Skew planar wheel antenna - is it circularly polarised or not?
Figure 10 of this excellent explanation boils down to this: 

If you ignore all the currents that cancel each other out due to symmetry, you can  physically guide (via the copper) three currents Ez of similar magnitude and phase in the z-direction and one equivalent current in the y-direction, the resulting two equal and orthogonal currents being spaced a quarter wavelength apart.  

So, with currents of equal magnitude and phase, orthogonal and spaced a quarter wavelength apart - voila, circular polarization!