Circularly polarized loop antennas with a parasitic element for bandwidth enhancement

In this paper, it is demonstrated that the bandwidth of circular polarization (CP) for a loop antenna can be significantly increased when adding one more parasitic loop inside the original loop. The axial ratio (ARles2 dB) bandwidth of the circular-loop antenna with a parasitic circular loop is found to be 20%, more than three times the AR bandwidth (about 6.5%) of a single loop. A bandwidth of 30% (ARles2 dB) with a gain of more than 10 dBi is obtained for a series-fed dual rhombic-loop antenna while a bandwidth of 50% for AR les 2 dB can be realized for the parallel-fed dual rhombic loop. This type of wideband CP antennas has two important features: i) easy to change the sense of CP; ii) able to implement an antenna array in a coplanar stripline circuit