planar spiral antenna

I can not understand how four beams are formed from sum mode and diference mode.In addition, how the four beams are processed to obtain the elevation angle and azimuth angle?who can give me some guidance?thanks!

iearch it in basic book of antenna and net


The effects of antenna shape and the feeding network on the cross-polarization discrimination of four-arm planar antennas are examined using a novel characteristic parameter, the polarization stability factor; this quantitiy is considered for both the four-arm planar Archimedean and logarithmic spiral antennas. It is shown that, if the size of the antenna is sufficiently large, high values of the polarization stability factor, which allow the radiation of very pure circular polarization with no need for extremely optimized feeding networks, can be obtained.

how the four beams are processed to obtain the elevation angle and azimuth angle?Does it need some specific circuits?

Corzine and Mosko have a book on this subject