Kwok L. Chung

Wang-Shaped Patch Antenna for Wireless Communications

A novel Wang-shaped patch antenna with a small aperture is presented in this letter. Stable radiation patterns and cross-polarization levels of -20 dB are achieved at a wide operational bandwidth of 1.7-2.5 GHz. Average gain, return loss, and efficiency are measured as 6.5 dBi, ≥ 15 dB, and > 80%, respectively, across the bandwidth. The antenna can serve simultaneously most of the modern wireless communication standards.

Microwave Non-Destructive Inspection and Prediction of Modulus of Rupture and Modulus of Elasticity of Engineered Cementitious Composites (ECCs) Using Dual-Frequency Correlation

This research article presents dual-frequency correlation models for predicting the growth of elasticity and flexural strength of engineered cementitious composites (ECCs) using microwave nondestructive inspection technique. Parallel measurements of microwave properties and mechanical properties of ECC specimens were firstly undertaken in the sense of cross-disciplinary experiments. Regression models were developed via means of nonlinear regression to the measured data. The purpose of the study is: (i) to monitor the flexural strength and elasticity growth; and (ii) to predict their mature values under the influence of different initial water contents, via microwave effective conductance at early ages. It has been demonstrated that both the modulus of rupture (MOR) and modulus of elasticity (MOE) can be accurately modeled and correlated by microwave conductance using exponential functions. The moduli developed as a function of conductance whereas the regression coefficient exhibited a linear relation with water-to-binder ratio. These findings have highlighted the effectiveness of the microwave non-destructive technique in inspecting the variation of liquid phase morphology of ECCs. The dual-frequency correlation can be used for structural health monitoring, which is not only for prediction but also provides a means of verification.

Intrinsic Sensing Properties of Chrysotile Fiber Reinforced Piezoelectric Cement-Based Composites

Lead-zirconate-titanate (PZT) nanoscale powder was first synthesized by the sol-gel method, then PZT and 0–3 type PZT/chrysotile fiber (CSF)/cement composite (PZTCC) wafers were fabricated after grind-mixing PZT powder with strontium carbonate and/or cement, ductile CSF in tandem with press-sintered process, respectively. The crystal structure (XRD), microstructure (SEM), piezoelectric properties after surface silver penetration, and polarization of the PZT and PZTCC wafer were investigated. Furthermore, self-sensing responses under either impulse or cyclic loading and micro-hardness toughness of PZTCC were also investigated. Results show that the incorporation of CSF and cement admixture weakens the perovskite crystalline peak of PZTCC; reduces the corresponding piezoelectric coefficient from 119.2 pC/N to 32.5 pC/N; but effectively bridges the gap on the toughness between PZTCC and concrete since the corresponding microhardness with 202.7 MPa of PZTCC is close to that of concrete. A good linear and fast electrical response against either impulse or cyclic loading of the PZTCC is achieved with their respective sensitivity, linearity, and repeatability to 1.505 mV/N, 2.42%, and 2.11%. The sensing responses and toughness of PZTCC is encouraging as an intrinsic piezoelectric sensor for real-time health monitoring of ductile concrete structures.

Polarization-Rotated Waveguide Antennas for Base-Station Applications

A novel base-station antenna element is proposed. It consists of an artificial surface composed of parallel strips rotating in the polarization direction and a segment of a rectangular waveguide. The surface is designed on a single-sided substrate, which has the same area as the aperture of the waveguide. To achieve the polarization rotation, the parallel strips on the surface are rotated by 45° with respect to the orientation of the waveguide antenna. By adding the surface, the linear polarization direction of the rectangular waveguide antenna rotates by 45° to comply with the requirements of the cellular industry. To verify the simulation results, the proposed antenna was fabricated and measured. Results show that the antenna has an operating bandwidth from 698–960 MHz, where a stable radiation pattern is achieved.

Determination of element spacing for wideband circularly-polarized antenna arrays

Wideband circularly-polarized arrays employing sequential rotated feeding technique to obtain wide axial-ratio bandwidth become mature. However, method to determine the element spacing of such arrays still remains fuzzy but crucial in designs of wideband arrays. Element spacing determines the maximum achievable directive gain at the center frequency but also affects array size. It hinders the compactness and hence losses of feeding network, mutual coupling, and sidelobe levels, etc. Study on these aspects that decide the optimized element spacing is found to be rare in literature. This paper presents a technique in this regard.

Microwave near-field noninvasive prediction of compressive strength of Engineered Cementitious Composites (ECCs)

This paper presents the results of a cross-disciplinary research project work, where microwave near-field noninvasive technique was used for the prediction of compressive strength of the state-of-the-art cementitious material, known as Engineered Cementitious Composites (ECCs). We mixed and cast specimens with water-to-binder ratios of 0.20, 0.255 and 0.30 for both the microwave noninvasive tests and mechanical destructive tests. Our results show that the compressive strength of ECCs at mature age can be determined by using microwave property of ECCs measured on the first day after casting. The inverse linear relations between compressive strength and microwave conductance were attained. The lower bound of microwave conductance in R-band was also established.