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Over the past years several attempts have been made to widen the bandwidth of single-fed circularly polarised patch antennas. This paper presents a novel structure to accomplish circularly polarised dual frequency operation by using two spur-line band-stop filters. This circularly polarised dual frequency operation can be obtained without increasing either the size or the thickness of the patch. Furthermore, through several experiments this paper also demonstrates that the location of the feed point establishes and controls a trade-off between axial ratio and impedance bandwidth.