The solution of the modal equation for electromagnetic wave propagation in a tropospheric duct, having a trilinear refractivity profile, shows the existence of two sets of roots in the complex propagation plane. One of these is of the whispering gallery type with very low attenuation and the other is of the waveguide variety with relatively high attenuation. When the contrast in refractive index is small, the lower-order waveguide modes have an attenuation comparable with the attenuation of the whispering gallery modes. A large contrast in refractive index causes the whispering gallery modes to dominate (surface or elevated ducts). Several examples of the altitude charts for the modal equation for propagation at 200 MHz through a medium having various contrasts in the refractive index give some physical insight into these two classes of modes.
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