Multi-Band Small Antennas for Mobile Terminals

The thesis presents several novel ideas of designing electrically small antennas for mobile terminals such as mobile phones. As the fifth generation wireless systems (5G) is coming soon, radio signals at sub 6 GHz and millimetre-wave (mmWave) frequencies will be employed in mobile communication. In this thesis, the author concentrates on the antennas at sub 6 GHz, because the signals at sub 6 GHz will still play an important role in 5G mobile communication due to the advantage of signal penetration through buildings. The research areas consist of main antenna and multi-input multi-output (MIMO) antenna technology including decoupling techniques and MIMO antenna unit.

First, a novel six-mode loop antenna as a main antenna is proposed for mobile phones. Loop antennas offer better user experience than monopole antennas, inverted-F antennas (IFA), and planar inverted-F antennas (PIFA) because of the unique balanced modes (1?, 2?, ...). However, the balanced modes also cause narrower bandwidth of loop antennas. In order to overcome the bandwidth problem, how to reach the upper limit of the existing operating modes and how to create more modes are explored. A novel monopole/dipole parasitic element, which operates at an unbalanced monopole-like 0.25? mode and a balanced dipole-like 0.5? mode, is firstly proposed. In order to validate the concept, one prototype with the dimension of 75×10×5 mm3 is designed, fabricated, and measured. The antenna is able to cover 660-1100 MHz, 1710-3020 MHz, 3370-3900 MHz, and 5150-5850 MHz, which is wide enough for almost all the service of mobile telecommunication systems.

Then, a multimode decoupling technique is proposed for wideband/multiband isolation enhancement in compact volume. Although decoupling techniques have been researched for many years, multimode decoupling technique remains a great challenge for mobile terminals. One difficulty in achieving multi decoupling modes is that the operating modes of closely-packed decoupling elements have very strong mutual effect, which makes the tuning complicated and even unfeasible. Thus, in physical principle, a novel idea of achieving the stability of the boundary conditions of decoupling elements is proposed to solve the mutual effect problem; in physical structure, a metal boundary is adopted to realize the stability. One distinguished feature of the proposed technique is that the independent tuning characteristic can be maintained even if the number of decoupling elements increases. Therefore, wideband/multiband high isolation can be achieved by isolating multi decoupling elements. To validate the concept, two case studies are given. In a quad-mode decoupling design, the isolation is enhanced from 12.7 dB to > 21 dB within 22.0% bandwidth by using a 0.295?0×0.059?0×0.007?0 decoupling structure.

Finally, a novel principle, namely differential/common mode (DM/CM) design, is proposed to achieve highly integrated MIMO antenna unit in mobile terminals. The inspiration comes from a dipole fed by a differential line which can be considered as differential mode (DM) feed. What will happen if the DM feed is transformed into a common mode (CM) feed? Some interesting features are found in the research. By symmetrically placing one DM antenna and one CM antenna together, a DM/CM antenna can be achieved. Benefitting from the coupling cancellation of anti-phase currents and the different distributions of the radiation currents, a DM/CM antenna can obtain high isolation and complementary patterns, even if the radiators of the DM and CM antennas are overlapped. Therefore, good MIMO performance can be realized in a very compact volume. To validate the concept, a miniaturized DM/CM antenna unit is designed for mobile phones. 24.2 dB isolation and complementary patterns are achieved in the dimension of 0.330?0×0.058?0×0.019?0. One 8×8 MIMO antenna array is constructed by using four DM/CM antenna units and shows good overall performance. The proposed idea of DM/CM design may be promising for other applications that need high isolation and wide-angle pattern coverage.