Circulary-polarized (CP) planar antennas have been received much attention for mobile wireless communication applications since they are not only able to reduce the multipath effects but also provide flexibility for transceivers [1]. CP microstrip antennas are widely used but they are required to have broadband, small size, light weight, and low profile characteristics [2]. However, conventional CP microstrip antennas had some drawbacks such as narrow impedance and CP bandwidths [3][4]. Also it is difficult to implement a broadband CP microstrip antenna having small dimensions. To enhance the impedance bandwidth, CP antennas using slot structure instead of the microstrip structure were proposed [5]. Still in the open articles, the slot antennas which have wide CP bandwidth of more than 20% are quite few and their dimensions are larger than that of a microstip antenna normally.
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