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The mutual coupling between two elementary sources exciting a substrate integrated waveguide feed is analyzed. The feed consists of two partially reflecting grids made by vertical metallic pins connecting the upper and lower metallic plates of a parallel plate waveguide. The sources are the inner conductor of a probe-like transition. The dispersion diagram of the structure is derived by finding the poles of the current expression (in the spectral domain) on the partially reflecting grids. Two leaky-wave modes are found as a function of the geometrical parameters of the partially reflecting grids. It is shown that, by properly choosing the geometrical dimensions of the partially reflecting grids, the leaky-wave modes can be used to opportunely shape the field launched inside the parallel plate waveguide and considerably reduce the mutual coupling between the sources. The same results in terms of mutual coupling can not be achieved with a single reflecting grid, justifying the use of the proposed solution.