Investigation of frequency-selective properties of microwave wideband bandpass filters

A new approach allowing to improve frequency-selective properties for a fixed order of N filter is suggested. In all the studies, conducted with the help of numerical electrodynamic analysis of 3D models of microstrip filters based on a multimode resonator, the same substrate with dielectric constant ε=2.8 and thickness h=2 mm (material - FLAN) was used in calculations. The central bandpass frequency of microwave structures f ₀≈1.4 GHz was registered and as well as relative bandwidth Δf/f _o≈80%. The strip conductor of central multimode resonator in six studied filters of the sixth and eighth orders has the shape of an irregular meander being electromagnetically connected with four single-mode resonators, a pair of which is located to the left of it, and the other pair is to the right. It is shown that in single-mode quarter-wave resonators building-up the number of portions of identical parallel strip conductors, connected to a screen at one end and connected with each other by a strip conductor jumper, can increase the power of suppression at low-frequency stop band by more than 15 dB, as well as near high-frequency slope of passband by more than 10 dB. Therefore, the level of maximums of return losses in the passband of wideband bandpass filter ranges within a few dB.