Radiating waveguide and communication system

A radiating waveguide comprises an elongated waveguide 10 with a transverse cross-section dimensioned to carry only a dominant mode at a selected operating frequency. One wall of the waveguide forms a continuous non-resonant longitudinal slot 11 or equivalent segmented slot to produce a radiated field polarized perpendicularly to the slot 11. The slot extends substantially parallel to the axis of the waveguide 10 and has a transverse dimension that is sufficiently narrow to substantially avoid the radiation of a field polarized parallel to the slot. The waveguide may be used to communicate with multiple radio units, for example in a road, rail or tunnel system. In a digital communication system, the waveguide provides low bit error rates.