A Compact and Low-Profile Loop Antenna With Six Resonant Modes for LTE Smartphone

In this paper, a novel six-mode loop antenna covering 660-1100, 1710-3020, 3370-3900, and 5150-5850 MHz has been proposed for the application of long-term evolution (LTE) including the coming LTE in unlicensed spectrum and LTE-Licensed Assisted Access. Loop antennas offer better user experience than conventional planar inverted-F antennas (PIFAs), inverted-F antennas (IFAs), and monopole antennas because of their unique balanced modes (1, 2, ...). However, the bandwidth of loop antennas is usually narrower than that of PIFA/IFA and monopole antennas due to these balanced modes. To overcome this problem, a novel monopole/dipole parasitic element, which operates at an unbalanced monopole-like 0.25 mode and a balanced dipole-like 0.5 mode, is first proposed for loop antennas to cover more frequency bands. Benefiting from the balanced mode, the proposed parasitic element is promising to provide better user experience than conventional parasitic elements. To the best of the authors' knowledge, the balanced mode for a parasitic element is reported for the first time. The proposed antenna is able to provide excellent user experience while solving the problem of limited bandwidth in loop antennas. To validate the concept, one prototype antenna with a size of 75 10 5 mm 3 is designed, fabricated, and measured. Both simulations and experimental results are presented and discussed. Good performance is achieved.