Design of MIMO Antennas with Low ECC and Wideband Characteristics for 4G Mobile Handsets

Abstract

In this dissertation, the designs of Multiple-Input Multiple-Output (MIMO) antennas with low Envelope Correlation Coefficient (ECC) and wideband characteristics for 4G mobile handsets are proposed. This dissertation focuses on the approaches to broaden the bandwidth and reduce the ECC for small MIMO mobile antennas with frequency bands below 1 GHz. In order to broaden the bandwidths of small MIMO mobile antennas, I propose the method to jointly utilize both distributed elements (inductive coils and capacitive slots) and chip elements (chip inductors and chip capacitors) in the small MIMO antenna elements. This method can effectively improve the bandwidth of small mobile antennas with much simpler and more concise antenna structures, which makes the antennas more suitable for the massive and commercial production. Moreover, Positive Intrinsic-Negative (PIN) diodes are also applied to realize the frequency reconfigurable property for antenna elements to further expand the bandwidth. Besides the bandwidth, the ECC is also a key parameter for evaluating the performance of MIMO antennas. This dissertation also focuses on the investigation of ECC. A worst case estimate is obtained for the ECC of a small MIMO mobile antenna with operational frequency below 1 GHz. This worst case estimate is numerically derived with the help of spherical and exponential wave functions in this dissertation. The derived result confirms the value of worst case ECC can be estimated with the rotation angle between radiation patterns from different antenna elements for a MIMO mobile antenna with frequency range below 1 GHz. The worst case estimate could be attained directly through the use of amplitude of 2D-electric-field patterns without any additional phase and polarization information. Moreover, the effectiveness of this worst case estimate is also verified by comparing different real MIMO mobile antennas operating around 0.8 GHz. Based on the analysis, the approach to effectively reduce the ECC of a small MIMO mobile antenna is also got. Finally, as a real example, the compact reconfigurable wideband Inverted-F Antenna (IFA) elements are developed and proposed for a Long Term Evolution (LTE) MIMO mobile terminal. The proposed inverted-F antenna has a very simple planar arrangement. A capacitive coupling feed strip is employed to modify the input impedance matching. A chip inductor is connected in series to the feeding point to realize the wideband property. The whole MIMO antenna system consists of two similar inverted-F antenna elements. Antenna element #1 has a wideband property with a 6-dB bandwidth from 0.74 to 0.9 GHz. To further expand the bandwidth, antenna element #2 has a reconfigurable property through the use of a PIN diode. Antenna element #2 has a 6-dB bandwidth from 0.74 to 0.84 GHz when the PIN diode is on and from 0.79 to 0.9 GHz when the PIN diode is off. Moreover, the proposed MIMO antenna has a low radiation-pattern-based ECC, which is less than 0.15 over LTE bands 4, 5, 13, and 14.