First harmonic suppression of a bandpass filter with an Optimum Bandstop Filter

Parallel coupledline bandpass filters (BPF) are widely used in microwave systems. But their spurious harmonics deteriorate their stopband performances. An Optimum Bandstop Filter (BSF) is designed and used to suppress the first harmonics of a parallel coupledline BPF. Simulations show that the first harmonics of the BPF is suppressed for more than 30 dB with little disturbance to the passband.