What is Ground wave ? When is it used for communication ?

Explanation

Ground wave : To radiate signals with high efficiency . The antennas should have a size comparable to the wavelength `lambda` of the signal (at least`~ lambda//4`) . At longer wavelengths (i.e., at lower frequencies) , the antennas have large physical size and they are located on or very near to the ground. In standard AM broadcast , ground based vertical towers are generally used as transmitting antennas. For such antennas , ground has a strong influence on the propagation of the signal . The mode of propagation is called surface wave propagation and the wave glides over the surface of result of absorption of energy by the earth . The attenuation of surface waves increases very rapidly with increase in frequency . The maximum range of coverage depends on the transmitted power and frequency (less than 2 MHz) . Ground waves will propagate long distance over sea water due to its high conductivity .