A study on the modified septum of a TEM cell for an expanded test region

In this paper, the effects on performances by the modified septum structure of a TEM cell for the expanded test region is discussed. In order to compare and analyze these effects on performances, the boundary integral method is applied. The electromagnetic characteristics of a TEM cell described in this paper include the characteristic impedance of a transmission line, and electric field distribution inside of a TEM cell. Through the analysis of the TEM cell, it is found that the closer the both ends of a septum is approached to the outer conductors, the higher the charge distribution is. The electric field distortion near the outer conductors could be reduced, by modifying both ends of the septum. As a result of the study, a new septum structure which can improve the variations of electric field intensity is proposed, while maintaining the characteristic impedance of the TEM cell constant. When a septum of the NBS TEM cell is folded down with 44.5° angle at the point of 9.68° of overall length from each end of septums, and cutting off horizontally about 1.61° of original septum length to compensate impedance variations caused by fringing capacitance variations, the region of vertical 1 dB difference from theoretical value is 40% wider than a TEM cell proposed by the NBS