A compact microstrip antenna for CP

Circularly polarized microstrip antennas, including single-fed patches, are widely used as effective radiators in many communication systems. However, one of the most serious drawbacks of these antennas is that their axial ratio and impedance bandwidth are narrower compared to those of the conventional microwave antennas. A commonly-employed technique to achieve broadband axial ratio bandwidth is to employ four linearly polarized elements, with a 90/spl deg/ phase differential introduced with respect to the adjacent ones. Although this design exhibits wideband characteristics, it has two important drawbacks. First, the requirement of four rectangular microstrip patches makes the antenna structure large; second, feeding of all the four elements with proper phase requires a complex feed structure. In this paper, we present a new design to overcome these difficulties. In our approach, we use a short-circuited rectangular patch, instead of a conventional rectangular one, as the radiating element to reduce the size of the antenna. Instead of feeding all the four elements, we excite only the two adjacent patches directly and the remaining two in a parasitic manner. This technique reduces the complexity of the feed structure, and enables the manufacturing of the antenna in a cost-effective manner.