Optimize SIW Antenna Beamwidth

My current substrate-integrated waveguide antenna in ANSYS HFSS follows an IEEE reference design and already shows an acceptable S11 curve, yet the far-field pattern is still disappointing—the beamwidth is noticeably broader than the paper’s results... (Budget: ₹1500 - ₹12500 INR, Jobs: Antenna Design, Electrical Engineering, Engineering, Matlab and Mathematica, Mechanical Engineering, Technical Documentation)