This paper presents a study of the interferometric coherence aimed to the understanding of the main sources of phase error of an operative wide band system. This objective is accomplished by finding a theoretical expression of coherence based on a suitable model capable of including the roughness of the surface, which produces decorrelation besides the conventional spectral overlapping approach. The analysis is validated by means of numerical simulations and controlled experiments in anechoic chamber, with the aim of extrapolating the obtained results to airborne and spaceborne imaging radars.