Electromagnetic Interference Measurement Methods - Shielded Enclosure

摘要

The report summarizes the accomplishments on a program to develop test techniques and procedures for making radiated measurements in shielded enclosures which can be correlated with open-field measurements. A technique of shielding the probe antenna in all directions except that of the desired signal path by means of a metal hood lined with absorbing material was developed. An evaluation indicated that, over the frequency range from 200 MHz to 12 GHz, the technique is capable of reducing the multipath reflections in shielded enclosures to a level comparable with the reflections normally encountered in open-field measurements. The development, fabrication and evaluation of two hooded probe antennas covering the frequency range from 200 MHz to 12 GHz are described. The field-intensity calibration of the two hooded probe antennas is also described. At lower frequencies where the shielded enclosure dimensions and probe antenna spacings are small relative to the wave lengths involved, tests which correlate with open-field measurements can be made. In order to establish the critical or upper limit frequency for this low frequency range, theoretical and experimental studies were conducted to investigate the field distributions within shielded enclosures at low frequencies. These studies established that satisfactory measurements can be made with conventional probe antennas in the frequency range from 1 to 30 MHz. (Author)