Microwave gas discharge tubes (GDTs) obtain energy from microwaves to generate an ionized plasma to reflect microwaves. In order to study and analyze the breakdown characteristics of microwave GDTs, including the time-domain characteristics and power loss characteristics, a broadband microwave GDT sample in $S$ -band with an ignitor was prepared and a breakdown experimental platform with an operation frequency 2.458 GHz was designed and constructed. The experiment was carried out without changing the structure of the tube and the gas composition inside the tube. The ignition voltage pulse was synchronized with the input microwave pulse, so that the tube broke down at a time point in the second half of the microwave pulse. A broadband oscilloscope was used to collect the time-domain waveform data of the leakage and reflection signals before and after the breakdown, respectively. The data were statistically analyzed to focus on the breakdown time point and the change law of the breakdown waveform. The results indicate that the tube discharge process which broke down at a time point within a pulse is different from that under a whole pulse, and there is a prebreakdown process. The time required for the prebreakdown and the time point of breakdown of the tube are discrete and are related to the input power and the ignition voltage. The arc loss decreases slightly with increasing input power. When the tube structure and gas composition are fixed, the input microwave power and ignition voltage can be adjusted to enhance the stability of the tube and reduce the loss to improve the reflection efficiency.

The first full demonstration of a high-efficiency phase-locking mechanism was accomplished in the millimeter-wave (mm wave) bands. Two modular Ka-band coaxial magnetrons were fabricated and phase-locked. The coaxial magnetron consisted of a conventional structure and a carefully designed coupling port that included an RF window. The coupling ports of two magnetrons were connected by coupling bridges to phase lock the coaxial magnetrons in zero-phase and π-phase difference mode, reaching phase-locking efficiencies of 93.4% and 95.6%, respectively. Two phase-locking modes were both verified by a mode detection system based on a symmetrical E-plane T-junction and a high bandwidth, high sample rate oscilloscope. Signals from the oscilloscope confirmed that phase-locking occurred after 25 ns and was reproducible from pulse to pulse. With higher power and more coaxial magnetrons in arrays, 1 MW may be achievable at Ka-band.

A method for improving peak power gain and pulse compression efficiency of the passive microwave pulse compressor is proposed. The proposed passive microwave pulse compressor consists of a dual-mode polarizer and two parallel spherical resonant cavities. This method can effectively depress the mode contention, but also, increase the Q factor of resonant cavities. By contrast, the peak power gain and pulse compression efficiency of the pulse compressor with a single cavity are 6.52 and 22.6%, respectively, while they are increased to 7.4 and 52.43 % respectively using two parallel cavities. The latter offers a 13% improvement in peak power gain and a 30% improvement in compression efficiency over the former.

This paper proposes a simplistic, efficient, and low-cost method of millimeter-wave nondestructive testing (NDT) of dielectric material cracks based on millimeter-wave interference. A relationship between combining efficiency, phase difference, and amplitude difference was analyzed. We found that phase difference was the main factor that affects combining efficiency. A change in combining efficiency of more than 1% was caused by a phase-difference altering of greater than 1.2° in a specific range. A relevant model was simulated with CST, and the operating frequency and antenna spacing were optimized to enhance sensitivity of the measuring system. Then, a Ka-band NDT system was built to test the combining efficiencies of different cracks. The experimental results showed that for polytetrafluoroethylene (PTFE) plates with a thickness of 5 mm, cracks with a width of about 0.4 mm, which is about 0.07 λg, could be detected at 35 GHz. Experimental results, simulation results, and theoretical derivation are basically consistent. Large-scale online applications of this NDT method in various industries appear feasible due to the above characteristics.

A low-cost method for measuring the thickness of low-loss nonmetallic sheet is proposed. This method is based on the sensitivity of millimeter wave coherent synthesis efficiency to phase consistency. A4-size paper sheets with 0.08mm thickness were tested using an experimental system with a working frequency at 35 GHz, and the preliminary experimental results are reported. The thickness of a low-loss nonmetallic sheet less than 0.08 mm can be measured at 35 GHz.

An arbitrary multi-way radial power divider/combiner with high-power capacity based on the coaxial waveguide TE01-mode is presented in this paper. The study of the TE01-mode in the coaxial waveguide shows the radial power divider can operate in the base mode with a large diameter, which can allow arbitrary multi-way waveguides to be radially distributed along the circumference of the coaxial waveguide. An equivalent circuit model of the arbitrary multi-way radial power divider is used to analyze its design procedure including the matching section, which can improve the transmission coefficient and isolation between output ports. To verify these, a 4-way and a 20-way radial power divider/combiner with a matching section are designed; the simulation results show that the power capability is >43 kW within 0.5 dB insertion loss for the four-way radial power divider. The average value of isolation between adjacent ports has improved by 11.1 dB after using the periodic matching structure as well as 2.78 dB between opposite ports in the frequency range of 34.08–36.32 GHz. The insertion loss of the 20-way radial power divider is less than 1 dB in the frequency range of 33.4–37.6 GHz, while the return loss is greater than 25 dB.