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A novel compact monopulse cavity-backed substrate integrated waveguide (SIW) antenna is proposed. The antenna consists of an array of four circularly polarized (CP) cavity-backed SIW antennas, three dual-mode hybrid coupler, and three input ports. \mathrmTE{10} and \mathrmTE{20} modes are excited in the dual-mode hybrid to produce sum and difference patterns, respectively. The antenna is modeled with a fast full-wave hybrid numerical method and also simulated using full-wave Ansoft HFSS. The whole antenna is integrated on a two-layer dielectric with the size of 42\;\mathrmmm \times 36\;\mathrmmm. A prototype of the proposed monopulse antenna at the center frequency of 9.9 GHz is manufactured. Measured results show - 10\-\mathrmdB impedance bandwidth of 2.4%, 3-dB axial ratio (AR) bandwidth of 1.75%, 12.3-dBi gain, and - 28\-\mathrmdB null depth. The proposed antenna has good monopulse radiation characteristics, high efficiency, and can be easily integrated with planar circuits. [ABSTRACT FROM PUBLISHER]