A Cavity-Backed Circularly Polarized Spiral Antenna with Ultra-Wide Bandwidth and Compact Size

An ultrawideband circularly polarized planar Archimedean spiral antenna is proposed. The proposed antenna consists of a two-armed Archimedean spiral radiator, an exponential gradient balun and a truncated cone form of the back cavity. By embedding two pairs of lumped resistors, a radiator is obtained with improved impedance matching in lower frequencies. Moreover, unidirectional radiation and enhanced axial ratio bandwidth are realized by loading the back cavity. Simulation results indicate that the designed antenna achieves an ultrawide overlapped operational bandwidth ranging from $1.9-18.2 \ \mathrm{GHz}$, during which the antenna gain reach 3.0-6.8 dBi. Meanwhile, a compact size of $55 \times 55 \times 40 \mathrm{~ m m}^{3}$ is also produced.