The development of the next-generation wireless communication systems requires broadband and multi-band devices for faster data transfers. Meanwhile, there is a trend towards the miniaturization of handheld devices. These conflicting requirements must be met using low-cost solutions, that simultaneously maintain a high efficiency. Transmission-line metamaterials (TL-MTM) provide a conceptual route for implementing small resonant antennas. Typically TL-MTM antennas suffer from narrow bandwidths. Recently, [1] addressed the bandwidth problem by proposing a two-arm TL-MTM antenna resonating at closely spaced frequencies. Furthermore, a compact tri-band monopole antenna with single-cell metamaterial loading was shown in [2] for WiFi and WiMAX applications and a dual-band metamaterial antenna was proposed in [3] for WiFi applications. Another TL-MTM type of a dual-band electrically small antenna (ESA) fabricated on an FR4 board with thickness of 1.6 mm, has been recently reported in [4]. This antenna is based on the planar CPW monopole topology but with a single-cell metamaterial loading, as shown in Fig. 1 (a). The antenna comprises a two-arm fork-like monopole with a thin-strip inductor loaded on top of the monopole and an interdigital capacitor loaded on the right-side arm. The MTM loading creates a second resonance covering the lower WiFi band of 2.40 GHz - 2.48 GHz, in addition to the monopole resonance over the 5.15 GHz - 5.80 GHz upper WiFi band. At the lower WiFi band, the antenna no longer acts as a regular monopole along the vertical direction but rather as a slot along the horizontal direction. The MTM loading forces the current to wrap around the slot perimeter and induce an E-field distribution along the horizontal direction within the slot, both contributing to the slot-mode radiation. The monopole element has dimensions of 8.5 mm × 5.7 mm (or λ_0/14.4 × λ_0/21.4 at 2.45 GHz). A dual-band performance can be clearly seen from the HFSS simulation shown in Fig. 1 (b). The simulated radiation efficiencies are above 60% and 90% at the lower and higher WiFi bands, respectively. This design is single-layered, via-free and therefore can be easily fabricated at a low cost. In this paper, we describe a simple technique to reduce the mutual coupling between two closely-spaced antennas having the previously described topology shown in Fig. 1. This is important for multiple input multiple output (MIMO) applications in handheld units.
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