A channel decomposition method is presented to evaluate the diversity performance of on-body UHF antenna systems. The novelty of this method amounts to separating the human body and propagation channel for independent estimation of each contribution. The proposed method can be readily adapted to different channel environments (once the statistics are known) to considerably reduce computation time. This channel decomposition method is adopted herein to evaluate the performance of a diversified UHF body-worn antenna system under a selection diversity scheme. The design process included three human postures (standing, kneeling, and prone) to incorporate a wide range of general human activity for realistic antenna performance. Mounting locations are evaluated and multiple antennas are used to increase communication data throughput. To validate the method, a 4-channel diversity module was fabricated employing selection diversity.