectiveness of this design, a one-bit digital phase shifter is developed, simulated, and fabricated to achieve a 45-degree phase shift at 2 GHz. Experimental measurements confirm an input third-order intercept point (IIP3) exceeding 100 dBm under a range of test conditions, validating the proposed architecture’s linearity advantages.

Microwave doi: 10.3390/microwave1020006

Authors: Judith González-Lavín Ana Arenillas Natalia Rey-Raap

There is currently an effort to scale up sol–gel nanomaterials without compromising quality, and microwave heating can pave the way for this due to its heating efficiency, resulting in a fast and homogeneous process. In this work, the sol–gel synthesis of transition metal aerogels, specifically iron-based aerogels, is studied using a microwave-assisted sol–gel methodology in an open-system multimode device as a potential route to scale-up production. Different approaches were tested to evaluate the best way to increase yield per batch, with different vessel shapes and volumes. It is shown that the shape and size of the vessel can be determinant in the interaction with microwaves and, thus, in the heating process, influencing the sol–gel reactions and the characteristics and homogeneity of the obtained nanomaterials. It has been found that a wide vessel is preferable to a tall and narrow one since the heating process is more homogeneous in the former and the sol–gel and cross-linking reactions take place earlier, which improves the mechanical properties of the final nanomaterial. For mass production of nanomaterials, the interaction of the reagents with the microwave field must be considered, and this depends not only on their nature but also on their volume, shape, and arrangement inside the cavity.

Microwave doi: 10.3390/microwave1010005

Authors: Yilin Zhou Ruinan Fan Changjun Liu

The advancement of microwave wireless power transfer technology has positioned low-power rectennas as a research hotspot. This paper systematically reviews core technological progress in low-power rectennas, focusing on innovations in rectifier circuit topologies, nonlinear device models, antenna array optimization, and efficiency enhancement strategies. Current technical bottlenecks and future application directions are analyzed, providing theoretical references for space solar power stations, IoTs, and related fields.

Microwave doi: 10.3390/microwave1010004

Authors: Yanxing Zhang Jinling Zhang

We propose a new ultra-wideband antipodal Vivaldi antenna design based on the Klopfenstein curve, incorporating exponential slots, horns, and apertures to improve the antenna’s return loss and increase its gain in high-frequency bands. The antenna achieves high gain and wide bandwidth characteristics, with measured −10 dB bandwidth ranging from 2 GHz to 20 GHz, maximum gain of 14 dBi, and gain exceeding 10 dBi from 3.5 GHz to 14 GHz.

Microwave doi: 10.3390/microwave1010003

Authors: Mingyu Lu Charan Litchfield

It is well known that two pieces of electrical conductors behave as a waveguide when they are employed to transmit AC signals. Some experimental and analytical studies are reported in this paper to demonstrate that two pieces of electrical conductors also behave as a waveguide when they are employed to transmit DC signals in practice. Specifically, the speed of wave propagation is measured in the experiments, and the analytical studies are based on the theory of transmission line.

Microwave doi: 10.3390/microwave1010002

Authors: Haoran Zhao Qi Wang Wen Yue Wei Wang

This work proposes an all-quartz integrated lens antenna for the first time. The antenna feed and lens materials are both made of quartz. The antenna is designed to work at 26 GHz and has the advantages of small size and reduced surface wave loss. Two antennas, a hemispheric lens and an extended hemispheric lens, are demonstrated. The hemispheric lens has an area of 30 × 30 mm2 and a height of 16 mm, while the extended hemispheric lens, with the same area, has a height of 26 mm. The measured peak gain of the extended hemispheric integrated lens antenna is 15.49 dBi, and the simulated peak gain is 17.68 dBi. The electric-field distribution was analyzed, and two hemispheric lens and extended hemispheric lens antennas of the same size were designed on PCB substrate for comparative analysis. The measured results validate the impact of the surface wave effect on the gain of the lens antenna proposed in this study.

Microwave doi: 10.3390/microwave1010001

Authors: Changjun Liu

Microwave technologies have long been at the forefront of scientific and engineering innovation, driving advancements in wireless communications, radar, satellites, medical applications, wireless power transfer, and more [...]