Post-wall waveguide with a linear array of reflection-canceling slot pairs and center-feed is designed to cancel the frequency dependent tilting of the main beam and enhance the bandwidth of the antenna boresight gain. The array is fed at the center of the waveguide from the backside; the length of the radiating waveguide is halved and the long line effect in traveling wave operation is suppressed. Authors establish the array design procedure in separate steps to reduce the computational load in the iterative optimization by using Ansoft HFSS simulator. A center-feed linear array as well as an end-feed equivalent with uniform excitation is designed for 25.6GHz operation and measured. The measured performances confirm the design and the advantage of the centre-feed; a frequency independent boresight beam is observed and the frequency bandwidth for 3dB gain reduction is enhanced by 1.5 times compared to the end-feed array.