larization Polarization Fig. 7.3 Magnitude of the reflection coefficient over the ground (frequencies above 100 MHz). Grazing Angle a1 (degrees) Horizontal Polarization Vertical Polarization Fig. 7.4 Phase of the reflection coefficient over the ground (frequencies above 100 MHz). Grazing Angle a1 (degrees) 100 MHz Horizontal Polarization 3 GHz 100 MHz Vertical 3 GHz Polarization Fig. 7.5 Magnitude of the reflection coefficient over the sea. Grazing Angle a1 (degrees) Fundamental Laws of Electromagnetism Horizontal Polarization (all frequencies) Vertical Polarization 3 GHz 100 MHz Fig. 7.6 Phase of the reflection coefficient over the sea. Grazing Angle a1 (degrees) Opaque Screen Fig. 7.7 Fresnel diffraction parameters. amplitude and phase, by the complex function F, given by equation 7.24, where V is given by equation 7.25. F(v) = n e dt (7.24) V = h Nab 1/2 (7.25) Propagation Mechanisms The principles of propagation enable the communications engineer to produce convenient calculation techniques. This process is facilitated by recognizing three major propagation mechanisms, which tend to dominate in different wave bands: Space wave, below 10 meters (i.e., frequencies above 30 MHz) Sky wave, 10 to 200 meters (i.e., from 1.5 to 30 MHz) Surface wave, above 200 meters (i.e., below 1.5 MHz) Chapter 7 Electromagnetic Waves This division into three categories should not be regarded as rigidly defining the propa- gation mechanisms for the frequency ranges specified. First, it should be appreciated that the stated ranges are only approximate. Second, there can be a considerable overlap, where more than one mechanism is relevant. For example, MF broadcast stations normally provide coverage via the surface wave, but this coverage can be reduced, at night, by sky wave interference. Generation of Electromagnetic Waves Electromagnetic waves, as distinct from electrostatic fields or steady magnetic fields, involve a continuous net transfer of energy, or radiation, through a medium and are produced by current and charge distribut