Fully planar frequency independent square Archimedean spiral antenna with impedance transformer for ground penetrating radars

Abstract

AbstractAn 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 vertically‐connected 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 mm3 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.