AbstractMicrostrips are open structures, that tend to radiate as the frequency of the signal increases within the microwave range. Radiation becomes significant in the vicinity of discontinuities, particularly when parts of the structure resonate. This situation is encountered in the design of filters, which often are arrays of radiating dipoles. When circuits are placed within metal enclosures, the radiated waves are reflected on the walls, and produce interference.The mechanism of radiation is linked to the excitation of three kinds of waves: radiated waves, leaky waves, and surface waves. The latter remain trapped within the dielectric substrate and propagate with little attenuation (i.e., optical fibers), producing spurious coupling and diffraction.An analysis technique originally developed for patch antennas provides a means to rigorously analyze the radiation from unenclosed microstrip circuits. It is based on a mixed potential integral equation formulation (MPIE), making use of Green's functions provided by Sommerfeld integrals, for which an exact evaluation technique was developed. The currents flowing on the circuits are expanded over a set of subsectional basis functions, and their intensities are determined by a method of moments. This approach provides both the circuit parameters (impedance or scattering matrices), and its radiation characteristics (antenna pattern). It allows one to determine the effect produced by a radiating element on another one, even when the two are not closely located with respect to one another.