Improved axial ratio quality for radio telescope applications with a phased array feed

Circularly polarized radiation is frequently used in satellite communication. This paper discusses how circular polarization can be generated by means of an array of linearly-polarized antenna elements by applying the sequential rotation technique. A calibration method is used to improve the axial ratio from ±30° to ±60° off bore sight by using a 4-bit accuracy for the active components. A measurement campaign was organized based on a demonstrator developed at ASTRON. The calibration of the axial ratio is applied to the measured element patterns of the ASTRON egg-crate structure antenna array operating at 7.14 GHz. The results prove that the axial ratio can be improved significantly. Finally the same technique will be applied to a linear array operating at 20 GHz with sequential rotation configuration in front of the reflector to improve the axial ratio in the boresight direction.