New narrow-band dual-mode bandstop waveguide filters

A method is presented for constructing a bandstop waveguide filter containing multiple-coupled cavities. A proof is given of the complementary principle for such a bandstop filter and a corresponding bandpass filter. Through this principle, a method is described for synthesizing elliptic bandstop filters. Four-mode and six-mode filters are constructed, and experimental results confirm the complementary principle. In addition, a practical waveguide bandstop filter is achieved. It is pointed out that if a suitable coupling mechanism can be found, the complementary principle can also be applied to microstrip bandpass and bandstop filters. With regard to the complementary principle, it is pointed out that when a bandpass filter has the same multiple-coupled cavity structure as a bandpass filter and when the symmetry and identity conditions are achieved, then their transmission responses are complementary to each other. Here, the passbands correspond to the stopbands, and the transmission poles and zeros of one filter correspond to the zeros and poles of the other.