One method of measuring a circular polarized horn is to use a linear source antenna.  By measuring the gain relative to a linear polarized gain standard and noting the axial ratio by rotating the source, the gain correction is given by the scale.  For example, an antenna with a 3 dB axial has a circular gain 1.64 dB above the measured value as a linear antenna at the peak of the CP response.
CP_meas_corr.gif