Design Procedure of Continuous Profile Stopband Filters Implemented With Empty Substrate Integrated Coaxial Lines

This article presents a systematic procedure aimed to design continuous profile stopband filters assembled with empty substrate integrated coaxial lines. The coaxial filter is made of five low-cost substrate layers, where the central boards contain the filter profile. This structure exhibits unimodal TEM operation over a wide frequency band, it is not dispersive, it is shielded and cannot be interfered from external signals, and it can be easily integrated with other planar devices. Well-known scattering inverse methods based on the coupled-mode theory have been used for the dimensional synthesis of the proposed topology. Since these dimensions are obtained analytically, no further brute-force optimizations are needed. In particular, a nine-pole elliptic stopband filter operating in the $K_{u}$ band and covering the whole bandwidth of Hispasat 1E communication satellite is shown. The final manufactured filter prototype is large in the longitudinal dimension, but due to its continuous topology, it results to be rather insensitive to fabrication tolerances and errors. Measured results are in good agreement with full-wave simulations. The proposed empty substrate integrated coaxial technology is particularly suitable to realize stopband filters with moderate or wide bandwidths for next-generation satellite applications.