Orthogonal mode transducer

An orthogonal mode transducer for an antenna feed intended for dual polarised operation in at least two different frequency bands includes a primary waveguide (4) and a first coupling section (5) comprising a side-arm waveguide (7,8) coupled to the primary waveguide for coupling signals in the higher of two different frequency bands supported by the primary waveguide and having a predetermined hand of polarisation. A frequency and polarisation selective filter (11,12) is disposed in the primary waveguide, downstream from the side-arm waveguide in the receive direction. The filter reflects signals with the predetermined hand of polarisation in the higher frequency band but will pass orthogonally-polarised signals in the same frequency band and all signals in the lower of the two bands. A second coupling section (6) is similar to the first section, but its side-arm waveguide (13,14) and its filter (16,17) are displaced angularly about the axis of the primary waveguide by 90 DEG with respect to those of the first section. The second section thereby couples the orthogonally-polarised signals in the higher frequency band.