New equivalent circuits for inhomogeneous coupled lines with synthesis applications

Equivalent circuits for inhomogeneous coupled-line sections are presented which enable cascade synthesis procedure to be applied to the design of very-broadband inhomogeneous filters. Here, the term inhomogeneous means that the even and odd modes have different phase velocities and different electrical lengths. The method has been implemented in the design of inhomogeneous high-pass filters constructed in suspended substrate stripline, when the even-mode characteristic impedance is much greater than the odd-mode characteristic impedance. An approximate but highly accurate synthesis of a 3:1 bandwidth-inhomogeneous distributed high-pass filter is described. The procedure is almost purely analytic, and computer-aided design is required only for fine tuning adjustments. The theoretical feasibility of designing such filters for upper-pass bandwidths of greater than 8:1 is demonstrated