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The design of several universal mobile telecommunications system multi-antenna systems with radiators having a high isolation, a high total efficiency and a low envelope correlation is presented. First, two planar inverted-F antennas (PIFAs), closely positioned at the top edge of a small ground plane whose size is representative of the printed circuit board of a mobile phone, are described. A technical solution is then proposed to increase the isolation between the antennas and enhance their total efficiency when still keeping them closely spaced. The technical solution is based on an optimal neutralisation technique, applied between the antennas of the structure. Several optimal systems, based on different parameters, are fabricated and measured. The simulated and measured S-parameters are presented in addition to the gain radiation patterns, the surface currents on the structure and the theoretical and experimental total efficiencies. The envelope correlation coefficients are also computed using two different equations. It is then demonstrated that, with the help of the neutralisation technique, a system composed of two closely spaced PIFAs located at the top edge of a small ground plane can lead to a high total efficiency and a low envelope correlation coefficient. In conclusion, this technique can be easily implemented at the terminal side of a wireless link and may enhance its diversity gain and multiple input multiple output performance.