This paper presents the design of an ultra-wideband planar two-arm compound spiral antenna composed of an equiangular spiral and an Archimedean spiral, which respectively improve the radiator performance at low and high frequencies. A metallic cavity filled with absorber material, used to make unidirectional the radiation pattern of the antenna, also allows the use of a resistive termination to improve antenna’s performance at low frequencies. By using a Hecken approach, a microstrip to parallel-strip transition (balun) is designed to feed the spiral arms. In the range from L- to X-band, measured radiation patterns, axial ratio, and reflection coefficient attain good results. Finally, the application of the antenna in a direction finding system is described.