Abstract: A compacted antenna is proposed with combining single-arm helix and cone horn, which meets the requirements in a small space. The radiation part of the antenna is a single-arm spiral (SAS) structure,which is directly fed by 50 W coaxial line through a small disc/ ground-plane, and this feeding method replaces the traditional balun which often used in conventional two-arm balanced-mode spiral antenna. Due to the spiral coil is fewer, the absorption resistance is loaded in the arm end to improve the performance of circular polarization. The conical horn structure is put ahead to improve gain of the antenna in the range of 45° beam, and the phenomenon of beam direction deviation is also reduced. The results of simulation and measurement show that the designed antenna has a good circularly polarized radiation performance in X band with the size of 40 mm′40 mm′24mm.