Radio atmospheric In the case of the ionospheric D-layer, it depends, in addition, on time of day, season, latitude, and the geomagnetic field in a complicated manner. VLF propagation within the Earth–ionosphere waveguide can be described by ray theory and by wave theory. When distances are less than about 500 km (depending on frequency), then ray theory is appropriate. The ground wave and the first hop (or sky) wave reflected at the ionospheric D layer interfere with each other. At distances greater than about 500 km, sky waves reflected several times at the ionosphere must be added. Therefore, mode theory is here
Earth bulge Earth bulge Earth bulge is a term used in telecommunications, meaning the curvature of the Earth's surface, which limits the range of communication methods that require a line-of-sight path, like high frequency radio waves, microwaves, or lasers. Assuming the Earth is a perfect sphere with no terrain irregularity, the line-of-sight distance to the horizon from a transmitter located at a given altitude above the surface can readily be calculated. Let R be the radius of Earth and h be the altitude of the transmitter's antenna. Line of sight distance d of this station is given by the Pythagorean theorem; Since