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An efficient mutual coupling reduction method is introduced for an extremely closely packed tunable dual-element planar inverted-F antenna (PIFA) array. High isolation can be achieved through a λ0/2 folded slot antenna formed by a slot on the ground plane and the neighboring edges of the two PIFAs. Direct coupling is blocked by the slot antenna through radiating the coupling power into free space. A measured isolation of more than 36.5 dB can be achieved between the two parallel individual-element PIFAs operating at 2.4 GHz WLAN band with an inter-PIFA spacing of less than 0.063 λ0 (center to center) or 0.0147 λ0 (edge to edge). Since there is only a narrow slot antenna formed between the PIFAs in the present method, the distance can be further reduced to less than 0.0016λ0 (edge to edge) with the maximal isolation of better than 40 dB. Both measured and simulation results show the effectiveness of the present mutual coupling reduction method.