Statistical behaviour of refractivity gradient in the tropics

The knowledge of the spatial behaviour of the refractivity in the lower troposphere is an essential information for the design of UHF and SHF radio links, as well as for the evaluation of interference between terrestrial communication systems. The paper presents results of refractivity measurements at 16 sites in several climatic regions in Brazil. An experiment underway in seven additional links, aiming to estimate the effective K factor of the Earth from attenuation measurements in diffracted links is also described. Depending on the site, different measurement methods were employed: radio sonde measurements or direct measurements using refractometers. The probability distributions obtained from measured data indicate that, in tropical regions, the media values of refractivity gradient are usually significantly higher than those found in temperate climates