A new technique is developed to couple the advantages of both the microstrip patch antenna and rectangular waveguide. An equilateral triangle is used as a radiating patch. This patch is fabricated on one face of a single-layer dielectric substrate with double-sided copper clad coated with tin, and on the other face an iris is fabricated and coupled to the waveguide. Antenna parameters such as return loss and bandwidth are studied for a circular patch. The results obtained are discussed in detail and explained. Matlab PDE toolbox is used to generate two-dimensional meshes. These meshes are converted into a three-dimensional form using subdomain numbers. RWG basis functions are used for MoM to calculate impedances. Reection coefcient and 2D current are obtained using the complex impedances.