Upper bound on pattern correlation of lossy N-port antennas in terms of modal efficiencies

An upper bound on pattern correlation of lossy multi-port antennas is derived. Contrary to previous derivations which were based on the per port analysis of multi-port antennas, analysis in this paper is based on the spectrum of the radiation matrix. The derivation holds for an arbitrary number of radiation modes. It is concluded that for any practical antenna design low pattern correlation is result of proper multi-port antenna matching and not a separate design goal. In order to illustrate this, the pattern correlation is calculated for three different arrangements of two monopoles. All three arrangements represent basic features of any practical 2-port Multiple Input - Multiple Output (MIMO) antenna design for small mobile terminals.