Tight upper bound on radiation pattern correlation for N-port MIMO antennas

A tight upper bound on pattern correlation for multi-port antennas is derived. It is valid for lossy antennas and an arbitrary number of ports. As opposed to previous work, the present derivation is based on modal analysis, considering the spectrum of the radiation matrix. The new approach yields a much tighter upper bound on pattern correlation than previously available from per port characterisation of multiport antennas. The result suggests that pattern correlation is generally small for reasonably well-matched antenna systems and therefore hardly ever relevant as an independent design criterion. Numerical results for simple model structures corresponding to 2-port, 3-port and 4-port multiple input-multiple output antennas are given to illustrate this conclusion.