Yongjun Xie

Although finite difference time-domain (FDTD) algorithms are potentially applicable to broadband wave propagation and radiation problems, various problems related to the absorbing boundary condition and complex structural meshing limit the accuracy of FDTD calculations. This paper develops an unconditionally stable Crank–Nicolson factorization-splitting (CNFS) algorithm with a higher-order perfectly matched layer (PML) scheme. The higher-order PML is formulated through the matrix exponential (ME) method, which requires fewer operators and less manipulation than the existing implementation. To analyze the fine details and curves, the sub-gridding technique is modified through the unconditionally stable CNFS algorithm. Introducing nonuniform mesh sizes inside the computational domains improves the efficiency without degrading the computational accuracy. The effectiveness of the algorithm is evaluated in wave radiation and propagation problems, including radar cross-section evaluation and antenna design. The numerical and experimental results favorably agree, confirming the effectiveness of the sub-gridding technique and ME-PML based on the CNFS algorithm.

In conventional surface resistance R_s measurement using cavity resonator, the 3-dB bandwidth method is frequently used for quality factor (Q-factor) extraction, in which the swept frequency bandwidth should be multiple of 3-dB bandwidth. However, sometimes, it is unable to measure resonator whose measurable bandwidth is smaller than 3 dB. Here, an x-dB bandwidth method is studied for Q-factor extraction and its feasibility is demonstrated by R_s measurements with a TE₀₁₁ mode cylindrical cavity resonator and a quasi-optical cavity.

With a low cost fabricated (3D printed dielectric part and metallization using adhesive copper tape) open-ended rectangular waveguide as transceiver probe, groups of μ-PCD (microwave photoconductive decay) experiments are presented to study the influence of excitation frequency on system performance such as sensitivity and signal-to-noise ratio (SNR). First, to observe effect of excitation frequency on the waveguide probe’s reflection coefficient, a μ-PCD setup based on a 1-port vector network analyzer is presented. Then, another μ-PCD setup based on conventional architecture is presented for carrier lifetime testing. A standard waveguide probe working at similar frequency band is also used as a benchmark probe. Experimental results show that the proposed 3D printed waveguide probe offers ~10x improved measurement sensitivity compared to the standard waveguide. For the used setup in this work, when excitation power is ≥10 dBm, by properly selecting the excitation frequency, the SNR of μ-PCD system can be improved by at least 10 dB. A detailed analysis of the mechanism of SNR improvement is also presented. The proposed low cost 3D printed dielectric-filled waveguide probe provides an alternative for μ-PCD systems.

Measurement of liquid’s evaporation rate is important for basic research, industrial, environment protection, daily life and so on. There are kinds of evaluation methods for evaporation rate, like mass sensors, CCD camera, laser interferometer, optical fiber, capacitance sensor and so on. Here, we show that microwave near-field sensing using a high sensitivity resonant coaxial transmission line probe can be used to monitor evaporation rate of both liquid droplet and liquid level. Due to the evanescent field around the tip of the coaxial probe, the proposed method is sensitive to mass, size as well as composition variation of liquid under evaluation. With a coaxial probe working at ~1.8 GHz, we demonstrate evaporation rate evaluation of both pendant and sessile droplet as well as liquid level. Effect of air flow and temperature on evaporation is also presented. Besides, low-cost version of the proposed method is verified. The proposed technique has potential applicability to many fields related with evaporation.

A high endfire gain periodic leaky-wave antenna (LWA) based on dielectric image line (DIL) and fed by a planar transition is proposed. The unit cell of the periodic DIL consists of two dielectric blocks with different permittivity, and space harmonics (SHs) are then generated. A revised formula for phase constant of the nth space harmonic is introduced for the periodic structure. Although the proposed antenna is grounded, it obtains high endfire gain with small cross section and experiences a slight deflection beam direction, effectively addressing the issue of conformity between the endfire antenna and metal surfaces. The maximum measured endfire gain is 16 dBi, achieved with a cross-sectional area of 0.34λ ₀ × 0.13λ ₀ 0.19λ ₀ The antenna can achieve good matching within the frequency range of 14.4 to 16.0 GHz.

Rotational speed measurement is important for many applications. Here, a noncontact rotational speed test method based on the detection of the periodically perturbed near-field microwave of an open-ended waveguide is proposed. Both simulations and experiments were conducted to verify the near-field microwave rotational speed sensor. The constructed rotation speed sensing system was composed of a standard open-ended WR-42 waveguide (in our measurements, a waveguide-to-coaxial adapter was used to represent an open-ended waveguide) working at ~18 GHz, a radio frequency (RF) circulator, a signal generator, a, RF detector and an oscilloscope. A rotating fan to be measured was placed close to the waveguide’s mouth and, thus, the waveguide’s reflection coefficient was periodically modulated by the rotating fan blades. Then, the RF detector converted this varying reflection coefficient into a direct current (DC) voltage, namely, a periodical waveform. Finally, the rotational speed of the fan could be extracted from this waveform. Measurements using both the proposed near-field microwave method and conventional optical transmission/reflection methods were conducted for verification. The effect of the rotating fan’s location relative to the waveguide’s mouth was also studied. The results show the following: 1. The proposed method works well with a rotational speed of up to ~5000 RPM (rounds per minute), and an accuracy of 1.7% can be achieved. 2. Metallic or non-metallic fan blades are all suitable for this method. Compared with the existing radar method, the proposed method may be advantageous for rotation detection in a constrained space.