Advanced Measurement Techniques Within Shielded Enclosures

This report summarizes the results of an experimental program to develop measurement techniques for obtaining accurate radiated interference data on electronic and electrical equipment when tested within the confines of shielded enclosures. Specific objectives were to investigate various observations, conclusions and recommendations evolving from prior work in this area. Although the initial measurements were made in a full size enclosure most of the experiments were performed using frequency scaled cavities at frequencies up to and including the first resonant frequency. With linear source and receive antennas, the severe E Field null observed by other investigators appeared at frequencies somewhat above fco/2 for the dominant waveguide mode. As noted by Mendz (IBM), the null moves toward the source as the frequency is increased. In addition, the location of the null relative to the source is a function of the enclosure geometry, the location of the source and the length of both the source and receive antennas. However, when either monopole was replaced by a loop the null did not appear. Also, the null was not observed with the monopole source antenna located on the bottom wall and a monopole receive antenna probing along the top wall. Regardless of the antenna configuration, the standing wave at the first resonant frequency is a problem. Reasonable success in reducing the amplitude was realized by loading the cavities with lossy, vane attenuators.