Dual-Band Low-Profile Hybrid Antenna for Bidirectional Radiation

This paper presents a novel dual-band, low-profile hybrid folded antenna capable of generating bidirectional independent beams. The proposed design operates as a liked-folded reflectarray antenna at the low band, producing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i>-polarized high-gain beams in the lower hemisphere, and as a folded transmitarray antenna at the high band, generating <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i>-polarized high-gain beams in the upper hemisphere. A prototype of the dual-band hybrid folded antenna was designed, fabricated, and measured. Measurement results show a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i>-polarized beam in the upper hemisphere, with a maximum gain of 31.2 dBi at 29 GHz, a peak aperture efficiency of 35.5%, and a 3-dB gain bandwidth of 8.2%. Simultaneously, an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i>-polarized beam is generated in the lower hemisphere, exhibiting a maximum gain of 22.9 dBi at 12.7 GHz, a peak aperture efficiency of 28.9%, and a 3-dB gain bandwidth of 26.3%. The proposed antenna retains the low-profile and high-gain advantages of traditional folded antennas, addressing space utilization limitations, effectively extending the conventional unidirectional dual-band folded structure to support bidirectional operation.