What is Axial Ratio of an Antenna?

The Axial Ratio (AR) of an antenna is defined as the ratio between the major and minor axis of a circularly polarized antenna pattern. If an antenna has perfect circular polarization then this ratio would be 1 (0 dB). However, if the antenna has an elliptical polarization then this ratio would be greater than 1 (>0 dB). 

This ratio tells us the deviation of an antenna from the ideal case of circular polarization over a specified angular range. For example: ‘Axial Ratio: <1.25 dB for +-20° from main beam’- this indicated that the antenna deviation from the main beam is less than 1.25 dB in the that given 20°.

Typically, axial ratios are quoted for circularly polarised antennas. Since a circularly polarised field is made up of two orthogonal electric field components of equal amplitude and 90 degrees out of phase, the closer the axial ratio is to 0 dB, the better. On the other hand, if two plane waves are of different amplitude and exhibit a 90° phase difference, that wave is said to be in an elliptical polarization. Therefore, it is always larger than 1 (>0 dB) in an ellipse.

The axial ratio for pure linear polarization is infinite because the orthogonal components of the field are zero.