1277250 Filters WESTERN ELECTRIC CO Inc 11 Nov 1969 [18 Nov 1968] 55044/69 Heading H1W Filter circuits are constructed using a cascade of bimodal networks all of which are characterized by the same two normal modes, and including means, connecting adjacent pairs of the networks, for converting all of the wave energy from one of the normal modes in one network to the other of the normal modes in the next network. In the Fig. 2 embodiment, each network section 11, 12, 13, 14 is a section of a twinconductor coaxial transmission line having an outer conductor 32, the sections being coupled by phase-inverting transformers 24, 25, and 26 connected between one of the inner conductors and the outer conductor. The filter is energized by a signal source 18 through a hybrid transformer 19 such that, with the source connected to port 1, as shown, conductors 30 and 31 of section 11 are energized in phase i.e. in the symmetric mode, such that section 11 behaves as a length of transmission line of characteristic impedance Z 1 . The signal in conductor 30 undergoes a 180 degree phase shift in transformer 24, such that the signals on the two conductors of section 12 are 180 degrees out of phase i.e. section 12 is energized in the antisymmetric mode, and behaves as a length of transmission line of characteristic impedance Z 2 , where Z 2 is less than Z 1 . Thereafter successive sections are energized alternately in the symmetric and antisymmetric modes, providing characteristic impedances of Z 1 and Z 2 respectively. The filter of Fig. 4 is constructed of a cascade in which each bimodal network section is formed by a pair of quadrature hybrid couplers, the two couplers of each pair being rotated through 90 degrees with respect to one another, so that a pair of ports 3 and 4 symmetrically located with respect to one of the axes of symmetry is coupled to a pair of ports 1 and 3, symmetrically located with respect to the other axis of symmetry. Reference is made to further filters constructed by using a conductively bounded transmission line with discontinuities introduced at locations determined by the different field distributions for the different transmission modes, or employing the different response of gyromagnetic materials to oppositely rotating circularly polarized waves together with an array of longitudinally spaced half waveplates.