Articles | Volume 15
21 Sep 2017
 | 21 Sep 2017

Effects of Aperture Size on Q factor and Shielding Effectiveness of a Cubic Resonator

Stefan Parr, Stephan Chromy, Stefan Dickmann, and Martin Schaarschmidt

Abstract. The EMC properties of a cubic metallic shield are highly affected by its resonances. At the resonant frequencies, the shielding effectiveness (SE) collapses, which results in high field strengths inside the cavity. This can cause failure or even breakdown of electronic devices inside the shield. The resonant behaviour is mainly determined by the quality or Q factor of the shield. In this paper, the effects of the aperture size on the Q factor and the SE of an electrically large, cubic shield are analysed. At first, a method is developed in order to determine the Q factor based on the resonance behaviour of the shield in time domain. Only the first resonance of the shield is considered therefore. The results are evaluated for different aperture diameters and compared with theory for the Q factor. The dominant coupling mechanism of electromagnetic energy into the shield is thus identified.

Then the effect of aperture size on the SE is analysed. The excitation of resonances is very probable if the interfering signal is an ultrawideband (UWB) pulse, which constitutes a typical intentional electromagnetic interference (IEMI) scenario. Therefore, the relation between aperture size and SE is analysed using the theory of the transient SE for a broadband signal with a constant spectral density distribution. The results show, that a worst case aperture size exists, where the SE has its minimum.

Short summary
Electric equipment can be shielded against high power electromagnetic fields via metallic cavities. At certain frequencies, called resonances, the shielding effectiveness collapses. In this paper the impact of different sized windows in the shield is analyzed. The results is: at a certain size of the window, approximately one third of the shield's size, the shielding effectiveness is at its minimum. Furthermore, the quality factor of shields with different windows sizes is analyzed.