Design of filtering impedance transformer with high impedance ratio

A kind of impedance transformer with high impedance ratio based on tapped coupled-resonator filter theory is proposed. Differently with the published ones, the proposed filtering transformer incrementally takes impedance property of the filter into consideration. Thus, the high impedance ratio can be achieved. By proper design of impedance and type of resonator, two prototypes with impedance ratios as high as 400:50 and 800:50 and 3-dB fractional bandwidths of 20.2 and 22.8% are realised.