In the present study the Split aperture decoupling method is applied to pillbox antennas in order to have low side lobe and high beam crossover levels at the same time. These two constraints cannot be satisfied simultaneously for any passive and lossless multi-beam antenna systems with a single radiating aperture. In order to validate the proposed concept a novel Substrate Integrated Waveguide (SIW) multibeam Pillbox antenna with two radiating apertures has been designed at 24.15 GHz and manufactured using standard Printed Circuit Board (PCB) fabrication process. The measured results agree with simulations showing a good scanning performance at 24.15 GHz over an angular sector of ±40°. The crossover level between adjacent beams is about -3dB with a side lobe level lower than -24dB for the central beam and better than -11dB over the entire field of view. © 2016 European Association of Antennas and Propagation.