A multiband four-antenna system with high isolation for the mobile phone applications is presented in this paper. The four antennas consisting of a main antenna and three auxiliary antennas are located on a 135×65×0.8 mm3 FR4 epoxy board. The main antenna is an improved monopole antenna which can cover LTE700/2300/2500, GSM850/900/1800/1900/UMTS, and 2.4-GHz WLAN bands. Three identical auxiliary antennas with a size of 40×6×0.8 mm3 are located vertically in the sides and bottom of the circuit board, respectively. By adopting three slots in each radiating patch of the auxiliary antennas, they can successfully cover 704-746 MHz and 1880-2690 MHz band with S11 less than -6 dB. The isolation among the four antennas is better than 13 dB. The measured efficiency of the main antenna in the operation band can reach 40%, and the efficiency of auxiliary antennas can reach 25% at upper frequency band and about 10%-20% at 704-746 MHz band.
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