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.

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.

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