An ultra-wideband unidirectional circularly polarized planar self-complementary and compact Archimedean square spiral antenna is proposed in this paper. Compared to the conventional center-fed or verticallyconnected coaxial line spiral antennas, the proposed antenna has a completely planar feed structure. The traditional impedance transformer (balun) in the third dimension is moved to the planar plane based on a wideband microstrip line to coplanar stripline transition to realize a completely planar spiral antenna. This impedance transformer is integrated with the spiral antenna to achieve a planar structure and to adapt the impedance into a conventional 50 Ω coaxial cable. The performance of the proposed antenna is simulated and measured. The proposed antenna has a compact size of 90 × 90 × 1.6 mm 3 on an FR4 substrate and can yield a large −10 dB impedance bandwidth ranging from 2.14 to 9.8 GHz which is equivalent to a fractional bandwidth of 102.13% with unidirectional radiation patterns. The VSWR is less than two in the operating bandwidth and the axial ratio (AR) is less than 3 dB in the frequency range of 1-8 GHz. The simulated radiation efficiency ranges between 20% and 80%. The characteristics of the proposed antenna make it suitable for ground-penetrating radar applications and many others.