EMC Chamber Quiet Zone Qualification for Applications Above 1 GHz Using Frequency Domain Mode Filtering

Abstract

Anechoic chambers used for electromagnetic compatibility (EMC) measurements above 1 GHz are qualified based on the site voltage standing wave ratio (SVSWR) method as per the international standard CISPR 16-1-4. With one antenna at the fixed position, some distance away from the quiet zone (QZ), the SVSWR is acquired by moving a dipole-like antenna along several linear paths that are located at the edge of the QZ. To reduce test burden, the SVSWR method under-samples the measurement by design, in that only six discrete points along each 40 cm linear travel path are measured. As a result, the test results are generally overly optimistic. In this article, we propose to use a novel cylindrical mode coefficient (CMC)-based frequency domain mode filtering techniques to obtain the VSWR. Here, we measure the vector pattern cut of the dipole-like test antenna with an offset placement at the outer edge of QZ. The antenna is then mathematically translated to the rotation center, whereupon a bandpass filter that tightly encloses the test antenna mode spectrum is applied. Two approaches are studied herein for translating the rotation center. One is by applying a path length correction to both magnitude and phase, and the other is by performing a cylindrical far-field transformation on the quasi-far-field data. The difference between the mode filtered antenna pattern and the original perturbed pattern is used to derive the chamber SVSWR. In contrast to the conventional technique, the proposed, novel method does not suffer from positional under-sampling, so it is well-placed to be applied at microwave frequencies and above.