Due to the typically low bandwidth of patch antennas, circularly polarized patches (such as the truncated rectangular patch) may not achieve good axial ratio at the same frequency that a good impedance match is achieved. By increasing the thickness of the substrate, the impedance bandwidth of the patch may be increased, thereby improving the impedance match at the point of optimal axial ratio. With a pin-feed, however, the increase in pin-inductance with a thicker substrate limits this impact. One approach to negating the impact of this pin-inductance is by introducing a series capacitance - realized as an annular slot on the patch - centered on the connection point of the feed-pin.
By using this configuration, a practical antenna on a thick substrate with wide impedance bandwidth and nearly-perfect circular polarization at broadside can be achieved....Typical circular gain pattern at the center frequency; typical axial ratio at broadside and typical S11 versus normalized frequency