In this paper, a Ku-band microstrip antenna is analyzed and will be designed. The characteristics of the patch are analyzed and optimized with Ansoft HFSS. A different patch is evolution of the classic rectangular patch. It has a square loop outside the rectangular patch. We can call it window-patch. The construction of the antenna is resulted from considering efficiency of the spillover efficiency (ηs ) and the illumination efficiency (ηi ). Once the unit element has been fully characterized and a reflectarray designed, an important aspect in the reflectarray analysis is the computation of the radiation patterns. In this literature, a method of Array Element Factor is used to calculate it. For the reflectarray antenna, the ideal range of phase shift should realize 0 ∼ 360◦ , but sometimes things go contrary. It would be discussed the effects brought by the lack of the phase shift and electromagnetic wave incident angle.