Simple Semi-Analytical Septum Design for Improved Matching in Open TEM Cells

Abstract

Open transverse electromagnetic (TEM) cells are used for many applications. Usually, such cells are built of metal sheets with linear profiles. Due to these simple geometries, the impedance matching is limited. To improve the matching, a simple semi-analytical method for an advanced septum profile is developed. Dominant TEM-mode propagation and an expression for the characteristic impedance of a stripline are considered. The characteristic impedances of the feed and the central stripline are equal to a given, desired value. To minimize reflections, the characteristic impedance of the TEM mode is enforced to be constant over the entire structure, especially along the tapered sections. In this way the proposed method returns the septum width as a function of the longitudinal coordinate along the cell such that each cross-section has a characteristic impedance equal to the given, desired value. A septum designed according to the proposed method is manufactured and installed in a TEM cell prototype. The 30 MHz -1 GHz frequency range is considered to cover CISPR bands C and D. Both measurements and full-wave simulations indicate an improvement in the return loss of 4.6 dB from 10.6 dB to 15.2 dB with respect to the previously installed linearly tapered septum.