A dual-band inverted-L-folded-antenna with a parasitic wire

Low-profile antennas, such as inverted-L and F antennas, are widely used in mobile communications. With the introduction of two parasitic wires, a horizontal dipole antenna and an inverted-F antenna over a large ground plane show wideband impedance characteristics (King, H.E. and Wong, J.L., 1972; Nakano, H. et al., 1998). A dipole antenna with a parasitic wire has two resonant frequencies (Ebine, Y. and Kagoshima, K., 1988). However, an inverted-L antenna with a parasitic wire over a large ground plane shows neither wideband nor dual-band characteristics, because the input resistance is low at the fundamental resonant frequency. An inverted-L-folded antenna with only one parasitic wire over an infinite ground plane is introduced to realize dual-band characteristics. A folded-monopole antenna has higher input resistance than that of a monopole antenna at the fundamental resonant frequency. Therefore an inverted-L-folded antenna over an infinite ground plane may be matched to a 50 Ω feed cable at the fundamental resonance. To get dual-band operation, one parasitic inverted-L wire is placed at the side of the inverted-L-folded antenna. It is found that the fundamental and the second resonant frequencies strongly depend on the lengths of the inverted-L-folded antenna and the parasitic wire, respectively.