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Planar dielectric resonators (DR) with height significantly lower than the operating wavelength are investigated as a new class of disk dielectric resonators of the millimeter wavelength range with whispering gallery (WG) modes. It is known that in the open state, such resonators based on the WG modes are not excited due to high radiation losses. It is shown that the solution of this problem is the partial shielding of the disk dielectric structures with a plane-parallel screen by placing the disk between two flat conducting mirrors. It has been established that by introducing an air gap between the flat base of the dielectric disk and one of the conductive mirrors, it is possible to increase the quality factor of the planar DR almost twice. One of the reasons for this is the partial displacement of the resonant field of the WG modes from the dielectric region to the air gap, where the dielectric losses are lower. In addition, an increase in the air gap in the range of optimal values, comparable to half the working wavelength, leads to a decrease in ohmic losses. The above causes an increase in the quality factor of planar DR as an air gap is extended.

In recent years, the interest in damage identification of structural components through innovative techniques has grown significantly. Damage identification has always been a crucial concern in quality assessment and load capacity rating of infrastructure. In this regard, researchers focus on proposing efficient tools to identify the damages in early stages to prevent the sudden failure in structural components, ensuring the public safety and reducing the asset management costs. The sensing technologies along with the data analysis through various techniques and machine learning approaches have been the area of interest for these innovative techniques. The purpose of this research is to develop a robust method for automatic condition assessment of real-life concrete structures for the detection of relatively small cracks at early stages. A damage identification algorithm is proposed using the hybrid approaches to analyze the sensors data. The data obtained from transducers mounted on concrete beams under static loading in laboratory. These data are used as the input parameters. The method relies only on the measured time responses. After filtering and normalization of the data, the damage sensitive statistical features are extracted from the signals and used as the inputs of Self-Advising Support Vector Machine (SA-SVM) for the classification purpose in civil Engineering area. Finally, the results are compared with traditional methods to investigate the feasibility of the hybrid proposed algorithm. It is demonstrated that the presented method can reliably detect the crack in the structure and thereby enable the real-time infrastructure health monitoring.

© 2019 IOP Publishing Ltd. Guided waves are traditionally used in different non-destructive testing applications because of their cost-effectiveness and piezoelectric patches that are easy to incorporate into the structure as transducers. The non-destructive evaluation of interfacial defects such as debonding in a composite structure is critical for safety and long-term use. A new guided wave technique to detect a variety of debondings in carbon fibre reinforced concrete structure has been developed and experimental testing has been carried out to verify the proposed approach. Five composite specimens with different debondings have been prepared. The received guided wave in the specimen with a perfect bonding is taken as a reference. This signal is compared with the received signal under different debonding conditions. The debonding is quantified using three damage indices: correlation coefficient, change in peak-to-peak and root mean square deviation. The results demonstrated that these indices could be a good indicator of the debond conditions as they correlated linearly with the extent of the debonding. The proposed method is effective in detecting interfacial defects in an existing structure without special preparation.

A recently developed microwave dual waveguide sensor is applied for the measurement of gap in concrete-based composites in this paper. The sensor includes two waveguide sections embedded in a metal plate and opened in the structure under test. A transceiver that generates, transmits, and receives microwave signals, and a measurement unit that measures reflection and transmission coefficients is the other components of the sensor measurement system. The sensor is designed, built, and tested with concrete-metal and concrete-dielectric layer-metal specimens. Then, the sensor with the specimens has been modeled using Computer Simulation Technology Microwave Studio. Extensive simulations are performed to determine the complex dielectric permittivity of concrete in the area of measurement from the measured data, and to carry out the parametric study of concrete-based composites. It is demonstrated that the measurement and simulation results are in good agreement. It is also shown that the proposed system can measure relatively small gaps (0.2–2 mm) in the concrete based composite structures at the $X$ -band. Measurement and simulation results show that the proposed sensor can be used for detection and evaluation of gap in composite structures, such as concrete-filled steel tubes and concrete members covered by dielectric coatings.

A miniaturized antipodal Vivaldi antenna to operate from 1 to 30 GHz is designed for nondestructive testing and evaluation of construction materials, such as concrete, polymers, and dielectric composites. A step-by-step procedure has been employed to design and optimize performance of the proposed antenna. First, a conventional antipodal Vivaldi antenna (CAVA) is designed as a reference. Second, the CAVA is shortened to have a small size of the CAVA. Third, to extend the low end of frequency band, the inner edges of the top and bottom radiators of the shortened CAVA have been bent. To enhance gain at lower frequencies, regular slit edge technique is employed. Finally, a half elliptical-shaped dielectric lens as an extension of the antenna substrate is added to the antenna to feature high gain and front-to-back ratio. A prototype of the antenna is employed as a part of the microwave imaging system to detect voids inside concrete specimen. High-range resolution images of voids are achieved by applying synthetic aperture radar algorithm.

A small antipodal Vivaldi antenna with rectangular slits at sun-shaped configuration and a half elliptical-shaped dielectric lens is designed for microwave and millimeter-wave imaging applications. The slits are developed to extend low end of frequency band and increase antenna gain at lower frequencies, while the lens is applied to this antenna to feature high gain at higher frequencies, high front-to-back ratio, low sidelobe and cross-polarization levels, narrow half-power beamwidth, and modification on E-plane tilt of beam. A prototype of the antenna with small size of 30 × 55 × 0.508 mm3 is fabricated and employed as a part of microwave and millimeter-wave imaging system. Applicability of the proposed antenna for 3-D and 2-D images of targets such as rubber discs and metal rods at different depths inside construction materials including foam and plasterboard sheet-layered structures is demonstrated. High-range resolution images are achieved by applying synthetic aperture radar algorithm.

This article presents a tag antenna for radio frequency identification (RFID)-based wireless structural health monitoring systems. An ultra-high frequency (UHF) microstrip patch tag antenna is designed with a new generation RFID tag chip for the Australian UHF RFID frequency band. Simulation studies with optimized slits demonstrate the ability of the antenna to tolerate the influence of construction materials such as concrete and metal on its resonant frequency, maximum gain, and read range. Noncontact measurement of the resonant frequency and experimental verification of the maximum gain of the fabricated tag antenna show good agreement with simulated performance. Read range measurements are conducted to verify the performance of the tag antenna on metal structures. It is also shown that the proposed tag antenna can be easily modified to operate in other UHF RFID frequency bands. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2231–2237, 2016

Wireless power transmission in human tissue using microwave techniques has great potential for different biomedical applications. Characterisation and optimisation of these techniques using in vivo performance measurements are complicated and time consuming. This study investigates the electromagnetic performance of microwave wireless power transmission at 2.45 GHz. For this purpose, a computational model of a measurement system is proposed and used to characterise and optimise a microwave wireless power transmission technique capable of delivering microwave power to a localised area in the human tissue. This technique includes an implanted miniature encased microstrip ring disk antenna and an external microstrip patch antenna operating at 2.45 GHz. The results show that the microwave technique can be optimised by matching the radiation directions and polarisations of the antennas and by inserting a dielectric layer between the human tissue and the external antenna. It is demonstrated that the optimised microwave technique can deliver an electrical stimulus that can be used for biomedical applications such as nerve regeneration with a specific absorption rate that satisfies the IEEE Standards.

This paper presents a multifunctional imaging system with a sensing unit including integrated microwave and laser sensors. It focuses on the development of software and algorithm for automated control of the movement of the sensing unit attached to the 3-axis scanning system and optimization of the standoff distance between the sensing unit and the material under test. The sensing unit consists of a microwave antenna/sensor and two laser displacement sensors. Microwave sensor is used for data acquisition and imaging of the surface and hidden flaws in the metal. Laser displacement sensors are used for obtaining profile information of the structure and an automatic adjustment of the sensing unit at optimized standoff distance. The optimization of standoff distance is important mainly in non-plain machine parts as their non-planarity might mask indications of minute flaws such as cracks. For this purpose, a combined software program is developed for data acquisition, motion control and synchronization in the LabVIEW platform. Further, the developed program includes a signal processing module for post-processing of signals and image smoothing. All these modules are synchronized using various functions with a trigger. A suitable user-friendly graphic user interface is also developed. The applicability of the system is demonstrated through a non-contact detection of hidden flaws in metals used in machine parts.

© The Author(s) 2018. In this article, a piezoelectric-based sensory technique is proposed for detection of the gap between surfaces of a carbon fiber reinforced polymer plate and a concrete specimen and characterization of shrinkage of early-age concrete. The proposed technique uses the propagation properties of the guided waves in the carbon fiber reinforced polymer plate excited and received by piezoelectric transducers attached to an external surface of the carbon fiber reinforced polymer–strengthened concrete specimen. Measurements are conducted with fresh and hardened early-age concrete specimens and two carbon fiber reinforced polymer plates at different gaps. A piezoelectric actuator is excited using a sine burst signal, and the generated wave is received by a sensor after propagation along the specimen. The received signal at different gap values is used to detect a gap. To quantify the gap, damage indices, including correlation coefficient, peak-to-peak amplitude of resultant signal, and root-mean-square deviation, are used. The shrinkage of concrete is detected and predicted by comparing the damage indices at different gaps with the indices at different stages of early-age concrete. The proposed technique is relatively simple method using small transducers. It is one-sided, non-destructive, and cost-effective solution for gap detection and concrete characterization.

High-quality microwave and millimetre wave imaging of construction materials and structures requires ultra-wideband (UWB) techniques to provide high-range resolution as well as a reasonable penetration depth. A modified compact microwave and millimetre wave UWB antipodal Vivaldi antenna is designed and presented in this study. First, the conventional antipodal Vivaldi antenna is designed as a reference antenna. Then, to provide the desired frequency range (3.4-40 GHz) with increased gain at its lower frequencies, the slit edge technique is applied, thus creating a periodic slit edge antipodal Vivaldi antenna (PSEAVA). Finally, a trapezoid-shaped dielectric lens (TDL) as an extension of the substrate is added and optimised to increase gain and directivity at higher frequencies of the frequency range, creating PSEAVA with a TDL (PSEAVA-TDL). The results show that the PSEAVA-TDL has the highest gain (up to 16 dB) and front-to-back ratio (up to 37.5 dB), and the narrowest half power beamwidth (down to 11.7°). A prototype of the proposed PSEAVA-TDL with compact size of 40 × 90 × 0.508 mm 3 is fabricated and applied for the imaging of samples made of construction materials. High-range resolution images of the samples are obtained with this antenna by using synthetic aperture radar algorithm.

A modified compact antipodal Vivaldi antenna is proposed with good performance for different applications including microwave and millimeter wave imaging. A step-by-step procedure is applied in this design including conventional antipodal Vivaldi antenna (AVA), AVA with a periodic slit edge, and AVA with a trapezoid-shaped dielectric lens to feature performances including wide bandwidth, small size, high gain, front-to-back ratio and directivity, modification on E-plane beam tilt, and small sidelobe levels. By adding periodic slit edge at the outer brim of the antenna radiators, lower-end limitation of the conventional AVA extended twice without changing the overall dimensions of the antenna. The optimized antenna is fabricated and tested, and the results show that S 11

Ultrawide band (UWB) nondestructive testing and evaluation techniques can provide the detection of defects and damages in construction materials and members. The development of UWB antennas with proper dimensions and high performance for these systems is one of the major challenges. In this letter, an antipodal Vivaldi antenna (AVA) has been developed to be used in UWB techniques for nondestructive detection of cracks in concrete members. To obtain a desirable antenna performance including the wider impedance bandwidth, the higher directivity, and higher front-to-back ratio rectangular-shaped slits (RSS) and a half-circular-shaped lens (HCSL) have been added in the design of a conventional tapered slot AVA. A parametric study on the dimensions of the slits is provided to optimize performance and size of the proposed antenna. The optimum size of the proposed antenna is 40 × 80 × 0.508 mm3, and it operates over the frequency range from 3.4–40 GHz. It is shown that the RSS increase impedance bandwidth especially at the lower frequencies. Directivity of the proposed antenna at higher frequencies has been increased by the HCSL. The feasibility of the proposed antenna for nondestructive evaluation of concrete members is demonstrated by investigation into its performance with concrete specimens possessing an air-gap inside a concrete specimen. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1573–1578, 2015

© 2017 IOP Publishing Ltd. Previous studies have successfully demonstrated the capability and reliability of the use of Smart Aggregate (SA) transducers to monitor reinforced concrete (RC) structures. However, they mainly focused on the applications of embedded SAs to new structural members, while no major attention was paid to the monitoring of existing RC members using externally mounted SAs. In this paper, a mounted SA-based approach is proposed for a real-time health monitoring of existing RC beams. The proposed approach is verified through monitoring of RC beams under flexural loading, on each of which SA transducers are mounted as an actuator and sensors. The experimental results show that the proposed SA-based approach effectively evaluates the cracking status of RC beams in terms of the peak of power spectral density and damage indexes obtained at multiple sensor locations. It is also shown that the proposed sensor system can also capture a precautionary signal for major cracking.

An ultra-wideband elliptically tapered antipodal Vivaldi antenna designed for civil engineering applications is presented. It is based on design of a conventional antipodal Vivaldi antenna (CAVA) which impedance bandwidth is limited at low end of frequency band. To extend impedance bandwidth, inner edges of top and bottom radiators of the CAVA have been properly bent; however, its gain and front-to-back (F-to-B) ratio is low at the low frequencies. To enhance gain and F-to-B ratio, the comb-shaped slits on edges of the radiators of CAVA are applied. The obtained results exhibit the impedance bandwidth of 1.65-18 GHz, gain of 6.7 dB at 1.65 GHz, and F-to-B ratio of 42 dB at 13.5 GHz that are higher than those parameters of the CAVA. Applicability of the proposed antenna for detection of void inside concrete beam is demonstrated. First, models of the proposed antenna and concrete beam possessing void are created in computer simulation technology and numerical study is performed. Then, a prototype of the antenna is fabricated and employed as part of microwave imaging system to verify simulation results and to detect voids inside concrete beam.