Analysis of the suspended strip in elliptical cross-section by separation of variables

The coaxial suspended strip configuration in elliptical cross-section is found in transitions from ordinary coax to planar circuits and other geometries. Using elliptical coordinates makes the problem separable and leads to closed forms solutions for the fundamental TEM mode and the TE/TM higher modes: it is emphasized that the presence of the thin strip is dealt with analytically in this approach. Mode patterns and TEM characteristic impedance are reported. The cutoff frequencies vs. excentricity are computed for the first eight modes by both the present and the F.E.M. method finding excellent agreement.