Hairpin filters with tunable transmission zeros

In the previous work, it was found that filter designed with 0/spl deg/ feed structure could have two extra transmission zeros in the stopband. In this paper, a method for tuning the frequencies of these two transmission zeros using impedance transformers is proposed. These zeros can be tuned to reject unwanted signals near the passband, such as image signals, This new approach has been experimentally verified with three second-order Butterworth filters.