Determination of frequency range limit when using Stripline as emission or immunity test methods

摘要

In the automobile industry the tests of electronic devices against electromagnetic interferences or susceptibility request today economical and unfailing test methods. For component testing the open "Transversal Electromagnetic (TEM)-Wave Guides" e.g. Stripline method [3; 4; 5; 6], respond in face of costs and reproducibility adequately well to these demands. The others closed wave guides like TEM-Cell, GTEM-Cell [3; 4; 5] may not be excluded by these considerations. Many of global automotive manufacturers and suppliers have dealt with the practical use of such open or closed wave guides where a special attention has been paid to the Stripline as a test method by national or international EMC committees. The test procedures and conditions defined for getting correlations with the vehicle results have been reported in [3; 4; 5; 6]. Furthermore theoretical calculations of closed TEM-Wave-Guides particularly the basic computations of the Transversal Electromagnetic (TEM)-Modes are also reported in [1]. The mutual influences of electromagnetic "Higher Order Modes" on the TEM-Modes related to the open TEM-Wave-Guides (e.g. Stripline) still remained weakly investigated [1; 2], because of computing complexities of the last one. Today questions of using such wave guides are more and more related to the limitation of the operating frequency ranges which can permit to reduce the mutual influences mentioned above. Further the determination or identification of the influencing parameters belongs to today important EMC tasks. The construction of open or closed TEM-Wave-Guides is not homogenous along the propagation direction because of the discontinuities at one or both sub parts as shown in an example of Stripline in Figure 1-1 of Annex A. Therefore it is plausible that upper from some frequencies (cut off frequencies) the electromagnetic "Higher Order Modes" can affect the propagation of the TEM-Modes under the main part of the Stripline (s. Figure 1-1 of Annex A). To avoid this, a frequency limitation should be taken into account. The main goal of this paper is to give theoretical backgrounds for better understanding of open or closed TEM-Wave-Guides. For evidence of that it is to determine from simple ways the frequency ranges where it is sure that the TEM-Modes and the electromagnetic "Higher Order Modes" are enough decoupled to each other and have not mutual influences. That means, in the frequency ranges the last ones shall not be able to propagate. As an example of open TEM-Wave-Guides a 90OMEGA-Stripline - common in the automobile industry - will be investigated.