Millimeter-Wave High-Gain Antenna for 5G Backhauling with Auto-beam-tracking Function

Licentiatavhandling, 2023

As microwave links are increasingly deployed within the millimeter-wave frequency range, E-band (70/80 GHz) links have been installed on a global
scale. One major technological challenge in this high-frequency spectrum is the significant increase in path loss between receivers and transmitters. To
address this issue, antennas with a larger aperture and higher gain are commonly employed. Unfortunately, these antennas feature an extremely narrow
beamwidth, making alignment exceedingly difficult and potentially compromising link reliability, especially when the supporting mast of the antenna
experiences swaying, disturbances, or twisting. This issue has emerged as a significant challenge in current high-frequency communication system, urgently requiring a perfect solution. This thesis introduces several ultra-high gain dual-reflector antennas that utilize feed defocusing technology for beam steering to address link disruption caused by mast sway, specifically designed for 5G backhaul applications.
Firstly, two types of monopulse antennas are presented: 1. A dual-polarized monopulse antenna based on a planar magic-T structure, which reduces manufacturing complexity due to the use of gap waveguide (GW) structures. The feed system provides SUM and DIFF beams for data transmission and tracking, respectively. 2. A dual-polarized monopulse antenna based on phase shifters and couplers, where phase compensation technology is employed to achieve DIFF beam steering. As part of the design, three compact, low-loss, reconfigurable phase shifters based on GW technology have been developed. These phase shifters achieve varying rates of phase change through the adjustment of the lengths of the transmission lines and the dimensions of slow-wave structures. To achieve precise beam tracking, an electromechanical automatic control system has been designed for these antennas. All measured outcomes have closely validated our theoretical forecasts.
Secondly, in order to have a low-cost solution without using DIFF receivers for tracking, a dual-reflector antenna system with beam steering through feed
defocusing has been developed. Our design goal is to avoid the use of coaxial cables for connections between the defocusing movable feed and the Tx/Rx
ports since the cable connection leads to cable bending, reduced reliability, and increased likelihood of failure. For this purpose, we propose 1D and 2D mechanically movable feeds combined with a Gregorian reflector. In this design, we first introduce three types of reconfigurable transmission lines: shielded rectangular waveguides, movable rectangular waveguides, and universal-joint circular waveguides. These innovations extend the applications of GW tech- nology into the mechanically reconfigurable domain. For precise beam track- ing capabilities, two cost-effective and environmentally sustainable mechani- cal tracking mechanisms are proposed: a gravity-driven balance system and a gravity-driven spring system. All measurement results closely confirm our theoretical predictions.

Finally, this thesis proposes other two innovative GW structures: 1. A novel concept of patterned distributed pins designed to stabilize the phase perfor- mance of Half-Mode Gap Waveguides, even with manufacturing inaccuracies, thus streamlining manufacturing processes. 2. A newly developed Ball-pen GW, which utilizes commercially available low-cost pens for mechanical re- configurability, offering an easily manufacturable solution.

In summary, the techniques and solutions presented in this thesis aim to offer a strategic approach to addressing the challenges encountered in 5G backhaul systems and other mmWave antenna systems. It is our aspiration that these advancements will contribute to the optimization and enhancement of backhaul networks.