A General Design Technique for Coupled Lines Array Filters

A general synthesis procedure for bandpass filters made up of coupled lines arrays is presented. This procedure allows a greater accuracy with respect to the traditional approximated design methods, without resort to the optimisation of the filter frequency response. The method requires the computation of the admittance matrix of a coupled line array, which is obtained from the quasi-static capacitance and inductance matrices per unit length of the array. The proposed method allows to take into account in the synthesis of the filter both the couplings with the external loads (such as a tap in the first/last line) and the various discontinuities present in the structure (shorts, open ends, T junctions, tuning screws, etc.). The procedure can be applied to several filters of practical interest, such as comb and interdigital, in different environment (coaxial, slab, microstrip); some microstrip interdigital filters, designed with the new method, have been fabricated for testing purposes; comparisons between measurements and simulations are reported.