This paper presents a circularly-polarized dual-band antenna for Ka-band satellite communications which operates simultaneously in both civil and military downlink (17.7-21.2 GHz) and uplink (27.5-31 GHz) bands. The antenna is composed of a metallic ridged cavity that is perforated with a crossed-slot. This cavity is excited in sequential rotation through four feeding coaxial cables, thus producing circular polarization (CP). The required feed distribution is obtained by a dedicated beam-forming network (BFN) able to provide phases to generate right/left-handed CP in the downlink/uplink-bands, respectively. The prototype of the antenna cavity is 3D printed basing on stereolithography (SLA), while the BFN is realized in printed circuit technology. Good agreement is obtained between theoretical and measured results, thereby validating the antenna concept and confirming the strong potential of SLA to conceive compact RF components.