EM radiation through aperture of metallic enclosure with a PCB inside

Enclosure technique is widely used to control electromagnetic (EM) radiation from electronic devices. As the aperture of the enclosure causes leakage of EM wave, desired shielding effectiveness (SE) may not be satisfied. In this paper, as a basic study to establish EMC guideline for shielding enclosure with practical PCBs (Printed Circuit Boards), influence on EM radiation through aperture of metallic enclosure with a PCB inside was investigated. A typical metallic enclosure, 300 times 200 times 100 mm³, with an aperture, 200 times 32 mm² and PCB of 230 times 55 mm² are used to experiment and calculation. The PCB is put in the enclosure and the placement of the PCB, angle of the PCB alpha, are changed. Firstly, angle-pattern and frequency responses of far-electric fields were discussed. It was demonstrated that PCB placement influences resonance and strength of the EM radiation from aperture of the enclosure. Horizontal and vertical component of EM radiation is the smallest in five test placements at the ldquoalpha = 270 degreesrdquo case in which signal trace is turned opposite side of aperture. Secondly, frequency response of SE, which defined as the ratio of EM field level of ldquoPCB onlyrdquo case, which means without enclosure, to ldquoPCB in enclosurerdquo case, is discussed. The alpha = 270 case has the largest SE. Finally, the electric field distribution was also discussed by calculation to understand the details of the effect of PCB placement. It was demonstrated that electric-coupling between PCB and wall of aperture side in the alpha = 270 case is weakened. This study is the basic but new considerations to realize method for suppressing EM radiation from a PCB with practical engineering stand point.