Review of broadband circularly polarized microstrip antennas

In this paper, conventional methods to increase the bandwidth of linearly polarized microstrip antennas (MSAs) are extended for broadband circular polarization (CP). Dual feed gap-coupled MSAs, direct-coupled MSAs, electromagnetic coupled MSAs, aperture coupled MSAs with cross-slots of equal or unequal arms, inclined or V-loaded inclined slots are presented for wideband CP with moderate gain. These configurations yield an axial ratio (AR) bandwidth up to 15% and VSWR bandwidth of around 30%. The AR bandwidth is increased significantly by feeding the array in sequential rotation. In this case each element or a group of elements are sequential rotated and fed with differential phase, which is same as the rotational angle with respect to each other. The elements of the array could be either circularly polarized or linearly polarized. The gain of these configurations is higher than a single element since many elements are used in an array configuration.