Microwaves have been widely used in the treatment of materials, such as heating, drying, and sterilization. However, the heating in the commonly used microwave applicators is usually uneven. In this paper, a novel multi-material turntable structure is creatively proposed to improve the temperature uniformity in microwave ovens. Three customized turntables consisting of polyethylene (PE) and alumina, PE and aluminum, and alumina and aluminum are, respectively, utilized in a domestic microwave oven in simulation. During the heating process, the processed material is placed on a fixed Teflon bracket which covers the constantly rotating turntable. Experiments are conducted to measure the surface and point temperatures using an infrared thermal imaging camera and optical fibers. Simulated results are compared qualitatively with the measured ones, which verifies the simulated models. Compared with the turntables consisting of a single material, a 26%–47% increase in temperature uniformity from adapting the multi-material turntable can be observed for the microwave-processed materials.

We present a 3D printed metallic K-band (18-26.5 GHz) passive frontend for satellite communication. It is intended to be used as a unit cell in a large antenna array. The proposed frontend is composed of a two-stage one-by-four power divider, rectangular-to-circular waveguide tapers, and a linear four-element conical antenna array, which are too complex to be fabricated by traditional machining process in a whole piece. Taking the advantage of the 3D printing technology in realizing complex structures, we achieve a highly-integrated passive module. Good agreement is achieved between simulation and measurement. The proposed frontend has impedance bandwidth 19 $\times$ 21 GHz, the maximum gain of 15.5 dBi at 21 GHz, and desirable radiation patterns on both E- and H-planes. The influence of fabrication tolerance like the surface roughness and dimensional tolerance are observed and analyzed. Compared with traditionally fabricated metallic microwave passive devices, the proposed work has a shorter turn-around time and a lower cost. Compared with dielectric 3D printed microwave devices, it features more simplicity in terms of process and better physical robustness. It opens up new possibilities for microwave device fabrication.

This letter presents a reconfigurable rectenna with an adaptive matching stub for 5.2- and 5.8-GHz Microwave Power Transmission (MPT). The proposed rectenna consists of a broadband receiving antenna and a dual-frequency rectifying circuit with a reconfigurable stub, where a depletion-mode Field-Effect Transistor (FET) is installed. Due to the FET used as a switch, a matching stub can automatically adapt to the dual-frequency operation. According to the load DC voltage at the two frequencies, the automatic on-off change-over is achieved, and the stub is connected to or disconnected from the rectifying circuit for good matching between the receiving antenna and the circuit. Simulated/measured conversion efficiency of 55.1%/54.9% and 54.2%/52.2% are obtained at 5.2 GHz and 5.8 GHz, respectively, under the input power of 6.5 dBm. With the advantage of convenient tuning, simple structure and ease of integration, the proposed rectenna can be widely used in dual-frequency MPT system. Also, the adaptive matching stub can be implemented for other reconfigurable structures.

Conventional microwave heating has serious problems such as non-uniform heating and low efficiency. A novel magnetron microwave sweep frequency heating method is proposed to improve microwave heating uniformity. In this method, the frequency-sweeping signal is injected into the magnetron by the injection frequency-locking technique, and the microwave sweep frequency heating of the magnetron is realized. In this paper, a complicated injection frequency locking system is given and analyzed and a multiphysics calculation model based on the finite element method for electromagnetic waves and heat transfer is established. The calculation of microwave sweep frequency heating is realized by the combination of COMSOL and MATLAB. The results show that the sweep frequency heating has an obvious superiority. An experiment is carried out to verify the simulation results. The simulation results are in agreement with the experimental data. Moreover, the effect of sweep bandwidth and sweep interval on heating uniformity is discussed.

A 2450-MHz microwave-induced room-temperature atmospheric-pressure plasma jet (RTAPPJ) based on a coaxial structure is proposed for applications of plasma biomedicine. This device is a simple structure with small size, low-power requirement and high controllability. It can induce an argon discharge at atmospheric pressure with as low as 15-W microwave power. The optical emission spectrum of the RTAPPJ shows various highly reactive radicals such as OH and O. The appropriate operation condition can reduce the temperature at the downstream of the flame to be around 30 °C. This portable device is designed to be suitable to manipulate different medical treatments.

For the discharges sustained by the travelling electromagnetic waves, the wave propagation characteristics are of great importance to discharge maintenance and stability. This study aims to investigate the propagating electromagnetic modes of the travelling wave in a cylindrical discharge tube bound by a metallic enclosure based on a microwave plasma torch. In certain particular circumstances, it is found that the cylindrical discharge tube with metallic enclosure is able to change from a one-conductor circular waveguide to a two-conductor-like coaxial waveguide. Such a change allows the travelling wave to propagate in the transverse electromagnetic mode, which has rarely been noticed before. Regarding this transition, a new criterion is proposed to determine the formation of the two-conductor-like waveguide structure. Existence conditions of different propagating modes of the travelling wave are further discussed with respect to different glass tube wall thicknesses and dielectric properties. The results indicate that it is possible to control the propagating modes of the travelling wave for different purposes by choosing a proper glass tube.