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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]

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 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]

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]

Microwave selective/localized heating has been widely used in special areas, e.g., medical treatment and food processing, but the control of selective/localized heating in restricted space, e.g., microwave ovens, is difficult to accomplish. In this article, a Giuseppe Peano fractal phased-array antenna is designed, and the phase of each radiator is derived to achieve the arbitrary spatial division of the electromagnetic field in a cavity based on the superposition principle of waves. Then, a multiphysics model of microwave heating is established by coupling the electromagnetic field and heat transfer to simulate the directional control of microwave heating in a cavity. An experimental system is built, and the heating results are measured using an infrared thermal imager to verify the temperature distribution in different spatial regions. Finally, the feasibility of the model for different heating targets is discussed. [ABSTRACT FROM AUTHOR]

A microwave power-combining technique based on magnetrons has been widely researched to solve the urgent demands for high-power, low-cost microwave sources in microwave industrial applications. To achieve high resultant efficiency, conventionally, injection locking with static frequency is utilized to drag the output frequency of each magnetron source to the exact same frequency, and a phase shifter is required to adjust the output phase of each combining source. In this article, we propose a novel phase shifterless microwave power system with two magnetrons utilizing a frequency-searching injection-locking technique. Coaxial cables with different physical lengths are used to realize phase adjustment under different injection frequencies. By sweeping the injection frequency in a small band, the phase difference between combining signals can be adjusted. Thus, high resultant efficiency can be obtained. When the injection frequency is swept, the phase difference between the output and injection signals has been analyzed theoretically, the phase difference between the different-length coaxial injection cables has also been analyzed. Furthermore, the relationship between the sweeping frequency bandwidth and requirements of the minimum length difference of the coaxial injection cables has been derived and analyzed. Experimental results show that a high resultant efficiency of 94.6% can be obtained with the proposed phase shifterless system. The system could maintain high resultant efficiency even when the free-running frequency and power of one magnetron are changed by adjusting its anode current. The experiments reveal that the proposed method works for different combining sources. [ABSTRACT FROM AUTHOR]

This paper proposes an S-band microwave transmitting system based on four-way master–slave injection-locked magnetrons, which has the advantages of low cost and high efficiency. The transmitting system consists of a microwave generator and an antenna array. The microwave generator provides four-way excitations with equal phases and amplitudes to the antenna array, where a radial divider is applied to produce and control the four-way injected signals to lock the slave magnetrons. The antenna array includes four pyramid horns and two layers of artificial materials with 2-D periodic corner-truncated square slots, which are arranged to enhance the gain and suppress sidelobes. The transmitting system is fabricated and measured. Experimental results show that the microwave generator is able to produce four-way excitations with the phase differences less than 12° and the amplitude errors less than 3%. The antenna array provides a gain of 27.2 dB, sidelobe levels less than −10 dB, and reflection coefficients less than −15 dB. The whole system operates stably at 2.46 GHz with a transmitting power of 3.5 kW. [ABSTRACT FROM PUBLISHER]

In some cases such as heating objects by microwave power, variation of the temperature in objects changes their dielectric permittivity and then leads to variation of the electromagnetic (EM) field. The commonly adopted method of simulation for the heating procedure is to repeatedly execute the full-wave simulation for each small time intervals, which results in a heavy computation burden. A fast algorithm for simulating 2-D sceneries is proposed for the first time. It starts with a full-wave simulation to get an initial EM field, and then corrects the EM field in an efficient way rather than repeating the full-wave simulation when the object's permittivity has slightly changed. A few sample numerical results are presented and analyzed, which verifies the accuracy, robustness, and effectiveness of the proposed fast algorithm. [ABSTRACT FROM PUBLISHER]