Analysis of effects of diffraction and interference on detection by microwave thermography

Abstract

In this paper, the effects of diffraction and interference on thermograms of steel bars in a reinforced concrete wall using microwave thermography are discussed. Three series of microwave thermography tests were conducted. Transmission approach was used for the first series of tests. The tests were carried out with a single rebar (12 mm diameter) vertically placed against a concrete wall of 1 m × 1 m × 6.5 cm. The sample was heated with an average power of 600 W for 3 min. The second and the last series of tests were carried out with a reinforced concrete wall using transmission and reflection approaches, with five angles of incident waves (0°, 15°, 30°, 45°, and 60°), and a heating power of 600 W for 5 min. The test results were analyzed based on the Snell–Descartes theory and data interpretation by multiple approaches. Detailed discussion clarifies how diffraction and interference affected the thermograms. In the reflection approach, the incident waves guided by the antenna were refracted after reaching the surface of the concrete wall, then transmitted into the concrete. The fractions of transmitted waves that reached the steel bars were reflected to the surface of the concrete wall, which made those parts of concrete hotter than the other parts without reflected waves. In the transmission approach, the interference of the diffracted waves made the concrete in the areas of wave superposition hotter than other areas and mainly affected the thermograms of the rear surface of the detected wall.