Franck Tchafa

ORCID Identifier(s)


Graduation Semester and Year




Document Type


Degree Name

Doctor of Philosophy in Mechanical Engineering


Mechanical and Aerospace Engineering

First Advisor

Haiying Huang


Structural Health Monitoring (SHM) is a vital technology that implements real-time condition monitoring or damage detection mechanisms for the safety assurance of structural systems such as power stations, aircrafts, bridges, dams, buildings, large machinery, and more. Without proper and timely maintenance, mechanical systems are prone to failures since they experience detrimental factors such as harsh environments, excessive loads, and vibrations. Several sensing technologies exist for SHM applications. Patch antennas are among the list of these technologies, and they are an attractive choice due to their compact size, light weight, ease of manufacturing, and low fabrication cost. Patch antennas have been studied not only for wireless communication but as sensors for condition monitoring. A typical patch antenna, consisting of a dielectric substrate, a radiation patch, and a ground plane, is an electromagnetic (EM) resonator with specific fundamental frequencies. These frequencies depend on the radiation patch dimensions and the antenna’s effective dielectric constant, which is a contribution of the dielectric constants of all layers (i.e., substrate and superstrate) and their respective thicknesses. Any change in the antenna dimensions or the material properties due to temperature, mechanical strain, and ash accumulation will cause a shift in the antenna resonant frequencies. Thus, it is possible to use patch antennas as sensors. The objectives of this research are to (a) extract multiple measurands (i.e., temperature, strain, ash thickness) from the antenna’s response; (b) demonstrate wireless interrogation of passive antenna sensors without electronics; and (c) establish a procedure for characterizing the dielectric properties of substrates. Measuring the dielectric property is important as substrate materials could be engineered based on the trade-off between the sensitivity of the antenna sensor to the three sensing parameters to achieve optimal sensor performance and facilitate simple data processing. Furthermore, these dielectric properties could be used in simulation tools to perform parametric studies on the substrate properties and to define the design space. In this dissertation, we demonstrate simultaneous measurement of multiple parameters using a single patch antenna. For simultaneous strain and temperature sensing, we exploit the two fundamental resonant frequencies of the antenna. The theoretical relationship between the antenna resonant frequency shifts, the temperature, and the applied strain was first established to guide the selection of the dielectric substrate, based on which an antenna sensor with a rectangular radiation patch was designed and fabricated. A tensile test specimen instrumented with the antenna sensor was subjected to thermo-mechanical tests. Experiment results validated the theoretical predictions that the normalized antenna resonant frequency shifts are linearly proportional to the applied strain and temperature changes. By comparing the temperature and strain inversely determined from the measured frequencies to actual thermocouple and strain gauge readings, we achieved measurement uncertainties of ±0.4oC and ±17.22 µε for temperature and strain measurements, respectively. Furthermore, we demonstrate simultaneous measurement of temperature and superstrate thickness using the antenna sensor. Since the temperature causes a change in the material properties, and the superstrate thickness variation causes a change in the antenna effective dielectric constant, the superstrate thickness and the temperature can be inversely determined simultaneously from the antenna frequencies. To demonstrate simultaneous measurement of these two parameters, the antenna sensor was designed, fabricated, and tested to characterize its performance. Using ash from charcoal briquettes as a superstrate, we achieved measurement uncertainties of ±0.58oC and ±58.05 µm for temperature and superstrate thickness, respectively. We also demonstrate wireless interrogation of a high-temperature antenna sensor without electronics. This specific achievement is a collaboration with Dr. Jun Yao. The sensor node is entirely passive and consists of an Ultra-wide Band (UWB) transmitting/receiving (Tx/Rx) antenna and a microstrip patch antenna serving as the temperature sensing element. A microstrip transmission line connecting the UWB Tx/Rx antenna and the antenna sensor delays the signal reflected from the sensing element and separates it from the background clutter. The operation principle of the wireless sensing scheme, the design, and simulations of the sensor node circuitry are discussed in this research. Furthermore, a digital signal processing algorithm that extracts the antenna resonant frequency from the wirelessly received signal is also described. Temperature tests were conducted to validate the performance of the wireless antenna sensor inside a high-temperature furnace. Finally, the broad range of microwave applications requires detailed knowledge of the dielectric properties of substrate materials. Due to the variability that exists in the material properties of fabricated printed circuit boards (PCBs), we investigate the use of a microstrip line resonator for dielectric measurement of PCBs. Using Rogers 3006 commercial PCB, we obtained a good agreement between the manufacturer specified dielectric constant of 6.15 and the measured dielectric constant of 6.17.


Wireless antennas, Temperature sensor, Strain sensor, Ash detection, Superstate thickness measurement, Thermal efficiency, Structural health monitoring, Temperature-compensated sensor


Aerospace Engineering | Engineering | Mechanical Engineering


Degree granted by The University of Texas at Arlington