Can you please explain the ratio's importance in a circularly polarized RFID reader antenna?
I am not an engineer, but I will offer an answer below. Any of our readers are invited to provide further clarification.
A circular-polarized antenna is used to read tags in random orientation. The axial ratio is the ratio between the minor and major axes of the polarization ellipse. If the ratio is one to one—that is, if the minor and major axes are the same—then the energy is emitted in a circular pattern. As the minor axis grows shorter and the major one becomes longer, the ellipse elongates. You can achieve a longer read range as the ratio changes, but it become more difficult to interrogate the tags in random orientation. As the ratio declines toward zero, the energy is emitted in an increasingly longer ellipse until you eventually end up with a linear-polarized antenna.
—Mark Roberti, Founder and Editor, RFID Journal