Wideband circularly polarized designs of circular microstrip antennas backed by a U-slot cut modified ground plane for GSM band applications

Proximity fed design of circular microstrip antenna backed by U-slot cut ground plane is proposed for wideband circular polarized response. The wideband response is attributed to the mutual coupling between degenerated TM11 and modified TM21 resonant modes of the circular patch. With a 900 MHz frequency spectrum and a substrate thickness of 0.088λcAR, antenna offers an axial ratio bandwidth of 115 MHz (12.57%), with a reflection coefficient bandwidth of 132 MHz (14.56%). Against the U-slot in the patch, the slot in the ground plane achieves more than two times the % axial ratio bandwidth. A reduction in the substrate thickness is achieved either by using the bow-tie shape of the ground plane or by employing a shorted microstrip line feed. The U-slot cut bow-tie shape of the ground plane achieves an optimum response at a substrate thickness of 0.069λcAR, and yields an axial ratio bandwidth of 94 MHz (10.21%), whereas the shorted microstrip line-fed design achieves an optimum response at a substrate thickness of 0.042λcAR and yields an axial ratio bandwidth of 28 MHz (3.2%). Both the designs achieve reflection coefficient bandwidths of greater than 13% with a broadside radiation pattern and a gain of above 6 dBic. An experimental validation of the obtained results has been carried out that shows close agreement against the simulations. With the obtained results, the proposed antennas can find applications in various GSM frequency band applications.