Guide EM waves with atmospheric ducts

Atmospheric ducts are dielectric waveguide-like regions which form in the troposphere due to climatic effects. EM waves may propagate in a duct over great distances with considerably less loss than over similar free-space distances. Meteorological measurements have shown that ducts may occur more than 50 percent of the time in some areas of the world. For this reason, designers and users should be aware of the effects of atmospheric ducting. Ducts can sometimes hinder the operation of radar by creating dead spots called radar holes. Attention is given to a trilinear elevated duct model, data measured on actual ducts lying above Crete, and the results of a waveguide mode model at 149 MHz.