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
History of radio proposed electromagnetic or Hertzian waves as a navigation aid or means of communication, with Crookes writing on the possibilities of wireless telegraphy based on Hertzian waves in 1892. After learning of Hertz' demonstrations of wireless transmission, inventor Nikola Tesla began developing his own systems based on Hertz' and Maxwell's ideas, primarily working toward a means of wireless lighting, and power distribution. Tesla, concluding that Hertz had not demonstrated airborne electromagnetic waves (radio transmission), went on to develop a system based on what he thought was the primary conductor, the earth. In 1893 demonstrations of his ideas, in St. Louis, Missouri