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The microwave-to-plasma energy conversion efficiency and the ease of plasma self-ignition are critical factors affecting the applications for microwave plasma sources (MPSs). This study presents a novel MPS utilizing dielectric wedges for self-ignition and improved energy conversion. Firstly, we crafted a dielectric wedge with a gradient refractive index, guiding the electric field from air to dielectric materials and facilitating microwave propagation along the dielectric in a waveguide. Through electromagnetic simulation, we explored how the size and permittivity of the dielectric wedge affect the electric field distribution. Then, the MPS based on the dielectric wedge was designed. In this configuration, a dielectric tube encloses the discharge tube, connecting to dielectric wedges to guide electromagnetic waves to the plasma. We analyzed the MPS performance using the Drude model, evaluating microwave energy conversion efficiency across various electron densities and collision frequencies. The results were compared with a commonly used MPS based on a tapered waveguide, demonstrating the proposed MPS has wider applicability across different operation conditions. Finally, experiments under low pressures were conducted using various gases, showing an average energy conversion efficiency of approximately 40% higher than the tapered waveguide MPS. The experiments also indicate the proposed MPS has a greater capability of self-ignition at lower power levels. These findings highlight the efficacy of incorporating dielectric wedges to enhance MPS performance, making it conducive for broader industrial applications. [ABSTRACT FROM AUTHOR]

This study investigates the counterintuitive behaviors of a microwave‐induced atmospheric pressure plasma jet (APPJ) under the influence of an external electrostatic field applied by one or a pair of parallel electrode plates subjected to DC voltages. The findings demonstrate that the plasma jet consistently deflects toward the electrode plate with an electrostatic potential, irrespective of the direction of the applied field. The deflection becomes more pronounced with increasing voltage on the electrode plate until the ionic wind generated by the high voltage significantly affects the jet's behavior. Remarkably, a negative voltage induces a greater deflection compared to a positive voltage. To further investigate this discovery, the dielectric properties and the non‐neutral characteristics of the APPJ are analyzed, and the simulations of the electric field distribution reveal a non‐uniform distribution, which plays a crucial role in understanding the mechanism behind the observed behaviors of the plasma jet. This study provides a comprehensive understanding of the underlying mechanism driving the observed phenomena and sheds light on the collective behavior of plasma jets under non‐uniform electric fields. The findings of this study offer valuable guidelines for investigating and controlling the behavior of APPJs. [ABSTRACT FROM AUTHOR]

A compact high-power four-port circulator aiming to simplify the conventional, complex, and bulky injection-locking magnetron system is proposed. To reduce the performance deterioration and the risk of ferrite rupture under long-term high-microwave-power condition, the method of breaking a monolithic ferrite into three discrete ferrites in a conventional three-port circulator is proposed. To miniaturize the size and cost of the four-port circulator, a butterfly-shaped waveguide structure is proposed, with a stub inserted into the cavity at the central point and with no connecting waveguide. Multiphysics simulation results show that the temperature coefficient of variation (COV) at the surface of the discrete ferrites is 12.4% lower than that of a monolithic ferrite circulator, with input microwave power of 10 kW. The size of the proposed four-port waveguide circulator is 27% less than the assembly of two three-port circulators, and way smaller than a conventional differential phase shift circulator (DPSC). The simulated and measured S-parameters match well, and the measured power capacity of the fabricated circulator is higher than 3 kW (limited by the experimental condition). A magnetron is successfully locked using only one designed compact circulator. The research in this paper promotes the development of injection-locking magnetron and provides a design example for the compact, high-power circulator. [ABSTRACT FROM AUTHOR]

Permittivity is an important aspect of the design of microwave devices. In order to design microwave devices that are more suitable for the excitation of air plasmas, this paper presents a method for the on-line measurement of the relative permittivity of an atmospheric pressure air plasma with 2.45 GHz microwaves. First, a ridged waveguide is designed and constructed to excite the air. Using the ridged waveguide, the plasma can be excited and measured simultaneously. An artificial neural network trained with simulated data is integrated into the system to invert the dielectric constant of the air plasma being measured. At a power of 786 W and an airflow rate of 1250 L/h, the real and imaginary parts of the permittivity are obtained to be −1244 and 60, respectively. The forecasted errors on the real part and the imaginary part of the relative permittivity are kept at around 5% and 3%. The Drude model is used to construct a relationship between the electronic and dielectric properties. This diagnostic method is simple to operate and has a high degree of generality. It should be useful in the design of microwave plasma devices and can greatly improve the efficiency of microwave plasma equipment. [ABSTRACT FROM AUTHOR]

Cylindrical loads are commonly found in microwave heating. However, cylindrical loads tend to have a thermal focus, leading to uneven microwave heating. In this paper, a novel microwave heating method was proposed by employing resonant rings to mitigate the thermal focus of cylindrical loads. Firstly, a multiphysics simulation model for microwave heating of a cylindrical tube is established, and the influence of the resonant ring around the cylindrical tube on the temperature distribution of load was analyzed. Subsequently, microwave heating experiments of agar gels with and without resonant rings were carried out. In agreement with the simulation results, the use of resonant rings reduces the coefficient of temperature variation (COV) from 1.064 to 0.793. In addition, the parameters affecting the heating uniformity of the cylindrical tube were discussed through simulations. [ABSTRACT FROM AUTHOR]

A wideband superdirective array, composed of a two-element circular monopole configuration, is introduced. The monopoles are placed in close proximity, facing each other on a metal ground. To ensure good matching at high frequencies, two pairs of elliptical patches are added to the sides of the monopoles, enhancing the surface current of the circular patch for wideband performance. To achieve equal amplitude excitation and the desired phase difference, a wideband power divider with a phase shifter is designed to feed the antenna array. Simulation and measurement results demonstrate that the proposed wideband antenna array, operating within the frequency range of 2.94–7.93 GHz, exhibits a maximum directivity of 8.36–10 dBi, with an antenna efficiency ranging from 47.86 to 83.18% across the bandwidth. The proposed array has the advantages of miniaturization, high directivity and wideband operation and can be widely used in various portable wireless communication systems, including WLAN (5.05–5.9 GHz), ISM (5.725–5.875 GHz), 5G communication (3.3–3.8 GHz), etc. [ABSTRACT FROM AUTHOR]

A novel planar lens structure is proposed by using metasurfaces to convert cylindrical‐like surface wave to inclined nondiffracting wave with a deflection angle. Based on the geometric optics, the refractive index distribution of the surface wave lens is derived. A square metal ring‐shaped metasurface cell is used for realizing small refractive index whilst two opposite E‐shaped metal patches are used to form the meta‐cell with large refractive index. By adjusting the size of meta‐cells, the desired refractive index distribution is obtained after optimization. Measured results show that the inclined nondiffractive surface wave can be achieved with the maximum propagation distance of 99.72 mm at the tilted angle of 26.8°, which are consistent with the full‐wave simulation and numerical calculation results. The proposed meta‐lens achieves compact size, large titled angle, and long nondiffraction propagation distance, therefore could be well utilized in surface wave communication and nondestructive detection of surface defects. [ABSTRACT FROM AUTHOR]

A planar metasurface Bessel lens has been proposed for non-diffraction surface wave deflection, which is composed of a planar flat lens and an asymmetric gradient refractive index lens. According to the desired refractive index distribution of the two lenses, a metasurface, consisting of sub-wavelength metallic square rings of different sizes, is constructed to realize the planar Bessel lens. Through the planar Bessel lens, the cylindrical-like surface wave radiated from a point source is converted to an inclined non-diffraction surface wave. The planar Bessel lens is fabricated, and the measurement results show good agreement with the full-wave simulation results, which would verify the correctness of the proposed theory. [ABSTRACT FROM AUTHOR]

A microwave atmospheric pressure plasma jet (MW‐APPJ) generator based on a substrate‐integrated waveguide (SIW) resonator is proposed. A thin, straight plasma needle with a length of 9 cm could be obtained. Higher than 90% of microwave feeding efficiency is obtained under a wide range of gas flow rates without a microwave matching network. As the gas flow increased from 1 to 14 L/min, a transition from laminar to turbulent mode is observed in the plasma jet's morphology. Furthermore, a preliminary surface treatment was tested for the device. The polyimide film became super hydrophilic after a 5‐min treatment with the developed SIW‐based plasma generator. Generally, the SIW‐based MW‐APPJ is low‐cost, electrodeless, and low‐profile along the plasma jet length direction. [ABSTRACT FROM AUTHOR]

In this paper, an oblique aperture ridge waveguide operating at 2450 MHz is proposed, and, using the ridge waveguide, a permittivity measurement system is constructed which can measure the permittivity of materials during microwave heating. The system calculates the amplitudes of the scattering parameters by using the forward, reflected and transmitted powers of the power meters, and it reconstructs the permittivity of the material by combining the scattering parameters with an artificial neural network. The system is used to measure the complex permittivity of mixed solutions of methanol and ethanol with different ratios at room temperature, and the permittivity of methanol and ethanol with increasing temperature, from room temperature to 50 °C. The measured results are in good agreement with the reference data. The system allows simultaneous measurement of the permittivity with microwave heating and provides real-time, rapid changes in the permittivity during heating, avoiding thermal runaway and providing a reference for applications of microwave energy in the chemical industry. [ABSTRACT FROM AUTHOR]

Microwave-assisted chemical reactions have been widely used, but the overheating effect limits further applications. The aim of this paper is to investigate the coupling degree of the electromagnetic field and thermal field in microwave-heating chemical reactions whose polarization changes as the reactions proceed. First, the entropy-balance equation of microwave-heating polar-molecule reactions is obtained. Then, the coupling degree of the electromagnetic field and the thermal field in microwave-heating polar-molecule reactions is derived, according to the entropy-balance equation. Finally, the effects of reaction processes on the degree of coupling are discussed. When the time scale of the component-concentration variation is much greater than the wave period during the chemical processes, the degree of coupling is sufficiently small, and the electric and thermal fields are considered as weakly coupled. On the other hand, the degree of coupling may change during the reactions. If the absolute value of the coupling degree becomes larger, due to the change in component concentration, this will lead to a transition from weak coupling to strong coupling. [ABSTRACT FROM AUTHOR]

Traditional tapered waveguide devices are limited by their power efficiency and produce unstable plasmas and small reaction zones. To overcome these drawbacks, a multi-ridge field compressed reactor operating at a frequency of 2.45 GHz is designed in this paper. The finite element method is applied to numerically compute its properties. A coupling model for the multiple physical fields is introduced to simulate the transient processes of a microwave argon plasma. Experiments with different flow rates and power settings are performed. This novel device can both sustain a long-length plasma and increase the power transfer efficiency from microwave to air and to the argon plasma, respectively, to 97.4% and 95.2%. Moreover, both devices are used to process the degradation of ethanol and benzene. The removal efficiencies are increased by 38.8% and 73.8% for ethanol and benzene, respectively, in the novel waveguide compared with the tapered waveguide. [ABSTRACT FROM AUTHOR]

This article reports experimental observation of rotating plasma discharges in a coaxial structure at a frequency of 2.45 GHz and under atmospheric pressure. In contrast to other cases, the discharge in our case spins without external static magnetic field or moving electrodes. Its formation and impact factors are experimentally explored. Results show that the rotating discharge, especially the peculiarities of its rotation velocity, is attributed to the combined action of gas flow and electromagnetic field intensity between the conductors. Raising the electromagnetic field intensity or reducing the gas flow rate in appropriate ranges can result in a notable increase of the rotation speed. Experiments also demonstrate that the rotation properties are independent of the coaxial dimensions. This article is supposed to provide a new thinking and guidelines for inducing rotating discharge in a coaxial structure without electrodes moving or external magnetic field. [ABSTRACT FROM AUTHOR]

A leaky wave antenna (LWA) based on half‐mode substrate integrated waveguide (HMSIW) is presented. Integrated phase inverters are used to eliminate the far‐field cancelation from the adjacent radiating element with half‐wave spacing. The ground plane is extended to reflect the quasi‐magnetic‐dipole radiation pattern from the backfire to the boresight for enhanced antenna gain. Two linear arrays with differential feeding are deployed back‐to‐back along their longitude, by which the far‐field radiation adds constructively that leads to further increased antenna gain. The proposed antenna features shorter electric length compared with counterparts of the same gain. Agreement is achieved between simulation and measurement. The proposed antenna has a measured impedance bandwidth of 11–12.8 GHz (15.6%), the measured maximum gain of 16.2 dBi @ 11.6 GHz. The antenna's beam scans from 20° to 50° in the boresight with the input signal's the frequency. By using the phase inverter, the extended ground plane and the back‐to‐back deployment, the proposed antenna produces equal gain with shorter antenna's electric length. [ABSTRACT FROM AUTHOR]

In this letter, an efficient broadband microwave rectifier with compact size for wireless power transfer is presented. It is proven that the proposed voltage doubler is easier to be matched than the series diode rectifier to achieve a wide bandwidth. In our analysis, the voltage doubler itself is already matched without the input matching network at the lower frequency band. For higher frequency band matching, a short section of microstrip line is connected to the input port of the voltage doubler. Thus, the proposed rectifier can operate over a broad bandwidth. To verify the design concept, a rectifier is fabricated, and the measurement results demonstrate that the power conversion efficiency is greater than 50% from 0.1 to 4.3 GHz at an 18 dBm input power level, and is higher than 70% within a wide bandwidth of 76.9%. In addition, when the input power is 18 dBm, the peak efficiency is 76.36% at 1.4 GHz. The results show that the proposed rectifier has the merits of compact size, broad bandwidth, and high efficiency. [ABSTRACT FROM AUTHOR]

In this work, an electromagnetic black hole based on gradient-index metamaterials is designed and fabricated by drilling holes in easily available materials to achieve efficient microwave heating in multimode cavities. This black hole shows superior performance in improving the utilization of microwave energy when the real part of the relative permittivity varies from 10 to 80 and the loss tangent of the heated material changes within 0.1–1. Different volume fractions of ethanol aqueous solution are used as test material to verify the effectiveness of electromagnetic black hole, and the experimental results are in good agreement with the simulation results. [ABSTRACT FROM AUTHOR]

In this article, the hybrid solar energy harvesting and wireless energy harvesting system is proposed. The hybrid energy harvesting system is comprised of the rectifiers, the solar cell panel, and the tree‐shaped antenna. The four tree‐shaped antennas are fabricated using three‐dimensional (3D) printing. The rectifiers are directly beneath the solar cell panel, and the every antenna connect to the corresponding rectifiers by the SMA adaptor. The key preponderance of this method is that the solar cell panel combined with the antenna to create multifunctional compact hybrid RF and solar energy harvesting system. Meanwhile, the four tree‐shaped antennas are used for enhancing the integral RF harvested energy. In addition to this, the 3D printed tree‐shaped antenna are nearly not affect on the sunshine received from the solar cell panel. It is demonstrated that in the lab, the maximum harvested power of the rectenna is 52.6% when the receiving RF power is 8 dBm, and the solar cell panel can achieve 7.3 mW power. The tree‐shaped antenna operates from 2.39 to 2.52 GHz for the reflection coefficient below −10 dB, and the peak gain is 6.0 dBi. Besides, when the RF input power is 8 dBm, the RF‐to‐DC conversion efficiency of the rectifier is 68.7%. The feasibility of the hybrid solar‐RF energy harvesting system used tree‐shaped antenna is demonstrated. [ABSTRACT FROM AUTHOR]

A low‐profile self‐hexplexing antenna based on the multimode substrate integrated waveguide (SIW) cavity is presented for mobile terminal applications. It has three feeding ports, each of which can generate two resonant modes. Taking the advantage of the truncated SIW resonant cavity, the footprint of the antenna remains compact in the multiple functional mode scenario. The proposed antenna operates at six different modes, namely TM10 mode at 3.55 GHz, TM01 mode at 4.85 GHz, TE110 mode at 2.45 GHz, TE220 mode at 5.6 GHz, TE110 mode at 3.4 GHz and TE120 mode at 5.35 GHz. Good agreement is achieved between the simulated and measured antenna performance. The excitation and field distribution of each mode is analyzed. Keeping the structure miniaturized, the proposed antenna realizes hex‐band characteristics without excessive feeding ports. It can be used for Microwave Access (WIMAX), the 5th Generation Mobile Communication Technology (5G) and Wireless Local Area Network (WLAN) applications, which is suitable for mobile handheld multiband terminals. [ABSTRACT FROM AUTHOR]

In this paper, a novel mushroom antenna with high gain using three dimensional printing (3D printing) is proposed. The proposed antenna is comprised of the circular microstrip patch antenna and the mushroom lens by using 3D printing (the mushroom lens include the hemispherical mushroom hairstyle lens and the bowl shaped mushroom stalk). And the cambered internal surface of the bowl shaped mushroom stalk is painted in sliver conductive adhesive. The circular microstrip patch antenna is installed in the bowl shaped mushroom stalk. The good performance characteristics of the high gain depend on the directive of the hemispherical lens and the bowl shaped internal surface. The antenna is fabricated and measured, the antenna operates from 5.56 to 6.0 GHz for the reflection coefficient ≤−10 dB, and the maximum gain is 16.5 dBi at 5.8 GHz. In addition to this, the antenna is low cost, very cross polarization and easy fabrication. [ABSTRACT FROM AUTHOR]

In this letter, an ultra-broadband rectifier is proposed and tested to get more RF energy from ambient. The rectifier using the Dickson charge pump increases the bandwidth and improves the dc voltage of the output load. The upper branch uses an inductor in series to counteract the capacitance of the imaginary parts of the diode and at the same time acts as a booster circuit to increase the output voltage. A shunt open-circuit stub acts as part of the matching network, tuning the imaginary part of the input impedance of the high frequency to realize the impedance matching of the ultra-broadband rectifier. The manufactured rectifier is measured and the result shows a fractional bandwidth of 145.5% (0.6–3.8 GHz) with an RF-to-dc power conversion efficiency (PCE) exceeding 40% at 8 dBm input power. And the measured efficiency remains over 50% across the frequency range from 1.6 to 3.4 GHz (including GSM1800, GSM2100, and WiFi). In addition, the rectifier made a maximum conversion efficiency of 66.6%. [ABSTRACT FROM AUTHOR]

In this letter, a multioctave voltage doubler rectifier using a T-type matching network is demonstrated. The voltage doubler topology is used to flatten the load impedance, which makes it easier to achieve broadband impedance matching. The T-type matching network is used for the broadband input impedance matching. The design procedure of the proposed broadband rectifier is provided and discussed. For validation, a rectifier prototype is fabricated, and the performance is evaluated. The measured results exhibit power conversion efficiency (PCE) of more than 50% over a frequency band of 0.2–3.2 GHz at 15 dBm input power and 70% from 1 to 2.7 GHz. Moreover, the peak PCE is 78.2% at 1.9 GHz when the input power is 18 dBm. [ABSTRACT FROM AUTHOR]

In this article, a cheap and simple method to determine the electron density of atmospheric microwave air plasma torch (AMAPT) is proposed, which avoids the complex and costly spectrum diagnosis methods. First, the incident and reflected microwave (MW) power of the AMAPT are measured to obtain the MW power reflection ratios. Then, a 3-D Gaussian electron density distribution is built to simulate the AMAPT, especially in calculating the MW power reflection ratios. By adjusting the electron density distribution to make the calculated MW power reflection ratios equal to the measured results, the electron density can get calculated. Finally, to verify this calculation, a system of a 2.45-GHz AMAPT is built up to measure the electron density with optical emission spectroscopy (OES) in different gas flow rates and gas compositions. The results show that the electron density determined by this method agrees well with the experimental measurements by OES with a relative error of 8% which is competent for engineering applications. The simplicity and accuracy make it a promising approach to obtaining the electron density of MW plasma in engineering applications. [ABSTRACT FROM AUTHOR]

Graphene has received intensive research interest in various areas due to its outstanding mechanical, electrical properties, high specific surface area and 2D flexible structure. Mechanical exfoliation generates defect‐free graphene, but suffers from ultra‐low yield. This drawback can be partially overcome by chemical vapor deposition method (CVD). However, conventional CVD imposes high temperature. In recent years, microwave plasma‐enhanced CVD (MPCVD) has been widely investigated as a viable strategy toward large‐area graphene synthesis. In this review, important issues during the process are summarized. Firstly, various MPCVD devices that have been explored for graphene synthesis are presented. Secondly, effect of carbon sources, nitrogen‐containing gases, and CO2 in the precursors are discussed. Thirdly, MPCVD enabled direct synthesis of graphene on metal substrates and non‐metal sustrates is discussed. Finally, the advances of MPCVD in lowering the graphene synthesis temperature are demonstrated. [ABSTRACT FROM AUTHOR]

A 7 × 8 Butler matrix (BM) beam‐forming network operating at Ka‐band is proposed to feed a 8 × 8 slot array. By merging the two central input ports of the BM into one port, the BM can provide output signals with equal phase, thereby generating a boresight beam. Measured results show that the proposed antenna has impedance bandwidth of 29.5–30.5 GHz, while the gain lies in the range of 14.7–21.8 dBi. Seven beams are realized, including a boresight beam, covering the range from −58° to 58°. In order to verify this concept, the feeding network and antenna are designed and manufactured. The measured results are in good agreement with simulation. It is a capable candidate for 5G mobile terminal applications. [ABSTRACT FROM AUTHOR]

Microwave-driven electrodeless ultraviolet (UV) lamps have the advantages of high efficiency and high power. However, the conventional microwave system is slightly oversized, which restricts the use of the lamp in a narrow space. A miniaturized microwave-driven electrodeless UV lamp based on a coaxial slot antenna was developed in this study. First, the structure of slots was optimized using a finite-difference time-domain algorithm such that high efficiency of radiated energy could be achieved. Second, a complex model based on the Drude model and the electromagnetic theory was established to simulate the interaction between the microwave and UV lamps. The efficiency and uniformity of the UV lamps were analyzed. Finally, an experimental system was built, and the computed results agreed well with the simulation results. The efficiency of the miniaturized microwave-driven electrodeless UV lamp reached 91.8%. [ABSTRACT FROM AUTHOR]

A novel two‐dimensional atmospheric pressure plasma jet device is proposed for large‐area processing. A finite element algorithm is adopted to obtain the optimized dimensions of the designed device based on microwave power reflection coefficient |S11| and electric field intensity. The plasma parameters of electron density ne and electron temperature Te within 0.1 ms after microwave power input were analyzed. When plasma is fully excited, ne is close to 3.5 × 1018 m−3, and Te is calculated to be 2 eV. Besides, an experimental system is established, and microwave power of 70 W can process 30 L of argon per minute, which shows a high energy efficiency. Two plasma jets with a width of 10 mm and a length of 5 mm are successfully excited. In addition, reflected power measured by the power meter is only 9.5% of incident power, which is in good agreement with simulation results. [ABSTRACT FROM AUTHOR]

Here, FeAlOx catalytic deconstruction of polyethylene in a domestic microwave oven is reported. With the starting weight ratio of FeAlOx to polyethylene at 1:1, the concentration and yield of H2 reach up to 67.85 vol% and 48.1 mmol g−1plastic, respectively. CNTs@Fe3O4/Fe3C/Fe composite, which exhibits excellent microwave absorption properties, is generated simultaneously. The minimum reflection loss (RLmin) of the solid product reaches −54.78 dB at 15 GHz with an effective absorption bandwidth of 4.5 GHz at the thickness of 1.57 mm. [ABSTRACT FROM AUTHOR]

A filtering antenna composed of a monopole and two stacked circular substrates integrated waveguide (SIW) cavity for 5.8 GHz WiFi application is proposed in this letter. Cross‐coupling between the feeding probe and the second SIW cavity resonator is utilized to generate a radiation null at the frequency above the working band. By properly designing the location of the feeding probe and the size of the coupling slot between the two SIW resonators, the frequency of the radiation null can be adjusted. Experimental results of the fabricated filtering antenna show good agreement with the simulated performance. The measured filtering antenna response has a return loss greater than 10 dB with a fractional bandwidth of 16.4% centered at 5.48 GHz while there is a radiation null at 6.85 GHz. Omnidirectional patterns are achieved in horizontal plane with a maximum gain of 1.1 dBi. The proposed filtering antenna has the merits of compact size and good stopband radiation suppression. [ABSTRACT FROM AUTHOR]

Traditionally, the signal power is divided equally between dual functions in wireless information and power transmission (WIPT), which, intuitively, is not an optimal solution since the power sensitivities for communication and charging node pose different constraints for signals. To address this challenge, a two-channel rectenna with an asymmetrical coupler feeding network (ACFN) has been proposed for WIPT under the power-splitting and time-sharing (PS and TS) schemes. The proposed two-port rectenna consists of a receiving antenna integrated with ACFN where one output port is connected to a rectifying circuit, whereas the other is used for information recovery. Different from the conventional PS scheme with equal division, the proposed rectenna can adjust ratios of power at two outputs. That is, routing high power to the radio frequency (RF)-dc port, and transferring low power but of a sufficient signal-to-noise-ratio (SNR) to the port being used for signal demodulation/decoding. As a result, the simulated/measured maximum RF-dc conversion efficiency of the rectenna can reach up to 73.9%/70.4% for quadrature phase shift keying (QPSK) modulated signals and 63.9%/60.7% for continuous wave (CW) signals. In addition, good isolation between the two output ports ensures low interference against information decoding. [ABSTRACT FROM AUTHOR]

A substrate integrated waveguide multibeam antenna fed by a double-fold 7×8 Butler matrix (BM) beam-forming network (BFN) is presented. The proposed antenna is composed of a 7×8 BM feeding network and an 8×8 slot array. By merging the two central input ports of the BM into one port, the BM can provide output signals with equal phase, which can generate a boresight beam. In order to reduce the size of the antenna, the BM is folded twice into a three-layer structure. Measured results show that the proposed antenna has impedance bandwidth of 29.5 ∼ 30.5 GHz, whereas the gain lies in the range of 13.2–17.3 dBi. Seven beams, including a boresight beam, pointing from −58° to 58° are achieved. Compared with counterparts, the proposed antenna features boresight radiation and miniaturized size. It is a good candidate for the fifth-generation (5G) mobile network applications. [ABSTRACT FROM AUTHOR]

Continuous-flow microwave heating is widely used in chemical engineering and other fields. However, during microwave heating, the load’s electrical permittivity change will affect microwave heating efficiency. In this article, a WR430 asymmetric propagation waveguide is proposed based on gradient-index metasurfaces (GIMs), which can achieve high-efficiency continuous-flow heating with a large range of permittivity. First, asymmetric microwave propagation is realized in a WR430 3-D waveguide model, and the influence of metasurfaces index, thickness, and discrete element size is analyzed. Second, the asymmetric propagation waveguide is realized by fluted medium based on the permittivity equivalent method. The results show that the microwave inside the waveguide can only propagate along one direction at 2.4–2.6 GHz. Finally, the proposed waveguide is applied to a continuous-flow heating system. Compared with the single-mode cavity, multimode cavity, and waveguide water load, the proposed method can improve heating robustness and efficiency with different loads. Experiments show that the microwave absorption efficiency is stable and greater than 90% in the proposed system when the permittivity of the load changes drastically. The proposed system has a simple structure and strong applicability, which can replace the three-stub tuner and get rid of the circulator and water load for continuous-flow heating. [ABSTRACT FROM AUTHOR]

Graphite‐derived carbon materials have been widely used in metal‐ion batteries due to their good mechanical and electrical properties, cost effectiveness, light weight, and environmental friendliness. Though natural graphite has been commercially used in lithium‐ion batteries, the small interlayer spacing hinders its application in other metal‐ion batteries. As such, numerous works have been done to enhance the metal‐ion storage capability of graphite and its derivatives. In this review, structural engineering on graphite including expanded graphite, graphite intercalation compounds and porous graphite, and the corresponding electrolyte engineering for lithium‐ion batteries, sodium ion batteries, potassium ion batteries, and dual ion batteries are summarized, aiming to make a comparison between various strategies and give a suggestion on future work in this area. [ABSTRACT FROM AUTHOR]

In this letter, a compact broadband RF rectifier is proposed and tested for ambient RF energy harvesting (EH). An uncomplicated impedance matching network with multistage transmission lines (quasi T-shaped) is presented. Quasi T-shaped network performs three different functions to complete the broadband matching process. The novel matching network widens working bandwidth using fewer transmission lines without via hole so that it has low loss and high conversion efficiency. After theoretical analysis and simulation, the rectifier is manufactured, and the measured results are consistent with the simulation results. The rectifier shows good broadband performance. The frequency range of rectifier efficiency over 50% is from 0.85 to 2.05 GHz (a bandwidth of 82.7%) at 5 dBm. The rectifier shows a bandwidth of 101.5% (0.8–2.45 GHz) with a power conversion efficiency (PEC) greater than 50% for an 8 dBm input power. In addition, the maximum conversion efficiency of the circuit is as high as 73.1% at the input power of 8 dBm. [ABSTRACT FROM AUTHOR]

Impedance matching plays a vital role in a microwave atmospheric pressure plasma jet (MW-APPJ). However, real-time tuning of MW-APPJ impedance matching has rarely been investigated. In this study, we have found that the dissipated microwave power and length of a MW-APPJ are effected by the application of a DC-bias magnetic field. By placing a MW-APPJ in a gap between two electromagnets, the consumed microwave power varied with the insertion position of the MW-APPJ. When the MW-APPJ was fixed at the center of the electromagnets, the consumed microwave power decreased as the DC magnetic field intensities increased, from 12 to 750 mT. The variation of consumed microwave power was much higher at lower gas velocities, which indicates that the DC magnetic field has a relatively strong effect on microwave plasma with lower gas velocity. The length of the plasma jet also decreased with increasing DC magnetic field, which was mainly due to the variation of the consumed microwave power. To explain these interesting phenomena, the effective permittivity of microwave plasma and the equivalent circuit model of a generator under DC-bias magnetic field have been analyzed. The effective permittivity of the plasma is related to its cyclotron frequency, which is a function of the DC-bias magnetic field. The variation of the effective permittivity changes the load impedance of the plasma generator. Therefore, the reflection coefficient of the plasma generator is adjusted and the length of the plasma jet varies with the change in consumed microwave power. [ABSTRACT FROM AUTHOR]

This article presents a new type of microwave-induced atmospheric-pressure low-temperature dielectric barrier discharge source, based on the principle of parallel plate transmission lines (PPTLs). This source is a simple structure with small size, large plasma discharge area, and low discharge temperature. Experiments show that linear plasma discharge with width of 15 mm can be obtained with input power of only 25 W at 2.45 GHz. The temperature of sample placed 1 mm vertically away from the nozzle and treated for 3 min is about 45 °C even with 60-W microwave input power, which proved its thermal safety even for the biomedical applications. Emission spectroscopy diagnosis indicates that the electron density of plasma can be controlled easily by adjusting the discharge gap and the dielectric thickness. It makes this source available for various applications. This work provides a new approach to obtain uniform linear plasma. [ABSTRACT FROM AUTHOR]

This letter presents a broadband rectifier that can be applied to harvest ambient radio frequency (RF) energy. The matching network consists of three-stage transmission lines. A cross-shaped match stub of two one-eighth-wavelength transmission lines is proposed to realize efficient and broadband impedance compression. The proposed circuit has been optimized and manufactured, and the measured results show that the rectifier can achieve high RF-to-dc conversion efficiency and broadband characteristics. The simulation and measured results were basically consistent. When the input power is 8 dBm and the terminal load is 1.6 $\text{k}\Omega $ , a bandwidth of 81% (1–2.35 GHz) with an efficiency above 50% and a bandwidth 69% (1–2.05 GHz) with an efficiency above 60% are achieved. The highest conversion efficiency of the test circuit is up to 73.2% at an input power of 8 dBm. Moreover, when the input power changes from 1 to 11 dBm, the efficiency of the rectifier circuit is always above 50%. [ABSTRACT FROM AUTHOR]

A device, which is designed based on strip line, is proposed for dual-channel plasma jets. Based on electromagnetic theory and DRUDE model, finite difference time domain algorithm and finite element algorithm are adopted to obtain the dimensions of designed device. An experimental system is established, and microwave power of 70 W can process 30 L of argon per minute, which is extremely efficient compared with traditional microwave plasma jet. Meanwhile, reflected power is measured to be 8% of incident power using a power meter, which is in good agreement with simulation results. [ABSTRACT FROM AUTHOR]

Dramatic shortages of filtering facepiece respirator supplies generally occur following the outbreak of a pandemic such as COVID-19. Here, the decontamination and reuse of respirators are considered. Among decontamination methods, microwave irradiation has great potential because of easy access of microwave ovens. However, can a respirator be heated in a microwave oven for a certain time and then be reused? Herein, we demonstrate that dry microwave irradiation cannot heat infectious aerosols or droplets up to their deactivation temperature. The microwave absorption performance of a single aerosol or droplet was analyzed theoretically. The multiphysics simulation results indicate that a single aerosol or droplet can be barely heated under dry microwave irradiation. Experiments were carried out using a traveling wave system to verify the simulation. Following this, we simulated multiple aerosols and droplets on a respirator material, with the results indicating that the aerosols and droplets were at the same temperature as that of the respirator. Experimental measurements using a microwave oven demonstrated that the temperature increase of an N95 respirator under dry heating is less than 10 °C, which is far less than the temperature required to deactivate the COVID-19 virus. Although dry microwave heating cannot be used to heat the aerosols or droplets, microwave-generated steam has proved effective in deactivating infectious biological organisms. Therefore, to successfully decontaminate a used respirator in a microwave oven, a reservoir with a small amount of water beneath the respirator (or a steam bag to accommodate it) is essential to the decontamination process. [ABSTRACT FROM AUTHOR]

Microwave plasma chemical vapor deposition (MPCVD) has received tremendous research interest in fabrication of carbon nanotubes (CNTs) due to its unique advantages of high reactivity, rapid heating, no pollution, good controllability, etc. It would be meaningful to summarize the efforts that have been devoted in this area. Up to now, no such review has been seen in the literature. In this review, a summarization and discussion of the MPCVD devices applied in CNTs fabrication are firstly given, followed by the discussion on effect and affecting mechanisms of H2 plasma pretreatment, nitrogen atoms in the reacting gases and microwave power. Notably, the parameters of as-synthesized CNTs products and the corresponding synthesizing MPCVD conditions are listed out. Finally, the capabilities of MPCVD in facilitating atmospheric-pressure, low-temperature and in situ growth of CNTs are reviewed. This review can give a comprehensive grasp of current progress and understanding of MPCVD in preparation of CNTs and may provide a useful guidance for readers to design their fabricating systems to obtained required CNTs using MPCVD. [ABSTRACT FROM AUTHOR]

In this article, a novel lens antenna with wide beamwidth using three‐dimensional printing is proposed. The proposed antenna is comprised of a concave lens by using three‐dimensional printing, a lantern shaped patch with a parasitic ring, and eight loaded monopoles around the center. The antenna operating frequency is 5.8 GHz, and the prototype is implemented. The measured half power beamwidth are 142° and 130° at the xoz and the yoz planes, respectively. Besides, the antenna is low cost, easy to fabricated, and very low cross polarization. Measured results agree well with the simulation results. [ABSTRACT FROM AUTHOR]

A dual-band sensor based on a planar rectangular cavity loaded with pairs of improved planar resonator is used to measure the permittivity difference of liquids with a small volume ($2.5~\mu \text{l}$). The planar cavity is connected by two microstrip transmission lines, constituting a two-port device with two branches. A pair of interdigital capacitor (IDC)-based split ring resonator (SRR) is coupled to the middle part of each branch. The distance between the IDC–SRRs is large, and their coupling can be neglected. Thus, two resonance frequencies are produced (4.15 and 9.18 GHz). One frequency is generated from the resonance of the two IDC-based SRRs (4.15 GHz), whereas the other results from the coupling of the rectangular cavity and the two IDC-based SRRs (9.18 GHz). Frequency splitting is used to detect changes in the dielectric characteristics of the liquids. If the two branches (IDC-based SRRs) of the proposed sensor are placed on the same liquid, only two resonance frequencies are obtained. When two different liquids are loaded by the sensor, the resonance frequency will be split into four. A planar rectangular cavity is used instead of a splitter configuration to motivate the two IDC–SRRs. This novel design affords a dual-band miniaturized sensor. [ABSTRACT FROM AUTHOR]

The development of microwave heating technology promotes the improvement of industry quality and efficiency, but the drawbacks of microwave heating, such as uneven heating, prevent its wide application in industry. In this paper, a double‐layer ring tube is proposed for improving the heating uniformity and achieving the target temperature. The gap between the inner and outer tubes is evenly divided into four regions, and the cooling liquid flows through the gap to control the temperature of the heating target placed in the inner tube. Maxwell's equation, heat conduction equation, Navier‐Stokes equation and the mass conservation equation are coupled to simulate the heating process, and the experimental data and simulation data are compared to verify the correctness of the model. Meanwhile, the heating uniformity of structure with or without heating exchanger is compared, and the effect of different fluid velocities, different sizes of the reactor and different dielectric constant of heating target on the heating performance is discussed. [ABSTRACT FROM AUTHOR]

Lens is an effective antenna solution, the radiation pattern of the antenna can be enhanced through the dielectric lens, the lens antennas tend to be directive rather than omnidirectional that is. However, in this article, a monopole antenna integrated within a three dimensional printing mushroom shape lens and pasted the copper foil on the part of the surface of lens is proposed for wireless electromagnetic energy harvesting system at 5.8 GHz to the realization of a wide beam antenna. The bandwidth is from 5.33 to 6.07 GHz with the reflection coefficient below −10 dB. The half power beamwidth is 197° in x‐z plane and 175° in y‐z plane, respectively. And the peak gain is 2.47 dBi. Simulation results agree well with measured results. [ABSTRACT FROM AUTHOR]

Microwave‐driven electrodeless UV lamps are widely used in industry. However, the ultraviolet light intensity of the traditional UV lamps is non‐uniform. In this article, a novel microwave‐driven UV lamp structure for underwater sterilization is developed, which equalizes the UV light intensity of each lamp at different positions. Based on the electromagnetic theory and the Drude model, finite difference time domain algorithm is adopted to obtain the dimensions of this new structure. An experimental system is established, and four UV lamp tubes are successfully excited and intensity of UV light at 253.7 nm is tested by spectrometer. Results show that the UV radiation of each lamp is almost same. The sensitivity of the structure is analyzed by changing the microwave power and electron density of the UV lamp. It can be found that significant changes in microwave power and electron density exhibit little effect on the uniformity of light intensity. Meanwhile, the microwave power should be large enough to excite the UV lamp and electron density may affect the S11 a little. Therefore, this structure can be applied to different types of UV lamps. [ABSTRACT FROM AUTHOR]

Microwave wireless power transmission (MWPT) via radio waves has been an important research topic of the WPT systems, where the overall power transfer and conversion efficiency is a key factor to evaluate the system performance. The state-of-the-art work on MWPT has only taken the end-to-end power transfer efficiency in free space or the power conversion efficiency of the receiver (rectenna) into consideration separately when optimizing the power transfer efficiency of the overall system. To comprehensively consider the system-level matching of the entire MWPT system, a novel space matching concept, including aperture field matching, power matching, and impedance matching, is proposed for optimizing the overall power transfer efficiency. By using the space matching method in this work, the optimal designs of the subsystem components such as the transmitting antenna, receiving antenna, and rectifying circuit can be separately obtained to avoid the complex systematic optimization of the large-scale and complicated MWPT system, therefore, significantly improving the overall power transfer efficiency by 19.4% compared with the traditional methods, and reducing the complexity in large-scale system-level design. This work will have valuable implications in the system design of MWPT applications. [ABSTRACT FROM AUTHOR]

This paper investigated the influence of gas humidity (1%, 3%, 8%, 10%, and 12%) on the characteristics of a microwave-induced atmospheric plasma jet. The plasma discharge was generated by a microwave solid-state source with a H2O–Ar mixture gas flow of 8.1 L/min. The variation in energy efficiency, O and OH concentrations, rotational temperature of heavy species, shapes of plasma plumes with different humidities, and microwave input powers were recorded and analyzed. The results showed that the concentrations of O and OH increase monotonously with gas humidity at higher input powers while they fluctuate with gas humidity at lower input powers. With an increase in the H2O/Ar ratio from 1% to 12%, the energy efficiency of the plasma generator decreases, and the plasma plumes become shorter and thinner. The rotational temperature of plasma at the nozzle also showed positive correlation with increasing humidity. Adding more input power would make all the values of these parameters increase. This paper is supposed to be helpful for the research of the interaction mechanism of mix gas plasma and microwave power and for improving the effect of plasma treating biomedical materials. [ABSTRACT FROM AUTHOR]

A K‐band (18–27 GHz) dielectric‐loaded waveguide antenna is designed as the feed for a parabolic reflector. The antenna is composed of a waveguide launcher and a dielectric load. The waveguide launcher is fabricated by selective laser melting using aluminium alloy. The dielectric load is fabricated by stereolithography using resin. The aperture field distribution of the antenna is tuned by extending the E‐ and H‐walls of the waveguide launcher to radiate a sidelobe‐reduced main beam in the boresight from −90° to + 90°. The agreement is achieved between the simulation and measurement. It has impedance bandwidth of |S11|<−10 from 18 to 27 GHz, the maximum gain of 11.8 dBi @ 19 GHz, and 3‐dB gain bandwidth from 18 to 25 GHz. For the sidelobe reduced, simple design and low cost, the proposed antenna features a good candidate as the feed for a parabolic reflector. [ABSTRACT FROM AUTHOR]

In this letter, a compact broadband high-efficiency RF rectifier is presented for ambient RF energy harvesting (EH). A novel impedance matching network with an additional quarter-wavelength short-circuited stub was designed to achieve broadband impedance matching. After the branch was added, the parallel resonance point on track $S_{11}$ did not move, and the remaining points were compressed. The measurement results show that the proposed topology could realize a broadband high-efficiency rectifier for ambient RF EH, and the measured results were basically consistent with the simulation results. The measured efficiency is as high as 71.5% for an input power level of 8 dBm. A 1–2.4 GHz wide frequency band with an efficiency above 50% is achieved for an input power level of 5 dBm with a terminal load of 1.6 $\text{k}\Omega $. Moreover, the circuit had wide dynamic input power characteristics, whose measured values remained above 50% with an input power of 3–14 dBm. [ABSTRACT FROM AUTHOR]

The nonreciprocal bandpass filter has been achieved by utilizing the nonreciprocity characteristics of the magnetostatic surface wave (MSSW) in planar ferrite slabs. In this article, the dispersion relation, impedance, and attenuation of MSSW between the pure and metallized ferrite slabs have been analyzed and compared. Theoretical analysis indicates that the ferrite slab metallized by a thin conductive layer would introduce significant attenuation on the metallized interface, which is MSSW propagating on the isolated direction, while MSSW on the other surface is barely affected. Therefore, nonreciprocal bandpass filter with enhanced isolation can be achieved with metallized ferrite slab. Simulated S-parameters show that a notch in the isolating direction can be obtained at the passing band of the transmitting direction. Experimental results show that the nonreciprocal filter with metallized yttrium iron garnet (YIG) slab (by depositing 100- $\mathbf {nm}$ copper film) exhibits more than 10-dB higher isolation than that with pure YIG slab, while the center frequency, insertion loss, and 3-dB bandwidth maintain almost the same. The center frequency of the filter with metallized YIG slab can be tuned from 7.26 to 8.38 GHz with insertion loss of less than 3 dB, and in-band isolation of more than 30 dB as the dc bias magnetic field is increased from 1.8 to 2.3 kOe. Besides, the nonreciprocal filter has ultra-wide out-band suppression since it utilizes the narrow frequency band of MSSW. The −10-dB out-band rejection exceeds 36 GHz in the upper band and 1 MHz in the lower band. [ABSTRACT FROM AUTHOR]