antenna-theory.com

[Riaan]

Practical spiral antennas have very good (e.g. 3 or 4 dB) wideband axial ratio (AR) out to 90 degrees off boresight. Numerical analysis however almost invariably predicts an infinite AR at 90 degrees. This is because the spiral is typically modelled as an electric current on the spiral tracks, which will have only an E(phi) component when viewed from the side (spiral located in the xy-plane). An electric current in the xy-plane cannot radiate an E(Theta) component.

Can anyone explain why this is the case? Surely the basic spiral (no cavity or feed mechanism) must radiate good circular polarization (i.e. AR) by itself. Is the numeric modelling possibly in error?

Can anyone refer me to a good discussion on the basic operation of the spiral antenna?