VLF/LF/MF Whispering gallery propagation studies

The propagation of very low frequency (VLF) and low frequency (LF) radio waves in the Earth-ionosphere waveguide is affected profoundly by the Earth's curvature. In particular, for frequencies greater than about 20 kHz, some modes are possible for which the energy is concentrated near the base of the ionosphere and the field strength near the ground is small. It is useful to think of such modes as being composed of waves repeatedly reflected at the inside surface of the ionosphere. Using analogy with sound waves, we can call these modes whispering--gallery modes. Since they depend only very slightly on ground conductivity, these modes are also referred to as Earth-detached modes. The Navy currently is pursuing efforts leading to a feasibility demonstration of a balloon gateway communications system that would employ a balloon-to-balloon cross link based on the whispering-gallery propagation modes. This report discusses some recent whispering-gallery theoretical advances and simplifications.