Excerpt from Excitation of a Tem Cell by a Vertical Electric Hertzian Dipole
This report describes theoretical and experimental analyses developed by staff of the University of Colorado at Boulder in collaboration with the Electromagnetic Fields Division of the National Bureau of Standards (nbs), under a contract sponsored by nbs. Professor David C. Chang heads the University team. Dr. Mark T. Ma of nbs serves as the technical contract monitor. The period covered by this report extends from July 1979 to July 1980.
The work described in this report represents a further aspect of establishing a theoretical basis for the technical analyses of transverse electromagnetic (tem) transmission line cells developed at nbs. The general purpose of pursuing theoretical studies is to evaluate the use of tem cells for (1) measuring the total rf radiated power by a device inserted into the cell for test, or (2) performing necessary susceptibility tests on a small electronic device.
The particular topic addressed herein discusses the manner in which a vertical electrical Hertzian dipole excites a tem cell under the assumption that the gap between the septum and the side wall is small. This premise is consistent with the cell geometries designed to provide a good impedance match, presently of interest to nbs, and leads to significant simplifications in the mathematical derivation. The formation of the problem also allows a vertical offset for the septum position so that the size of the test area may be increased to accommodate larger pieces of test equipment.
Approximate expressions are found for the field distribution inside a tem cell and for its characteristic impedance. The latter result consists of a dominant gap dependent logarithmic term, plus a correction series that accounts for the vertical offset of the septum. This vertical offset is allowed to be arbitrary, and therefore the results contained in this report will supplement previous efforts which were restricted to small offsets. In addition, because of the delta function nature of the source considered here, the results lend themselves naturally to an analysis of more complex source configurations such as a practical monopole via Greens function methods. The monopole result may then be used to model probes inserted into a tem cell to measure or excite fields.
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