Angular bandpass filters: an alternative viewpoint gives improved design flexibility

An alternative approach for the design of angular bandpass filters composed of metal strip gratings is proposed. In the classic approach, frequency selective surfaces (gratings) are used as resonant elements and the spacing between them is adjusted to obtain the required electrical performance. Following this implementation, however, the electrical coupling to the individual resonators can not be fully controlled so that very limited design freedom is obtained. Following the approach that we propose, the resonant element is the spacing between surfaces while the individual grating geometries are adjusted to obtain the required inter-resonator couplings. With this arrangement the structure becomes effectively equivalent to a closed waveguide filter, all electrical parameters of interest can be individually controlled and considerably extended design freedom is achieved. The basic resonator performance is analyzed and complete filter design examples are discussed. Furthermore, it is shown that, following the approach proposed, the frequency behavior of the structure is completely decoupled from its angular behavior in the design process thereby further extending the design freedom