Implications of anomalous propagation in the evaporation duct for radars at X and Ku band

Abstract

The radio refractive index of the atmosphere is governed by the combination of atmospheric temperature, pressure and humidity. Over oceans, humidity gradients can cause an effect known as the evaporation duct. Such a duct has the property of trapping radio waves between the sea surface and the top of the duct, which can result in extended range or radio black spots. The amount of channeling is dependent on the carrier frequency, the duct structure and various transmitter and receiver properties including antenna tilt and height above sea level. Knowledge of the duct and how it affects propagation is therefore of some importance to people using or designing maritime radio equipment for use in maritime and littoral environments because it allows disadvantages to be overcome while making use of the advantages of extended range. This paper discusses research being undertaken at James Cook University on duct height determination and propagation within it. The paper also discusses current topical studies within the International Telecommunications Union (ITU) on sharing between radar and satellite links in the 13.75–14-GHz band and implications to this work arising from a predominant duct.