A model to calculate EM fields in tropospheric duct environments at frequencies through SHF

Non-monotonic variations of the atmospheric refractivity with height often enable energy to be propagated in a duct mode with little attenuation over long distances. A duct model employing a waveguide-mode method of analysis was developed to calculate the field strength when the refractivity variation with height, is known. The developed numerical procedure methodically determines all significant eigenmodes for elevated and surface ducts at all frequencies through SHF. Computer model results for beyond-the-horizon ducts compare favorably with measurements taken at frequencies between 65 MHz and 3.3 GHz.