Adaptive Multiport Antennas for Handsets

Generally, the main factors driving the evolution of modern handset antennas are the requirements for increasing the data capacity and the visual appearance restricting the volume of the antennas. To increase the data transfer rates, new bands with wider frequency ranges are used together with an increasing number of multiple-input multiple-output (MIMO) antennas. The appearance of the device is an important factor in the smartphone industry. However, nearby conductive objects, such as the screen or metal rim of the device, hinder the operation of the antennas. Therefore, new innovations and techniques are required to reach these difficult goals. This thesis studies whether multiport antenna techniques can be used to address the aforementioned challenges. In the first part of this thesis, new design methods for multiport handset antennas are presented. The first method can be used to design switch-reconfigurable antenna systems. The rim is used for MIMO operation on different frequency bands with switches and the proposed design method makes the optimization process efficient. The proposed antennas achieve 30-40 % total efficiency in the 700-960 MHz band and 25-75 % total efficiency in the 1.7-2.7 GHz and 3.0-4.0 GHz bands. The second method utilizes multiport antennas and characteristic modes to utilize an unbroken metal rim for MIMO antennas. The proposed antennas achieve an efficiency of 15-59 % at low band and 25-80 % at higher bands. The second part of this thesis focuses on the interaction between the user and multiport antennas. First, the effect of the user's hand on the operation of the antennas designed in the first part is investigated. Given the promising results obtained, the study is extended to include the user in the design process from the beginning. A design process for hand-immune antennas based on the characteristic modes of both the metallic antenna structure and lossy dielectrics of the user is presented. The resulting antennas achieve a total efficiency of more than 30 % at low band with the user holding the device. A common problem even with multiport antennas is the ability to achieve a wide enough bandwidth. The third part of this thesis presents a method for realizing antennas in extremely small volumes inside smartphones by taking advantage of the gap between the battery and the back cover of the device. An average total efficiency of 35 % is achieved across the 3.3-4.2 GHz band with an antenna height of only 0.75 mm. Following that, a more general design tool for accelerating the design process of multiport antennas is presented. Using the quality factor combined with a proper choice of feeding signals, the achievable performance of a structure can be estimated without the need for time-consuming matching network optimization. The new and computationally efficient design methods developed in this work enable the realization of antennas with improved performance. Moreover, the presented antenna designs demonstrate the benefits of multiport antennas in comparison to traditional solutions.