Design for Ka-band wideband bandpass filter with three-line microstrip structures

Presented in this paper is a systematic procedure for designing wideband bandpass filter based on parallel coupled three-line microstrip structures. An improvement for applying in practice is added. As compared with traditional parallel coupled wideband filters, the proposed three-line design requires less tight line spacing. Two filters with different line spacing are designed on substrates with low dielectric constants. Both of them have a fractional bandwidth (FBW) more than 40% at Ka- band.