Radio Wave Transmission Paths

Rays. In considering the transmission paths, it is convenient to assume that radio waves travel in rays. Of course, if a radio wave were visible, an oncoming radio wave would appear to be magnetic and electric lines of force crossed at right angles. This is called a wave front. It is much simpler to represent a radio wave by a ray, or line, perpendicular to the wave front, showing the direction of propagation of the wave. This has been done in Fig. 3.

Ionospheric Wave. If a transmitting antenna directs a component of its total radiated energy upward toward the ionosphere, under conditions often encountered, the ionosphere reflects much of this energy back toward the earth. This is indicated in Fig. 3. The ionospheric wave (or sky wave) is defined¹ as "a radio wave that is propagated by reflection from the ionosphere."

Tropospheric Wave. At very high radio frequencies and beyond, the troposphere commonly has much influence on radio transmission. A tropospheric wave is defined¹ as "a radio wave that is propagated from a place of abrupt change in the dielectric constant or its gradient with position in the troposphere."

Ground Wave. Particularly important in standard radio broadcasting is the ground wave defined¹ as "a radio wave that is propagated over the earth, and is ordinarily affected by the presence of the ground. The ground wave includes all components of a radio wave over the earth except ionospheric waves and tropospheric waves. The ground wave is somewhat refracted by the normal gradient of the dielectric constant of the lower atmosphere."

Figure 3. This shows the various paths by which radio waves travel. This drawing is considerably simplified from the actual condition that may prevail at a given time. The transmitting antenna is marked T, and the receiving antennas at various points are R1, R2, R3, etc. The abbreviations are DW, direct wave; GRW, ground-reflected wave; SW, surface wave. Reflections actually occur in the ionosphere and in the surface layers of the earth.

The ground wave is often considered⁶ to be composed of two components, a surface wave and a space wave. The surface wave is the component of radiation that travels entirely along the surface of the earth (Fig. 3). The space wave is composed of a direct wave "that is propagated directly through space"¹ and a ground-reflected wave, which is "the component of the ground wave that is reflected from the ground."¹ These wave components or rays also are shown in Fig. 3.

Wave Diffraction. When a radio wave encounters an obstruction in its path, diffraction occurs. Thus, a large building or a hill will not cause a sharp "radio shadow," and reception may be possible behind a large building or below the top of a range of hills. Diffraction is one factor causing radio waves to bend with the surface of the earth.

Wave Refraction. When a radio wave passes from one medium having a certain dielectric constant and velocity of propagation to another medium having a different dielectric constant and velocity of propagation refraction occurs. Because of refraction, the path of a radio wave is bent, and, if the bending is sufficient, wave reflection may occur.

Absorption. If a radio wave were traveling in free space, a spreading would occur, yet there would be no energy dissipation in the usual sense. But a radio wave does not travel in free space. Thus, radio waves near the surface of the earth suffer an energy loss in this surface, and in buildings, trees, hills, etc. Also, a radio wave in the troposphere may suffer energy loss by absorption, probably caused by water vapor. Absorption of energy also occurs in the ionosphere.