Re: phased array
In general, once given the hypotesis of neglecting mutual coupling and border effects (large arrays), one can consider whatever kind of antennas to realize a PA.
Probably, the reason why the spiral antenna is not considered is the same of the short dipole antenna: infact spiral antenna can be compared to a short magnetic dipole, therefore it has a very poor radiating behavior.
Tipically, B.Cs textbook on antenna arrays consider only the half wavelength dipole antenna case..and since no equivalent in terms of magnetic dipole exists (there is no half wavelength magnetic dipole antenna) the spiral antenna case is disregarded.
This is my opinion.
Bye.