Antenna design based on quasi-degenerate characteristic modes of unbroken metal rim

A simple and efficient antenna technique for both single antenna and multiple-input multiple-output (MIMO) antenna design utilising an unbroken metal rim is analysed in the frequency band <1 GHz for mobile phone applications. A 110 mm × 55 mm × 5 mm unbroken metal rim has two characteristic modes (Modes 1 and 2) that resonate around 1 GHz, and these two modes are quasi-degenerate and orthogonal to each other. An uncontacting feeding structure is proposed to excite and utilise the characteristic modes of the unbroken metal rim for antenna radiation. In MIMO antenna design, it is possible to achieve high isolation and good diversity performance by properly exciting orthogonal modes with the proposed feeding technique. Two different cases for MIMO designs are presented to verify the feasibility of the proposed technique. In both two cases, the simulated isolation is ~10 dB, and the 3:1 VSWR bandwidths for the MIMO antennas are wider than 150 and 60 MHz, respectively. The measured results show that the envelope correlation coefficients for each case are <0.2, indicating good diversity performance. The measured radiation patterns for the two MIMO antennas are orthogonal, again verifying the diversity performance.