A study of the diffraction of an electromagnetic wave on a dielectric coating with a diaphragm over an infinite waveguide antenna array has been carried out. The metal diaphragm is located at some distance from the surface of the antenna array. The influence of the geometric dimensions of a metal diaphragm and its distance from the grating surface on the modulus of the reflection coefficient of the incident wave in waveguides is investigated. Dielectric coatings with different values of the dielectric constant are considered.