Research on Multifunctional Metasurface Antennas for Millimeter-Wave and Terahertz Applications

Abstract

The rapid growth of wireless communication and sensing technologies in the millimeter-wave (MMW) and terahertz (THz) frequency ranges has sparked considerable interest in developing multifunctional antennas capable of operating in these attractive frequency bands. Metasurface-based antennas have emerged as a promising solution to tackle the challenges associated with increasing frequencies, including high atmospheric attenuation loss, limited fabrication tolerance, and confined system volume. This thesis focuses on the exploration and development of metasurface-based multifunctional antennas for enhanced performance in MMW and THz applications.

First, a novel wideband metasurface is proposed for individual phase and amplitude manipulation. The meta-atom functions as a receiver and transmitter (R/T) construction, transforming input linearly polarized (LP) waves into outcoming circularly polarized (CP) waves. The receiver section utilizes a modified E-patch to obtain corresponding LP waves. The top transmitter layer, with connected arms, generates CP waves. Power transmission is enabled through a metallic via connecting the top and bottom patches. The angles of the transmitter and receiver patches individually manage the transmission phase and amplitude, respectively. A transmitarray antenna is designed and fabricated using this metasurface, resulting in high directivity and around -25 dB sidelobe level (SLL) performance. Measurement findings prove that the antenna realizes a high-gain of 28.1 dBic at the designated frequency of 60 GHz with a broad 3-dB gain bandwidth (32.2%).

Second, a wideband and ultra-low-profile CP folded transmitarray antenna (FTA) is devised for utilization in the MMW region. The FTA consists of a multifunctional metasurface placed over a source-integrated low-cost metal sheet reflector. The polarization selective metasurface is modeled by individually engineering the forward and backward electromagnetic (EM) wave characteristics under orthogonal CP incidences. On the one hand, the metasurface operates in a reflection manner, combined with the metal sheet, confines EM waves between the two surfaces for numerous abnormal reflections, leading to a reduced antenna profile. On the other hand, the transmission manner of the metasurface compensates for the non-uniform phase distribution to make high-directivity radiation. A metasurface prototype with 19×19 meta-atoms is designed and produced, resulting in the FTA profile reduced to 0.2 of the focal distance. The measurements demonstrate that the FTA prototype has a wide 3-dB gain bandwidth (32.3%) and a high-gain of 24.2 dBic at 61.5 GHz.

Third, a wideband, low-cost, and low-profile CP beam scanning folded reflectarray-transmitarray antenna (FRTA) is presented operating at 60 GHz. The FRTA incorporates the Risley prism concept to achieve a 2-D beam scanning function by in-plane rotating two beam deflection metasurfaces. The dual-metasurface structure reduces the antenna profile using ray tracing principles. Both metasurfaces have the same phase gradient for beam deflection. The first metasurface can modify the forward and backward EM wavefronts based on CP states. The second source-integrated reflective metasurface converts incoming CP waves into orthogonal polarization. By controlling the reflection phase distributions, anomalous reflections occur between the metasurfaces, significantly reducing the antenna profile to 0.25 of the focal distance. A prototype antenna operating at 60 GHz is designed, modeled, and experimentally verified, demonstrating a wide scanning range of 0° to 40°. For broadside radiation, the fabricated FRTA achieves a realized gain of 22.4 dBic and a 3-dB gain bandwidth of 18.8% in measurements.

Fourth, a reflective metasurface operating in the THz range is proposed for frequency-controlled Bessel beam steering. The meta-atom is a multi-resonance shape with an extended reflection phase tuning range achieved by varying dimensions. The required phase distribution for producing the nondiffractive beams with fixed directions can be divided into four parts, including the phase to form Bessel beams, the phase for incident plane wave compensation, the phase to deflect the reflected beam, and a reference phase for optimization. A reflective metasurface comprising different meta-atom configurations is modeled to fit the calculated phase distributions at all designated frequencies. Under the LP incidence, the metasurface produces Bessel beams with tunable directions according to the working frequencies. The metasurface sample is fabricated through micromachining and verified by experiments, showcasing Bessel beam steering ability in the elevation plane.

Fifth, a dual-polarized space-time-coding (STC) metasurface is proposed for multiple modes of orbital angular momentum (OAM) generation in the MMW band. The meta-atom is loaded with PIN diodes to achieve precise 1-bit phase tuning capability. By applying a periodic time sequence to the meta-atom, continuous and independent control over the phase and amplitude can be achieved at one harmonic. Under the monochromatic wave incident at 26 GHz, the STC metasurface can generate the OAM beam with computer-programmable topological charges and phases. The measured near-field phase distributions agree reasonably with the calculated results, proving the multi-mode OAM generation ability of the proposed STC metasurface.

Date of Award	18 Nov 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Hou CHAN (Supervisor)