A comparison of 3, 9.6, and 18-GHz low-altitude over-the-horizon propagation (as influenced by the evaporation duct) is presented. Theoretical and experimental results are given for a 35-km over-water path, where the transmitters were located about 5 m above mean sea level and the receivers were located either 4-5 m or 18-19 m above sea level. Results are presented in terms of the one-way propagation factor in decibels at each frequency. Long-term cumulative frequency distributions of calculated and observed propagation factors are presented. Particular attention is given to the relative performance of each radio frequency to investigate frequency-diversity improvements that may be available on such paths. In most cases presented, there is a close agreement between theory and observations. It is concluded that substantial improvements in received signal levels are likely to be achieved in most areas of the world by a suitable choice of two frequencies in the 10-20-GHz range. >