Limitations of the Free Space VSWR Measurements for chamber validations

摘要

Free Space VSWR measurement has been the de facto standard method for anechoic chamber performance evaluation for more than 50 years. In this method, a probe antenna is first kept parallel with the boresight angle while traveling along a linear path to record the receive pattern. The probe antenna is then rotated to a different angle to record a standing wave pattern along the same path. Reflectivity, which is used as the chamber performance metric, is calculated as a function of probe rotation angle from the VSWR ripples. Reflectivity obtained this way is shown to be the ratio of the reflections seen by the probe antenna to the incident field at the probe antenna. We demonstrate that reflectivity is affected by the antenna patterns of the probe antenna. Reflectivity measured using a higher gain probe typically yields better (lower) reflectivity level than using a lower gain probe antenna.