Angle-Dependent Reflectivity of Microwave Absorbers at Oblique Wave Incidence

For frequencies in the GHz-range, anechoic chambers are usually evaluated using ray-tracing techniques to locate disturbing reflections off the chamber walls. Most approaches reduce this wave-absorber interaction to a specular reflection, although the absorbers may extend over several wavelengths in size and display a rough surface. In order to develop more realistic ray-tracing models, the reflection characteristics of absorbers must be evaluated based on physical wave phenomena. In this paper, a measurement method is proposed which extends the established NRL-arch to measure the angle-dependent reflectivity for non-specular cases. First measurement results of commercial pyramidal absorbers in the frequency range between 1GHz and 10GHz indicate that the assumption of specular reflections is not justified, as power is reflected over a wide angular range with approximately the same intensity. This effect is, to our knowledge, currently not implemented in ray-tracing methods. These results contribute to a better understanding of the properties of RF absorbers to improve the efficiency of their use.