Waveguide structures for double-beam leaky-wave antennas

The behaviour of two types of leaky-wave antenna is described, where more than one mode of propagation is supported in the leaky waveguide. The radiation pattern of such antennas contains one main lobe for each mode of propagation, and the design of a double-beam antenna is studied in some detail. First, consideration is given to a slotted double-ridge waveguide. It is shown both theoretically and experimentally that the structure can be designed so that pairs of higher-order modes cause radiation of nearly independent beams at angles which are closely spaced. When H02 and H03 modes are used for this purpose, the structure has the disadvantage of also supporting, at the operating frequency, the H01 mode and a slot mode. These undesired modes may be eliminated if an inductive, rather than capacitive, form of coupling is used between the constituent waveguides and if the radiating waveguide wall also has an inductive reactance. The reactance is realised by means of circular-cross-section coupling holes spaced periodically along the waveguide. Theoretical and experimental results for this structure are shown to compare favourably, and problems associated with the extension of the principle to antennas radiating more than two independent beams are briefly mentioned.