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Tactical ground-based electronic warfare systems have conflicting requirements for wide frequency coverage, good sensitivity and direction finding accuracy, and small size and weight. An experimental multi-band direction finding (DF) antenna was developed to investigate the feasibility of constructing a compact lightweight antenna array by superposing three sub-arrays in the same plane. Experimental measurements performed for the frequency range covered by the three sub-arrays revealed that ambiguities in the estimates can occur, particularly for the highest frequency band. Since these ambiguities are not observed with single arrays, further investigations were carried out. Full-wave analysis revealed mutual coupling between the sub-arrays as the primary cause of the ambiguities. A theoretical analysis of closely spaced coplanar sets of elements was performed to obtain a better understanding of the factors affecting estimation accuracy. This investigation demonstrated that the sensitivity of the DF technique to mutual coupling effects is an important consideration when multiple DF antenna arrays employing coplanar sets of closely spaced elements are used. Furthermore, DF techniques that can be used with mutual coupling compensation have potential performance advantages.