Numerical solutions of open-wall stripline geometries

Medium to high power radar antennas often adopt air spaced stripline as a suitable transmission medium. The authors present a method of accurately predicting the modal structure for a variety of geometries; in particular, coupled lines are examined. This technique uses an open-wall finite element solver with special infinite decay elements which permits open boundaries to be accommodated and the stripline to be modelled accurately. Computer predictions are presented which show that the higher order mode cut-off frequency is considerably lower than previously reported in the literature. It is demonstrated how the cut-off wavenumber of the first higher order mode of a pair of symmetrical coupled striplines reduces to that of a single wide line in the limit of small spacing.