A microstrip-fed quasi-spiral circularly polarized ultra-wideband antenna

A circularly polarized (CP) quasi-spiral antenna for ultra-wideband (UWB) applications is proposed in this paper. By carefully choosing the offset feeding point and adopting an impedance transformer, the developed quasi-spiral antenna can be directly fed by a microstrip tapered line. The measured 10-dB return loss bandwidth and 3-dB axial-ratio (AR) bandwidth are 72.4% (5.15 to 11 GHz) and 44.3% (6.37 to 10 GHz), respectively. The measured antenna gain is about 3 dBi. Moreover, it can realize almost omni-directional radiation covering the upper UWB frequency range (7 - 10 GHz).