The wave concept iterative process is a procedure used for analyses planar circuits, this method consists in generating a recursive relationship between a wave source and reflected waves from the discontinuity plane which is divided into cells. A high computational speed has been achieved by using Fast Modal Transform (FMT). In this paper we study a patch antenna and MESFET transistor, to determine the electromagnetic characteristics of these structures. 1. Introduction The MESFET or GaAs FET as it is also called is a high performance form of field effect transistor that is used mainly for high performance microwave applications and in semiconductor RF amplifiers. The abbreviation MESFET stands for Metal-Semiconductor Field Effect Transistor. GaAs FET standars for Gallium Arsenide, the substance from wich this FET or field effect transistor is made. [9], [10] The GaAs FET or MESFET shares many features with the standard junction FET or JFET, although the MESFET is able to offer superior performance, especially in the region of RF microwave operation, especially for use within RF amplifiers. [1] In this paper, an iterative method is applied to active circuit, it consist in successive reflection between the circuit plan and its two sides. It also has an alternative behavior between space and spectral domains. In addition the discontinuity plan is divided into cells and characterized by scattering operator matrix depending on the boundary conditions. [2] This analysis is applied to calculate the S parameters of rectangular waveguide including a MESFET transistor.