The analysis of the effects on the power pattern radiated by reflector antennas in presence of localized defects (also called bumps) of the parabolic surface is addressed by means of an innovative analytic strategy. The bump depths are supposed unknown or estimated with a given tolerance such that their deviations from the nominal surface can be expressed as intervals of values. The upper and lower bounds of the power pattern function including all possible patterns generated by the actual parabolic reflector with bumps having depths within the considered intervals are computed by means of the Interval Analysis. In order to show the effectiveness and efficacy of the proposed analytic tool, a benchmark example is reported and discussed.