As traditional horn antennas that could be used as feeds for reflector antenna systems, substrate integrate waveguide (SIW) horns could be used as feeds for planar antennas and arrays. In this letter, a phase-and-amplitude-corrected SIW horn by metal-via arrays is presented as a planar feeding structure. The compact SIW horn is applied to feed two antipodal linearly tapered slot antenna (ALTSA) arrays, forming sum and difference beams at X-band. The measured gain of the sum beam is 12.84 dBi, the half-power beamwidth is , the FTBR is 15.07 dB, and the sidelobe level is 20.79 dB at 10.1 GHz. The null depth of the difference beam is 44.24 dB. Good agreement between the simulation and the measured results is obtained.Index Terms-Antipodal linearly tapered slot antenna (ALTSA), array antenna, feeding system, planar horn, substrate integrate waveguide (SIW).