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, which can significantly impact antenna performance, such as frequency shifts and efficiency reduction, are 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 low band (LB) of 0.704–0.960 GHz, middle high band (MHB) of 1.71–2.69 GHz, and new radio (NR) bands of 3.3–3.8 GHz and 4.7–5.0 GHz 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.