High-Efficiency Deca-Band Foldable Smartphone Antenna Using Quarter-Wavelength Parasitic Branch

As foldable smartphones typically operate in two primary states—unfolded state and folded state, the structural changes between the two states can signif-icantly impact antenna performance, such as frequency shifts and efficiency reduction, is critical for practical applications. This paper proposes to use a quarter-wavelength parasitic branch to mitigate the effect of the structural changes between the two states. With the quarter-wavelength parasitic branch, a high-efficiency deca-band mobile antenna with similar performance in both the unfolded and folded states is achieved. Prototype testing shows negligible frequency shifts (approximately 0 MHz) in the LB (0.704-0.960 GHz), MHB (1.71-2.69 GHz), and NR (3.3-3.8 GHz and 4.7-5.0 GHz) bands between the unfolded and folded states. The efficiency reductions are 0.45 dB (from −4.98 dB to −5.43 dB) in the LB band, 0.28 dB (from −2.70 dB to −2.98 dB) in the MHB band, and 0.81 dB (from −2.34 dB to −3.15 dB) in the NR band. The proposed antenna, which demonstrates minimal frequency shifts and slight efficiency degradation in the unfolded and folded states, makes it highly promising for practical applications.