Abstract: A combiner for transmitting and receiving co-polarized microwave signals in a selected propagation mode in at least two different frequency bands, the combiner comprising a main waveguide dimensioned to simultaneously propagate signals in the different frequency bands, at least a portion of the main waveguide being overmoded; at first and second junctions spaced along the length of the main waveguide for coupling signals in the different frequency bands in and out of the main waveguide, at least the first junction being located in an overmoded portion of the main waveguide and having side-arm waveguide means associated therewith for propagating signals in one of the different frequency bands; filtering means disposed within the main waveguide and operatively associated with the first and second junctions, the filtering means having (1) a stopband characteristic for coupling signals in a first one of the frequency bands between the main waveguide and the first junction and the side-arm waveguide means associat