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

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 0.25 lambda 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 0.25 lambda mode, while the inductive strip has constructive effects on improving the impedance matching of the antenna´s dual-resonant 0.5 lambda mode. Note that the internal printed matching circuit applied here allows the excitation of a much lower first resonant mode (0.25 lambda mode) for the proposed loop antenna in the mobile phone.