Quarter-wavelength printed loop antenna with an internal printed matching circuit for WWAN operation in the mobile phone

It is a trend that the mobile phone employs several system modules for a variety of applications. The internal antennas for different system modules are usually required to be mounted in close proximity inside the limited space of the mobile phone. As a result, the internal antennas with a decreased size are hence attractive and usually demanding for practical applications. Recently, the loop antennas are found attractive to multiband mobile phone applications [1]–[3]. It is shown that the loop antennas can be formed by a narrow metal strip folded into a compact configuration and still show a wideband or multiband operation for WWAN operation. On the other hand, to further reduce the internal antenna size, several designs using an external matching circuit to widen the operating bandwidth have also been reported [4]–[6]; however, the studied matching circuit usually works for single band only. In this paper, we demonstrate a small-size printed loop antenna with an internal printed matching circuit operating at its 0.25-wavelength resonant mode as the fundamental mode for mobile phone application. The proposed printed loop antenna has a folded loop metal strip of length 85 mm (about 0.25λ at 900 MHz) printed on the top no-ground portion of the system circuit board. The printed matching circuit is encircled by the loop strip and comprises a coupling strip and an inductive strip. The coupling strip effectively compensates for the large inductive reactance of the input impedance of the antenna's 0.25λ mode, while the inductive strip has constructive effects on improving the impedance matching of the antenna's dual-resonant 0.5λ mode. Note that the internal printed matching circuit applied here allows the excitation of a much lower first resonant mode (0.25λ mode) for the proposed loop antenna in the mobile phone [6].