The authors present a novel technique for obtaining circularly polaris
ed (CP) dual frequency operation by using two spur-line band-stop filt
ers within the perimeter of a microchip patch antenna with optimum fee
d point location and aspect ratio. This circularly polarised dual freq
uency operation can be obtained without increasing either the size or
the thickness of the patch. Furthermore, through several experiments t
he authors also demonstrate that the location of the feed point establ
ishes and controls a trade-off between axial ratio and impedance bandw
idth.