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An approach to the design of multiple-input multiple-output (MIMO) arrays exploiting planar directive antennas is presented. It is well known that pattern orthogonality is a key aspect to reach low correlation, and thus to improve channel capacity in rich multipath environments. However, attention is often focused on reducing mutual coupling rather than optimizing the active element patterns. In this communication a planar MIMO array of printed Yagi-Uda antennas with integrated balun is presented. The end-fire radiation mechanism of the Yagi-Uda is exploited to obtain a triangular array of three sectoral antennas. This allows to achieve nearly orthogonal patterns, while keeping a low mutual coupling among radiating elements. A properly shaped ground at the feeding points allows to increase the isolation between the antennas, even in such a compact layout. A laboratory model has been characterized experimentally, and the effectiveness of the proposed design in terms of theoretical achievable capacity is demonstrated through numerical simulations considering IEEE 802.11n multipath fading channel models.