Performance-Enhanced Compact Multi-Band Antenna Integration on Terminal Frame Using Filter-Integrated Back Cover Strips

The increasing antenna density and limited clearance in modern smartphones severely restrict the bandwidth and isolation of frame antennas. While the large space on the smartphone back cover has been commonly used for implementing standalone planar antennas, much less effort has been devoted to harnessing its potential to assist the design of frame antennas. This work proposes a filter-integrated back cover strips concept, in which dedicated strips are designed for different antennas and operation bands to introduce additional resonant modes and controllable coupling paths, enabling simultaneous decoupling and bandwidth enhancement, while filters prevent cross-band interference and enable compact integration. A quad-antenna frame system covering the 4G B41 (2.5-2.69 GHz) and 5G N79 (4.4-5.0 GHz) bands is designed and experimentally validated. With the proposed back cover strips and only 0.75-mm frame clearances, the -6-dB impedance bandwidths are expanded by 2.1~2.4× in the B41 band and 2.4~3.6× in the N79 band, while the corresponding sideband efficiencies are improved by 7.2~10.4 dB and 3.2~5.6 dB, effectively activating frequency regions that were previously difficult to utilize. Meanwhile, the isolations are improved to above 11.2 dB across both bands. The proposed concept offers an effective and practical solution for high-performance multiple-input multiple-output (MIMO) frame antennas in smartphones.