Design of compact antennas for mobile device applications

Abstract

This thesis presents the design of compact antennas for mobile device applications. The three main designs of smartphone antennas, smartwatch antenna, and MIMO antennas are designed and studied. Firstly, two compact multiband loop antennas are designed for the smartphone application. The two loop antennas are designed within a very compact volume of only 5×8×60 mm3. The generation mechanism of the new 2-λ mode (4th resonant mode) of loop antenna is presented. It is found that distance between the feed point and the shorting point of the loop antenna could be used to generate and independently control the 2-λ mode (4th resonant mode). This method could improve the bandwidth of the loop antenna in the upper band without increasing the occupied volume, which means the available volume of the loop antenna occupied can be utilized more efficiently. To further improve the bandwidth of loop antenna, three parasitic elements are appropriately placed inside the antenna volume, and finally seven resonant modes are generated. These seven resonant modes form wide bandwidth in the lower band and upper band, which could cover GSM850, GSM900, DCS1800, PCS1900, UMTS2100, LTE2300, LTE2500 and LTE3500. The effects of phone case, phantom hand and head, and the metallic electronic component inside the loop are also investigated. Secondly, a cavity-backed annular slot antenna with high efficiency is designed for smartwatch with metallic housing. By using an angle of 22.5o between the microstrip feed line and shorting strip on the annular slot, a new lower resonant mode, the 1-λ mode, is generated. The new 1-λ mode makes the antenna small enough for current smartwatch applications in the 2.4-GHz WiFi band. Results show that, when the smartwatch is worn on the phantom hand, the measured antenna efficiency is about 57-66%, which is much better than the other designs. The effects of metallic components inside the watch and Specific-Absorption Rate (SAR) are also been analyzed. Lastly, two compact MIMO antennas are designed for the mobile devices. In the first design, a decoupling technique using a patterned ground structure is designed for small MIMO antennas. The antenna consists of two identical inverted-F antenna (IFA) elements with a distance of only 0.13λ for radiation and a diamond-shaped patterned ground resonator (DSPGR) placed between the antennas for decoupling. Results show that the DSPGR can dramatically reduce mutual coupling between the two IFA elements to below −20 dB in a wide operating frequency range of 2.18–2.65 GHz (19.5%). In the second design, a Compact Slot Multiple-input multiple-output Antenna (CSMA) for smartphone application is designed. The antenna has a slot radiator shared between two input ports and a T-shaped defect-ground slot (DGS) for decoupling. The two antenna elements of the CSMA and its decoupling structure are achieved using only one slot structure. A function for selecting the best dimensions of the DGS for optimum decoupling is derived using the transmission theory.