On Small Terminal MIMO Antennas, Harmonizing Characteristic Modes With Ground Plane Geometry

Abstract

multiple input–multiple output (MIMO) antenna systems are defined by fundamental figure of merits (FoM), such as branch imbalance, total efficiency, mean effective gain (MEG), and the correlation coefficient. Those FoM requirements are challenging, specially when applied to electrically small mobile devices, i.e., smart-phones, due to the form factor-reduced dimensions and multiplicity of adjacent frequency bands of operation. Uncorrelated antennas are especially important for MIMO antenna systems; other than few exceptional cases, the reduction of magnitude of complex correlation coefficient will increase the system capacity and data throughput. This work proposes an MIMO antenna system for mobile terminals, based on a realistic form factor and packaging implementation, with a very low magnitude of the complex correlation coefficient and an impressive isolation. An isolation better than 20 dB has been achieved using a folded monopole and a commonly adopted planar inverted F antenna (PIFA) on a platform with a very small form factor ( ${\bf{100}}\times {\bf{50}}\times {\bf{10}}\;\bf{mm}$ ). The proposed implementation is based on the synergy of two known techniques that consists in the: 1) reference ground plane geometry manipulation and 2) in the application of the characteristic mode theory to obtain orthogonal radiation modes. Since MIMO antenna systems at frequencies higher than 1.7 GHz are naturally proper isolated and decorreleated, this work demonstrates the proposed antenna topology enabling higher isolation and uncorrelated antenna system at 750 MHz, which is more difficult to achieve in form factors smaller than ${\bf{1}}\boldsymbol{\lambda}$ , while maintaining high total efficiency and adequate gain imbalance. In this paper, a simulation model and prototype ( ${\bf{1/4}}\boldsymbol{\lambda}$ long) measurement results are presented, demonstrating the implementation feasibility of such antenna system in realistic mobile device embodiment.