High isolation compact four-port MIMO antenna systems with built-in filters as isolation structure

In this work, we present a built-in filter method to enhance the isolation level between closely packed MIMO antennas. The built-in filter is composed of several slits etched on the ground of the antenna structure. The number and length of slits depend on both the resonance frequency and the structure of the MIMO antenna system. To illustrate its usefulness, two designs of compact planar four-port MIMO antenna systems with built-in filters are proposed. One MIMO system is composed of different types of antennas with similar radiation characteristics, such as polarization. The lateral size of the whole system is 0.64λ×0.48λ. To further reduce the size of the MIMO structure, polarization diversity is utilized in the second example. The dimension is only 0.4λ×0.4λ. In both cases, with the built-in filters, the isolation level can be improved by 10dB. In both MIMO systems, the measured port isolations between antennas are below −25dB, and the envelope correlations are smaller than 0.04, indicating good MIMO performance.