A ridge-waveguide filter with a signal input port at a first end and a signal output port at a second end contains a cascade assembly of metal-bounded ridge-waveguide sections with interspersed metal-bounded evanescent-mode coupling regions, and also contains low-loss ridge-waveguide port coupling networks to impedance-match the ends of the assembly to respective signal-port reference impedances. A frequency multiplexer with a composite-signal port and a plurality of channeled-signal ports is composed of a plurality of ridge-waveguide filters that are series-connected through a ridge-waveguide manifold containing a multiplicity of three-way waveguide junctions and quasi-lumped waveguide elements.