Circularly polarized broadband dual loop antenna fed by coplane stripline

A dual rhombic loop antenna with double parasitic loops to increase the bandwidth of circular polarization operation is proposed. The gap position of each loop controls the sense of circular polarization. The original dual rhombic loop radiates a CP wave at a lower frequency while the parasitic loop produces CP radiation at a higher frequency. A combination of the two frequencies leads to a bandwidth enhancement. A relative bandwidth of 23.7% (AR ¿ 2 dB) is simulated. For measuring, a balun from the coplanar stripline (CPS) to a microstrip line is designed. The balun has a little effect on the antenna. The measured AR, return loss and radiation patterns agree to the simulated results. The antenna achieves S11 (¿ 15 dB) bandwidth of 13.1%, AR (¿ 3 dB) bandwidth of 16.2% and gain of 11.5 dB.