Measurement of RF Absorber at Large Angles of Incidence using Spectral Domain Transformations

Pyramidal RF absorber, widely used in indoor antenna ranges, is designed to minimize reflectivity by creating an impedance transform from free space to the impedance of the absorber material. The pyramidal shape provides this transition quite well at normal incidence. It has been shown in [1] that pyramidal RF absorber performs very well up to angles of incidence of about 45 degrees off-normal, but at wider angles of incidence, the performance degrades significantly. Unfortunately, it is very difficult to perform RF absorber measurements at large oblique incidence angles. In such measurements, the reflected path and the direct path between the antennas are very close in length, making it difficult to use time-domain gating techniques to eliminate the direct coupling.In this paper, a novel approach for performing oblique RF absorber measurements is introduced based on spectral domain transformations. Preliminary measurements using this technique have been compared to RF simulations. Results appear to indicate that this approach is a valid way to perform RF absorber reflectivity measurements at highly oblique incidence angles.