Elevated duct propagation in the tradewinds

All of the maximum propagation (220 Mc) ranges observed in an elevated duct regime varied from 500 to 1200 miles compared to less than 400 miles observed with the same equipment elsewhere. The measurements were made at the optimum season (November) in a tradewind regime between Brazil and Ascension Island (
8\deg
S. latitude). The field strengths above 4000 feet altitude are as much as 40 db larger than those at lower altitudes. From the level at average duct height (6000 feet) the field decreases slowly to 10,000 feet, the maximum altitude investigated. The slow fading rate usually associated with duct propagation is not always observed even on the very long range runs. An extrapolation to propagation conditions in the month of March via refractive index measurements indicates quite minimal ducting conditions 10 per cent to 20 per cent of the time.