In this work we present a dualcircularly-polarized dual-port Ka-band feed to be integrated in a transmit-array (TA) antenna system. The feed gain is 12 dBi at Rx (19.7-20.2 GHz) and 14 dBi at Tx Ka-band (29.5-30.0GHz), with orthogonal circular polarizations in each band. Polarization switching is possible by changing the feeding port. A modular approach is followed to reduce the design and fabrication complexity. The feed comprises: i) a compact (10.9 x 10.9 x 18.0 mm) dual-port dual-band linearly polarized waveguide antenna; ii) a dual band polarizer that provides circular polarization with orthogonal sense between Rx/Tx bands and polarization switching; iii) a dielectric lens that increases the antenna directivity for a proper TA illumination and at the same time shifts the feed phase centre. Instead of being close to the aperture as usual, the phase centre at both bands is located at the lower face of the feed structure. This virtual focus approach allows to reduce the overall TA antenna height. Finally, the complete antenna system (feed and TA) is successfully validated through measurements. The antenna allows mechanical scanning up 50º degrees in the elevation plane with 23 dBi and 25 dBi in the Rx and Tx respectively, occupying a 195 x 145 x 100 mm3 volume