Compact wideband microstrip‐slotline bandpass filter with sharp selectivity and suppressed spurious harmonic

Abstract A novel compact wideband microstrip‐slotline bandpass filter (BPF) with roll‐off cutoff slop and extended upper stopband is proposed, which consists of two cascaded microstrip‐to‐microstrip vertical transitions. Under the influence of interdigital capacitor, a pair of transmission zeros (TZs) at both sides of passband are generated to effectively enhance shirt selectivity, the mechanism of cross‐coupling structure is theoretically interpreted in detail based on equivalent transmission line network. In order to well suppress the upper stopband spurious harmonic, there are additional five TZs are introduced by properly adjusting the electrical length of open‐ended slotline sections and etching a set of spurlines on the feeding lines. For experimental validation, two broadband BPFs with/without spurlines are designed, fabricated, and measured. The measured results demonstrate that the BPF with spurlines have more excellent electric performances compared with the one without spurlines. The return loss ≥17.38 dB, insertion loss ≤0.82 dB with the 3‐dB fractional bandwidth (FBW) of 134.52%, the extended upper stopband is up to 3.86 times center frequency with rejection level better than 20.63 dB, the spurious harmonic is significantly suppressed without increasing additional circuit dimension, the compact effective circuit size is only 0.95λ g *0.29λ g , λ g is guided wavelength at center frequency. Finally, the theoretical analysis is well verified by measured and simulated results, excellent agreement between them is achieved.