Abstract: The incline rectangular aperture antenna is one whose opening is tapered only on both narrow sides of a rectangular waveguide, placed on a perfectly infinite conducting plane, excited by the dominant H₁₀ mode in the rectangular waveguide. The electric and magnetic field components over the aperture and radiated field formula are simply analyzed. The main beam pointing lies off broadside, namely the major lobe pointing deviates from the perpendicular of the aperture and the axis of the excited waveguide. The principal plane patterns have been calculated and the input voltage standing wave ratio (VSWR) versus measured frequencies is given. It is found that the VSWR is related to the incline angle of the aperture, and can be improved using different incline angles.