Electromagnetic waves and propagation

Electromagnetic waves (EMW) are waves made of oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel. They need no medium and travel at the speed of light in free space (c = 3 × 10⁸ m/s). Radio waves, light and X-rays are examples; they are produced by accelerated charges, such as the current in an antenna.

Propagation

A transmitting antenna radiates the wave, which travels to the receiving antenna in one of three ways, depending on frequency.

          sky wave  .--ionosphere--.
                  /                 \
 Tx ----space wave (line of sight)--- Rx
  \_____ground wave (follows earth)___/

1. Ground (surface) wave propagation: the wave travels along the earth's surface, following its curvature. Used for frequencies below about 2 MHz (LF, MF), such as AM broadcasting. Range is up to a few hundred km, and it depends on the ground conductivity.
2. Sky wave propagation: the wave goes up to the ionosphere, which reflects (refracts) it back to earth. Used for HF (2 to 30 MHz), such as short-wave radio. It gives very long distance communication, even across continents, with the possibility of fading.
3. Space wave propagation: the wave travels in a straight line, directly or after reflection from the ground. It is used for VHF, UHF and microwaves (above 30 MHz), such as FM, TV, mobile and satellite links. It needs line of sight, so antennas are placed on tall towers. Range is about d ≈ 3.57 √h km for an antenna height h in metres, so tall towers give a wider coverage.