Multipath propagation at 11, 18, and 36 GHz due to ducting

Field strengths at 11, 18, and 36 GHz were measured at a 15 km and a 5 km radio link over land. At the transmitter site the stratification of the atmosphere was monitored with a sodar. Temperature inversions are detected in the layer from the ground to 600 m. Almost all duct layers are connected with temperature inversions. Statistics of layers in the layer close to ground and of multipath propagation are presented. Case studies show that the appearance of multipath propagation is observed with time delay between the frequencies 11, 18, and 36 GHz but not always in the same order. Refraction is caused by the vertical gradient of refractive index. Fading is set up by variations of that gradient and not by the changing heights of the duct layers.