On the design of circularly polarized U-slot cut square microstrip antenna

Circularly polarized microstrip antenna realized by cutting unequal arm length U-slot is discussed. In 2.3 GHz frequency band, it gives VSWR and axial ratio bandwidth of 9 and 4%, respectively. In the reported work, a detailed explanation about the working of circular polarized antenna in terms patch resonant modes and the equations/design procedure to realize similar antennas at different frequency is not explained. In this paper, an in-depth analysis which explains the effects of unequal lengths U-slot to give circularly polarized response is presented. First the equal lengths of U-slot reduce the resonance frequencies of patch TM01 and TM20 resonant modes. An unequal length in U-slot arms (asymmetrical U-slot) leads to excitation of new resonant mode whose current distribution is similar to TM01 mode of U-slot cut patch. This gives circularly polarized response. Further equations for various patch parameters for U-slot cut antenna are proposed. Using them U-slot cut patch was designed at 1500 MHz. At this frequency antenna yields VSWR and axial ratio bandwidths of 138 (9.6%) and 60 (4.3%) MHz, respectively. Thus proposed equations are useful for designing U-slot cut antenna at any given frequency on thicker air substrate.