Antennas
A number of different types of antenna elements can be utilized to produce circularly polarized (or nearly so) radiation; following Balanis, one can use dipole elements:
"two crossed dipoles provide the two orthogonal field components... If the two dipoles are identical, the field intensity of each along zenith ... would be of the same intensity. Also, if the two dipoles were fed with a 90° degree time-phase difference (phase quadrature), the polarization along zenith would be circular... One way to obtain the 90° time-phase difference between the two orthogonal field components, radiated respectively by the two dipoles, is by feeding one of the two dipoles with a transmission line which is 1/4 wavelength longer or shorter than that of the other", p.80;
or helical elements:
"To achieve circular polarization ... the circumference C of the helix must be ... with C/wavelength = 1 near optimum, and the spacing about S = wavelength/4." p.571;
or patch elements:
"circular and elliptical polarizations can be obtained using various feed arrangements or slight modifications made to the elements... Circular polarization can be obtained if two orthogonal modes are excited with a 90° time-phase difference between them. This can be accomplished by adjusting the physical dimensions of the patch ... For a square patch element, the easiest way to excite ideally circular polarization is to feed the element at two adjacent edges ... The quadrature phase difference is obtained by feeding the element with a 90° power divider", p.859.
Read more about this topic: Circular Polarization