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

摘要

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.