Extending Site VSWR to Millimeter Wave Using Cylindrical Mode Filtering

Abstract

Site VSWR is used to qualify the performance of an EMC chamber for above 1 GHz applications. It is traditionally measured by sampling the standing wave along a linear path at various quiet zone (QZ) locations per CISPR 16-1-4. The limitation due to data undersampling is well recognized. As the frequency range starts to extend to the millimeter wave band, the limitation becomes even more pronounced, to the point that it makes little sense to attempt to apply the same test methodology. A novel approach based on frequency domain mode filtering is presented here, which overcomes the difficulties in the traditional approach. In the proposed approach, an omni-directional transmit antenna is placed at the edge of the QZ, and a full rotation vector pattern cut is acquired. The data is then processed and decomposed into cylindrical modes, where a mode filtering algorithm is explored to separate the antenna response from the chamber reflections. The VSWR of the chamber can then be computed by comparing the filtered and unfiltered pattern. The mode filtering approach provides greater insights to the QZ performance, including showing the unperturbed antenna pattern, and the ripples caused by reflections separately. It is much less prone to undersampling because a dense angular step size can be easily accommodated using the existing turntables in EMC chambers.