Influence of Geometric Parameters and Permittivity of Stripline Filters on Resonator Coupling Coefficients

It has been proved that electromagnetic coupling coefficients K of resonators in stripline filters with homogeneous dielectric depend only on geometric parameters of the filter designs and are independent of relative permittivity ε_r (if the dielectric is two-layer, coefficients K depend only on geometric parameters of the filter designs and ratio ε_r2/ε_r1). It has been shown that the revealed earlier influence of permittivity ε_r and the operating frequency on K is only a consequence of the influence of the length of stripline resonators on K: the lesser is the length, the larger is coefficient K. It has been found that these propositions enable consideration of frequency characteristics of the same design of the bandpass filter in different frequency bands by changing εr and performing slight changes in outermost resonators. The results of computer simulation of the transfer of frequency characteristics of a stripline bandpass filter from 3 GHz to 6 and 12 GHz with retaining the fractional bandwidth and selectivity are presented.