An Innovative Parallel Feeding Approach to Eliminate Radiation Efficiency Local Minimum in Inverted-F Antennas With the Parasitic Branch for Metal-Bezel Smartphones

The inverted-F antenna with a parasitic branch (IFA&PB) is widely used in metal-bezel smartphones. Typically, the primary IFA branch is directly fed, while the parasitic branch is excited via electromagnetic coupling. However, the conventional single-feed approach often leads to a radiation efficiency local minimum, degrading performance. This paper proposes a novel parallel feed with two different feeding phases for the IFA&PB structure to eliminate this issue. A high-efficiency mid-high band (MHB, 1.71–2.69 GHz) antenna with this approach is demonstrated, effectively removing the radiation efficiency local minimum. The design is further extended by integrating a low band (LB, 0.704–0.96 GHz) antenna, resulting in an octa-band mobile antenna. A prototype achieved measured -6 dB impedance bandwidths of 270 MHz (0.69–0.96 GHz) and 1750 MHz (1.35–3.10 GHz), covering LB and MHB bands. The measured average efficiencies were -7.7 dB (17%) and -4.3 dB (37%), respectively. The proposed parallel feed method effectively eliminates the radiation efficiency local minimum, offering a promising solution for modern metal-bezel smartphones.