Planar antenna reflectors are attractive for the designs of both narrow-beam antenna arrays and frequency-scanning arrays. Planar implementation of reflectors can be performed through substrate integrated waveguide (SIW) technology. The magnitude and argument of the field reflected from such surfaces depend on the angle of the incident field. Thus, the reflector’s profile should be synthesized considering the effect of spatial dispersion of the reflection coefficient to maximize the efficiency of the reflection. In this paper, we show how to synthesize a reflector’s profile that is characterized by the effect of spatial dispersion of the reflection coefficient. For this purpose, we use a spectral representation of the field reflected from a surface with spatial dispersion of the reflection coefficient. Specifically, we investigate a phase error for an SIW-based parabolic reflector. We show that offset SIW-based reflectors are mostly affected by the effect of spatial dispersion of the reflection coefficient.