Propagation of Millimeter Waves in Composite Materials

Composite materials have become indispensable in a wide variety of technical fields, where they are viewed as the key technological component. Due to their structural complexity, these materials are prone to errors while in the manufacturing process, thus non-destructive inspecting becomes a crucial step in the processing chain and further maintenance of said material. However, a tomographic characterization of these components is often only possible to a limited extent-a capability which is essential to exploit the full potential of this technology. Measurement systems based on millimeter waves (mmWs) have the potential to achieve exactly that, by providing valuable information on the internal structure of such components. Yet, these non-destructive and non-contact methods have not found wide usage in practical applications. One reason for this is that the effects composite materials have on the propagation behavior of mmW signals have not been a major subject of study in the past. Therefore, a model for predicting the propagation properties of mmW signals in composite materials is presented here. Based on coupled wave analysis, the propagation of arbitrary electromagnetic signals in composite materials can be simulated. These results can be directly incorporated into the design of respective measurement systems. The introduced model is verified through a comparison of simulation and measurement results, obtained via a focused frequency modulated continuous wave (FMCW) radar, operating in the $H$ -band (220-325 GHz), directed at glass fiber reinforced plastic samples.