Polarization properties of scanning arrays

Some conceptual notions of "cross" polarization were formed from the point of view of mechanically-scanned aperture antennas, and attempts to apply them to electronically scanned arrays (ESAs) are not appropriate. This stems from the fact that it is not generally possible to define a direction of co-polarization in terms of an ESA's geometry alone, as it is with fixed-beam antennas. This paper presents several examples of array types, showing how their polarization states vary with scan direction. The following are the important conclusions: (1) An ESA's main beam polarization is the same as that of the embedded element pattern in the scan direction. It will have different polarization characteristics than an isolated element because the radiator structure can support current modes that are not excited equally by its own feed and coupling from nearby elements. (2) An ESA's polarization state almost always changes with scan. The amount of change depends on radiator type, array lattice, and scan plane. (3) The definition of cross polarization for an ESA must be made in terms of its function. The fact that its polarization state changes with scan does not automatically imply an increase in polarization loss.