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 1×8 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 18.6°, 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.