Higher order modes in coupled striplines: prediction and measurement

Finite element and resonant cavity methods are used to predict and confirm the cut-off frequencies of higher order modes in stripline. Infinite elements are used to treat open-wall boundaries. Single lines, symmetrically coupled and asymmetrically coupled lines are treated in turn. In the tightly coupled situation the cut-off frequency of the lowest TE mode degenerates to that of an abutted combined linewidth. Established approximation methods for single lines are used where possible. Some practical observations are made and selected finite element field plots are included for completeness.