Frequency diplexer using multimode bandpass filters with high stopband attenuation level

Multimode bandpass filters are proposed by using a hairpin resonator and two multimode resonators. The proposed bandpass filter has five-mode resonance in a passband and transmission zeros at both the lower and higher stopbands near the cutoff frequencies. Because the steep characteristics at the edge of the passband and the high attenuation level at the stopbands can be obtained due to the multimode resonance and transmission zeros, a frequency diplexer with adjacent passbands can be comprised of the proposed bandpass filters by using an open-matching methodology.