  We present the electromagnetic model of a dual circular polarization
antenna-feed system, consisting of a corrugated feedhorn, a polarizer and an
orthomode transducer. This model was developed for the passive front-end
implemented in the Q-band receivers of the STRIP instrument of the Large Scale
Polarization Explorer experiment. Its applicability, however, is completely
general. The model has been implemented by superposing the response of two
linearly polarized feedhorns with a $\pi$/2 phase difference, thus taking into
account for the effect of the polarizer, that behaves differently for the two
polarizations of the incoming electric field. The model has been verified by
means of radiation pattern measurements performed in the anechoic chamber at
the Physics Department of the University of Milan. We measured both arms of the
orthomode transducer, in order to check that the diagram at one port is the 90_
rotation of the diagram at the other port. Simulations and measurements show an
agreement at the level of fraction of a dB up to the first sidelobes, thus
confirming the model.
