Space-Wave Propagation

The signal strength at the receiving antenna is the vector sum of the two waves. The direct wave travels through the atmosphere and is affected by the troposphere. The ground-reflected wave suffers absorption and phase shift at the surface of the earth (page 451).

The length of the path and the heights of the antennas are other factors affecting the received signal strength. The wavelength is short at the frequencies under consideration (30 to 300 megacycles, and 10 to 1 meters), and the location of the antennas should be chosen with care. Once the site is selected, the exact location should be checked experimentally to determine the position of the strongest received signals. Methods of calculating the approximate signal strength with elevated antennas are summarized in reference 6. Reference 28 considers the effect of atmospheric conditions, hills, and buildings.

The following equation²³ for field strength applies approximately within the "radio path" horizon, equation 10,

where w is the power in watts radiated, h_t is the height, of the transmitting antenna in meters, h_r is the height above the earth of the receiving antenna in meters, λ is the wavelength in meters, and d is the distance in meters between the antennas. This equation holds for small dipoles (page 460) , for distances much greater than the antenna heights and for both vertical and horizontal polarization.

Channels for frequency-modulation broadcast stations start at 88 megacycles and extend to 108 megacycles. Certain television channels also arc in this vicinity. For data regarding frequency-modulation signal strengths at various distances, for different radiated power, and for various antenna heights, reference 29 should be consulted.