Multiple parameter radar measurements (dual frequency hail signal DFR-HS or differential reflectivity ZDR) can be strongly affected by the mismatch of the antenna illumination functions at the two wavelengths or polarizations. The resulting instrumental error can be estimated, with some approximations, from the independently measured reflectivity at one wavelength or polarization by using the actual radar illumination functions. This objective estimation of the antenna-induced error is made in some real cases and its possible conseqences on the data analysis are discussed.