Abstract: For the demands of miniaturization and lightweight design of feed sources in modern satellite communication systems, 5G, and RIS applications, a compact, dual-band circularly polarized horn feed operating in the K/Ka bands is proposed. This feed alters the current distribution by perforating specific locations on the septum, expanding the impedance and axial ratio bandwidth, with a relative bandwidth reaching 57.14%. Through proper filter selection and co-optimized design, as well as choosing appropriate septum perforation parameters, wideband circularly polarized radiation is achieved while maintaining high feed port isolation and reducing physical length. Measurement results show that the horn feed has a reflection coefficient S₁₁ below −10 dB in the K (17.5–21.5 GHz) and Ka (27.5–31.5 GHz) bands, in-band axial ratios below 3 dB, gain ranging from 7.88 to 12.70 dBi, and port isolation exceeding 42 dB and 29 dB in the K and Ka bands, respectively. Measured polarization isolation at 20 GHz and 30 GHz is above 20 dB. The overall feed size is compressed to only 61 mm × 21 mm × 21 mm. Compared to existing results, without including the radiating horn, the length is reduced to 51 mm, achieving a 43% size reduction; with the radiating horn included, the reduction reaches 65%. Measurement and simulation results show good agreement.