This paper presents a model which describes the operation and performance of the cophasing space diversity combiner used in radio propagation experiments on a Bell System one-hop line-of-sight path utilizing a 6 GHz, 8 PSK digital radio system. This model was confirmed by comparing the model predictions with the recorded spectrum observed in the 1977 field tests. Analysis of the model has demonstrated that a cophasing combiner maximizes power but does not minimize in-band amplitude variation with frequency which is the prime impairment for 8 PSK systems. This is the main reason that, at a bit error rate of1 \times 10^{-3}appropriate to voice circuit application, the diversity improvement observed was a factor of six relative to the expected nondiversity performance. A simulation showed an additional factor of two improvements for an ideal maximum power combiner. An ideal linear adaptive equalizer was also simulated for each antenna. The improvment relative to nondiversity performance without an equalizer was estimated to be in the range of 3 to 6. The combination of combiner and ideal equalizer would have reduced outage to a negligible amount.