Mutual coupling effects on antenna radiation pattern:: An experimental study applied to interferometric radiometers

Antenna pattern mismatches are one of the most important error sources in planned Earth-observation interferometric radiometers. From a low Earth orbit, the wide field of view, about +/-30 degrees, leads to the use of antennas with a large beam. In addition, antennas must be closely spaced to avoid, or at least minimize, aliasing effects in the formation of the synthetic brightness temperature images. The accuracy demanded of these systems requires the precise knowledge of all the antenna radiation voltage patterns (amplitude and phase), which may differ from their theoretical values due to mechanical and electrical tolerances in the manufacturing process and which can change due to the proximity of other structures, i.e., other antennas of the array or the mechanical support. Two approaches are found in the literature to interpret the impact of antenna mutual coupling on the performance of an interferometric radiometer: (1) a modification of the antenna voltage pattern and (2) a mixing of the cross correlations measured between the signals collected by the antennas. The main contribution of the present work is a detailed theoretical analysis of the impact of mutual coupling effects showing the equivalence between both approaches. Theoretical results are corroborated with a set of experimental measurements with two kinds of antennas. Theoretical and experimental results can be used in the design of the antennas of interferometric radiometers in order to predict the impact of mutual coupling on the system's performance and point out the importance of an accurate antenna pattern characterization.