An exponential increase in large-scale infrastructure facilitates the development of wireless passive sensors for permanent installation and in-service health monitoring. Due to their wireless, passive and cost-effective characteristics, ultra-high frequency (UHF) radio frequency identification (RFID) tag antenna based sensors are receiving increasing attention for structural health monitoring (SHM). This paper uses a circular patch antenna sensor with an open rectangular window for crack monitoring. The sensing mechanism is quantitatively studied in conjunction with a mode analysis, which can uncover the intrinsic principle for turning an antenna into a crack sensor. The robustness of the feature is examined when the variation of crack position associated with an aluminum sample and the antenna sensor is considered. The experimental results demonstrate a reasonable sensitivity and resolution for crack characterization.

A low-profile antenna with a high gain and broad bandwidth is proposed for Sub-6GHz communication in this paper. A narrow-band patch mode and a narrow-band dipole mode are shared in one radiator and simultaneously excited to broaden the bandwidth. A compact prototype with a projection size of 0.90 λ₀ × 0.78 λ₀ and a profile of 0.13 λ₀ (λ₀ is the wavelength in the free space at the center of the operating frequency) is fabricated and measured. The measurement demonstrates an impedance bandwidth of 67.50%, covering the frequency range from 2.75 GHz to 5.45 GHz and an average gain of 8.4 dBi in the operating band of 3.0–5.0 GHz.

A dual-band base station antenna is introduced in this paper. The proposed antenna is composed of baluns, bowtie patches, and a split ring. The two pairs of bowtie patches excited by the two orthogonal balun structures can achieve dual polarization. The split ring is used to yield two additional resonances to broaden the impedance bandwidth. In this way, a compact dual-band base station antenna is obtained with the size of 0.41 λ_c×0.41 λ_c×0.13 λ_c (λ_c is the wavelength in the free space at the lowest operating frequency band) and the average gain of 8.2 dBi. Moreover, the operating frequency bands of the proposed antenna cover 25152675MHz, 34003600MHz, and 48005000MHz, which can function as an element for macro- or microcells in sub-6GHz communications.

Fatigue cracks and other forms of damage can have a significant impact on the normal operation of metal facilities, necessitating the deployment of multiple sensors for monitoring within large structures. The arrangement of these sensors must take into account factors such as the shape, size, and complexity of the monitoring area, as well as the optimal positioning and spacing of sensor nodes. This requirement for comprehensive coverage while minimizing costs presents considerable challenges for structural health monitoring (SHM) techniques. In this article, the feasibility of crack detection with a simple microstrip line (ML) is studied in the millimeter-wave band. The detection sensitivity is 0.283/mm2, the precision is 13.61%, and the minimum crack depth that can be identified is 0.2 mm (when crack width $\ge 1.0$ mm). An equivalent circuit model for this type of traveling-wave sensor is established in conjunction with field analysis, and the accuracy of the model is verified by comparing full-wave simulation and the circuit model. The proposed sensor can act as a distributed sensor for the SHM applications.