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A compact tri-band planar monopole antenna is proposed that employs reactive loading and a ??defected?? ground-plane structure. The reactive loading of the monopole is inspired by transmission-line based metamaterials (TL-MTM), which enables the loaded antenna to operate in two modes. The first resonance exhibits a dipolar mode over the lower WiFi band of 2.40 GHz - 2.48 GHz, and the second resonance has a monopolar mode over the 5.15-5.80 GHz upper WiFi band. Full-wave analysis shows that the currents of the two modes are orthogonal to each other, resulting in orthogonal radiation patterns in the far field. The feature of a ??defected?? ground-plane, formed by appropriately cutting an L-shaped slot out of one of the CPW ground-planes, leads to the third resonance that covers the WiMAX band at 3.30-3.80 GHz. Air bridges at the intersection between the antenna and the CPW feedline ensure a balanced current. A fabricated prototype has compact dimensions of 20.0 mm ?? 23.5 mm ?? 1.59 mm, and exhibits good agreement between the measured and simulated S parameters and radiation patterns. The measured radiation efficiencies are 67.4% at 2.45 GHz, 86.3% at 3.50 GHz and 85.3% at 5.50 GHz.