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In this work, an analytical model for the study of passive intermodulation (PIM) on large reflector antennas is presented. Passive intermodulation, in the scattered field, arises when the scatterer is nonlinear, or when it presents junctions connecting linear materials. Its presence causes a degradation of some antenna parameters and, especially, the cross-polar level, which may rise by several tens of dB. It would then be useful to develop a technique to predict its influence, in order to take appropriate steps during antenna design. A heuristic model for the junction problem has previously been derived and validated with measurements in a time-domain physical optics (TD-PO) framework. These results are applied here to a TD-PO analysis of reflector antennas, in particular, for a satellite-communication antenna and for a radio-astronomy antenna.