Development of wideband circularly polarised square- and rectangular-loop antennas

New types of wideband circularly polarised (CP) square- and rectangular-loop antennas are developed and discussed. It is shown that the bandwidth for circular polarisation of a loop antenna can be significantly increased by adding a parasitic loop inside the original loop. The addition of the parasitic element can create one more minimum axial ratio (AR) point, which can appropriately combine with the original one, leading to a significant enhancement for the AR bandwidth. It is found that the AR (≤2 dB) bandwidth of a square-loop antenna can be increased from 6.5% to 20% by adding a parasitic square loop. By replacing the square loop with a rectangular loop, the AR bandwidth can be further enhanced. For a rectangular loop with an aspect ratio of 1/2, a bandwidth of nearly 50% for AR≤2 dB is obtained with a gain of 8.5-7.5 dBi. A broadband balun is introduced for impedance matching.