Abstract: For expanding the passband, two Antipodal Sinusoidally Tapered Slot Antennas (ASTSAs) with high gain and wide bandwidth are proposed based on the conventional ALTSA and fed by Substrate Integrated Waveguide (SIW). The Half Mode Substrate Integrated Waveguide (HMSIW) monopulse fed network is designed by making use of the principles of rectangular 3 dB directional coupler and 90 phase shifter. Then the symmetrical and unsymmetrical monopulse ASTSAs are realized with passbands both over 3.0 GHz and gains of sum beam about 9.0 dBi. The zero depths of different beam at 10.0 GHz both lower than -20 dB. There into, the unsymmetrical monopulse ASTSA has wider bandwidth and more balanced gain, which is widely used in the millimetre-wave directional-finding system.