The adjusting parasitic patch size technique for the broad-beam microstrip antenna array using the cavity-backed slot coupling is presented. The phase of each element of the microstrip array has been designed to emulate the reflection of waves on the surface of parabolic backscattering. In order to increase the efficiency of this array antenna, the back-slot cavity with an exciting probe will be employed for coupling the electromagnetic waves to the back of this array. The proper sizes and locations of patches and the optimized position of the cavity have been investigated by the Computer Science Technology (CST) Microwave Studio. The gain, the radiation pattern, the bandwidth, and the return loss are extensively analyzed. The fabricated antenna has the return loss of −22.39 dB, the bandwidth of 47 MHz (4.975–5.022 GHz), and the maximum directive gain of 5.6 dB at 5 GHz, and it can produce a wide beam width (half-power beam width around 130°). The antenna could be applied for wide applications in the wireless communications system. In particular, this realizing antenna covers the low earth orbit (LEO) satellite beam.