Direction finding and polarization estimation, using electrically long dipoles or electrically large loops

Abstract

In antenna array signal-processing algorithm development, research has focused on electrically "short" dipoles with physical lengths (L) under 1/10 of a wavelength λ. Such "short" dipoles have very small input impedances, rendering them to be poor radiators. Practical dipoles, with an electrical length of L/λ ε[0.1, 1], have notably larger input impedances, hence making them better radiators. This thesis will first present the measurement model (i.e. array manifold) of such practical dipoles, as a triad that is collocated in space and orthogonal in orientation. Using such a triad to estimate incident sources' bivariate azimuth-elevation directions-of-arrival and bivariate polarizations, closed-form algorithms will be pioneered. The triad's collocation gives a point-like spatial aperture, limiting the dipole-array's spatial resolution. To realize a large spatial aperture, electrically long dipoles can be positioned sparsely on a circular circumference, with each dipole oriented radially (or tangentially), to allow a rotational invariance with respect to the circle's origin. For such a circular array of sparsely spaced and differently oriented dipoles, this thesis will also develop the measurement model and will pioneer closed-form algorithms to estimate incident sources' bivariate azimuth-elevation directions-of-arrival and bivariate polarizations. For two electrically long dipoles, this thesis also pioneers signal-processing algorithms in closed forms, to estimate the polarizations of impinging sources. This is unlike the vast literature on crossed-dipoles polarimetry, restricted to electrically short dipoles. In this thesis, the two long dipoles are perpendicularly oriented, but may be collocated or may be separated by a known displacement. Using such a pair of electrically long dipoles for polarization estimation, this thesis proposes new closed-form formulas, and derives the associated Cramer-Rao bounds. Besides electrically long dipoles, electrically "large" loops with circumference (2πR) over 1/10 of a wavelength λ have likewise been neglected in the literature on antenna signal processing. This thesis will formulate the array manifold for a triad of electrically "large" loops, collocated and orthogonal. Then for such a triad of large loops, this thesis will pioneer closed-form signal processing algorithms to estimate the incident signals azimuth-elevation directions-of-arrival and polarizations.