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A compact planar E-plane monopulse antenna is proposed and realised at 36.5 GHz for millimetre-wave radar or direction-finding system application, which is built on the substrate integrated waveguide (SIW) technology. A phase shifter having a non-uniform SIW width configuration is self-consistently made of a periodically embedded via array and it can achieve good performances over a relative broad bandwidth. A 180deg directional coupler incorporating this proposed phase shifter structure is developed in the design of an integrated feeding network for the monopulse antenna. Four elements array based on the configuration of antipodal linearly tapered slot antenna is designed as radiator, offering a pair of 'sum' and 'difference' beams along the long direction of the substrate. Measured gain of the 'sum' beam is higher than 16 dBi, whereas the zero depth of the 'difference' beam is lower than -38 dB. This type of monopulse antenna presents an excellent candidate in the development of intelligent millimetre-wave directional- finding system.