Duct propagation is a special type and used for very high microwave frequencies. New phenomenon which occurs in super-refraction, also known as ducting. As the height above earth increases, the air density decreases and refractive index increases. Under certain special atmospheric conditions, a layer of warm air may be trapped above cooler air, often over the surface of water. So that refractive index will decrease far more rapidly with height than is usual. This happens near ground within 30 m of it. Due to this rapid reduction of refractive index, the microwaves completely bend back towards earth surface as shown in fig. Microwaves are thus continuously refracted in duct and reflected back by the ground, so that they are propagated around the curvature of the earth for distances which many of times exceed 1000km. The main requirement of formation of atmospheric ducts is the so-called temperature inversion. Temperature inversion is the increase of air temperature with height, instead of the usual decrease in temperature of 6.50C/km in the standard atmosphere. The Duct propagation is used for very high frequencies in GHz range.