Feed region modes in dipole phased arrays

Feed region modes are derived for a class of dipole phased arrays. The dipole and its balun are a linearly polarized version of the perimeter array radar (PAR) antenna element, and are modeled in strip-line geometry. Knowledge of the feed region modes is essential in determining the influence of supports on the element scan performance, and should shed light on the formation of blind spots in dipole arrays. It is shown that for practical spacings, the balanced strip-line feed structure supports a propagating transverse magnetic (TM) mode in addition to two transverse electromagnetic (TEM) modes. The propagation constant of this mode is scan dependent, and under inappropriate conditions its cutoff occurs before the onset of the grating lobe. Pending further analysis, it is conjectured that this mode cutoff may cause blind spots which limit the array scan coverage.