Recent studies showed that the loop antennas are promising candidates for multiband operation in the mobile phones [1]-[3]. These promising loop antennas can be implemented by directly printing on the system circuit board of the mobile phone [1] or fabricated by cutting from a metal plate/strip or printing on an FR4 substrate and then mounted above the system circuit board [2], [3]. In this paper, we present a multiband loop antenna with a very small occupied volume of 1 cm{sup}3 (40 × 5 × 5 mm{sup}3) only, yet capable of GSM (890～960 MHz), DCS (1710～1880 MHz), PCS (1850～1990 MHz) and UMTS (1920～2170 MHz) operation. The occupied volume of 1 cm{sup}3 is among the smallest in the conventional internal multiband mobile phone antennas [4]. The proposed loop antenna comprises a folded uniform loop strip attached on a foam base and a tuning pad connected to the loop strip. Good excitation of the antenna's first three resonant modes (0.5-, 1.0- and 1.5-wavelength modes) to cover GSM/DCS/PCS/UMTS operation can be obtained. Moreover, the proposed loop antenna is especially suited for application in the small-size mobile phone with a small groundplane length of 60 mm only. This behavior is owing to that the excited surface currents of the 0.5-, 1.0 and 1.5-wavelength loop modes on the system ground plane are smaller than that of the conventional internal mobile phone antennas such as the PIFAs. In this case, less dependence of the performances of the proposed antenna on the length of the system ground plane can be expected.
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