Dual-fed patches may be used to produce circularly polarized radiation but this requires the use of a feed network to provide equal excitations and a 90 degrees phase shift between the ports [Long et al]. The pin-fed circular polarized circular patch with trimming stubs antenna has the advantage of using a single coaxial feed, orientated at 45 degrees with respect to the stubs to produce circular polarization. Manufacturing tolerances can also be tuned out by trimming the stubs.
Similar to the elliptical patch, the basic resonant structure is perturbed such that two, spatially orthogonal resonant modes are induced by a single feed pin.
The axial ratio of the antenna has a narrow bandwidth, but approaches perfect circular polarization at the center frequency....
S. A. Long, L. C. Shen, D. A. Schaubert and F. G. Farrar, “An experimental study of the circular-polarized elliptical printed-circuit antenna”, IEEE Transactions on Antennas and Propagation, vol. AP-29, no. 1, Jan 1981, pp. 95–99.