The axial ratio is the ratio of orthogonal components of an E-field. A circularly polarized field
is made up of two orthogonal E-field components of equal amplitude (and 90 degrees out of phase).
Because the components are equal magnitude, the axial ratio is 1 (or 0 dB).
The axial ratio for an ellipse is larger than 1 (>0 dB). The axial ratio for pure linear polarization is infinite, because the orthogonal components of the field is zero.
Axial ratios are often quoted for antennas in which the desired polarization is circular. The ideal value of the axial ratio for circularly polarized fields is 0 dB. In addition, the axial ratio tends to degrade away from the mainbeam of an antenna, so the axial ratio may be indicated in a spec sheet (data sheet) for an antenna as follows: "Axial Ratio: <3 dB for +-30 degrees from mainbeam". This indicates that the deviation from circular polarization is less than 3 dB over the specified angular range.