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This paper describes the results and conclusions from a two-year field evaluation programme of 11 GHz 140 Mbit/s q.p.s.k. digital radio relay equipment conducted on three hops in the UK by British Telecom. In the first part of the paper the general features of system performance are described, including month-by-month outage results, error distributions, outage event durations, and error-free-secondperformance. The relative effect on performance of rainfall attenuation and multipath propagation are illustrated and discussed, together with effects of variations in hop length and the use of height diversity facilities. Worst-month outage results are compared with CCIR performance requirements to determine planning criteria for diversity. The second part describes in detail the system behaviour during multipath conditions, and examines whether the outage primarily resulted from noise, interference, or distortion conditions. The signature method of characterizing tolerance to distortion is described in order to explain observed results, and the effect of adaptive equalizer techniques is examined. Finally results from the evaluation of an automatic slipless protection switching system are presented, showing probability of slipless operation and performance improvements achieved in multipath conditions.