Fully metallic antennas for millimeter wave applications
Time: Tue 2021-06-08 14.00
Location: H1, Teknikringen 33, Stockholm (English)
Subject area: Electrical Engineering
Doctoral student: Qingbi Liao , Elektroteknisk teori och konstruktion
Opponent: Prof. Sean Victor Hum, University of Toronto
Supervisor: Oscar Quevedo-Teruel, Elektroteknisk teori och konstruktion
Modern societal demands for a high data throughput, short time latency and low energy consumption are difficult to satisfy using current wirelesscommunication techniques. The carrier frequency of previous wireless communication, such as broadcast, the global system for mobile communication, and wireless local area networks, are in the sub-3 GHz spectrum. Electromagnetic waves in the sub-3 GHz spectrum possess a long wavelength and small free space path loss (FSPL), but with a narrow absolute bandwidth. This absolute bandwidth limits the channel capacity according to Shannon theory. Under the assumption that relative bandwidth is fixed, the absolute bandwidth is proportional to the carrier frequency. That means, wireless communications with high carrier frequency can provide wide bandwidth and large channel capacity. Besides, the sub-3 GHz spectrum is already too crowded to have future advanced wireless communications. Nowadays, it is essential to move carrier frequencies to higher frequency spectrum. The millimeter wave (mmWave) frequency band can provide an extensive bandwidth but suffers high atmospheric attenuation and FSPL. The highattenuation and loss limit the propagation distance of mmWave to a few kilometers. Additionally there is a high attenuation due to precipitation, as the wavelength of mmWaves are of the same order in size as rain drops. Due to these losses, there are restricted applications in the mmWave band used for wireless communications. However, the electromagnetic spectrumshortage encourages new researches to look for solutions overcoming the drawbacks of mmWave.
Specific requirements on antenna designs are imposed by using mmWave communication, including manufacturing costs, integration, efficiency, scanningrange, and directivity. Antennas designed for the mmWave have a small physical size, which requires finer manufacturing resolution and increases manufacturing costs. To compensate for the high FSPL and attenuations, high directive antennas with low side lobe level are favorable. To improve the radiation efficiency, it is preferred to use fully metallic structuresas opposed to structures containing dielectrics for antennas operating in the mmWave range. This thesis investigates the innovative techniques for designing high performance fully metallic antennas in mmWave. Antennas made in gap waveguides and geodesic lens antennas have low manufacturing costs, low loss, and high directivity. The gap waveguide technology can be used to manufacture antennas in separated pieces. These pieces are united together afterwards. The manufacturing cost is reduced in this way. In gap waveguides, the radiation leakages from gaps between separated pieces are prevented using metasurfaces. The research emphasis is placed on the properties of glide-symmetric metasurfaces. Comparing with non-glide metasurfaces, glide-symmetric metasurfaces have an extended electromagnetic bandgap. On the other side, the geodesic lens antenna is designed based on geometrical optics (GO). The graded index lenses can be transformed to geodesic shapes through GO. Since the mmWave presents optical propagation characteristics, GO can be used as a good approximation. A ray-tracing model is developed to calculate the radiation patterns of geodesic lenses and its performance is verified by full wave simulations. Geodesic lens antennas implemented in parallel plate waveguides are in full metal and allow waves to propagate in vacuum or air.