Propagation MODES

Propagation MODES

A signal radiated from an antenna travels along one of three routes: ground wave,

sky wave, or line of sight (LOS). Table 5.3 shows in which frequency range each

predominates. In this book, we are almost exclusively concerned with LOS communication,

but a short overview of each mode is given in this section.

Ground Wave Propagation

Ground wave propagation (Figure 5.5a) more or less follows the contour of the

earth and can propagate considerable distances, well over the visual horizon. This

effect is found in frequencies up to about 2 MHz. Several factors account for the

tendency of electromagnetic wave in this frequency band to follow the earth's curvature.

One factor is that the electromagnetic wave induces a current in the earth's

surface, the result of which is to slow the wavefront near the earth, causing the wavefront

to tilt downward and hence follow the earth's curvature. Another factor is diffraction,

which is a phenomenon having to do with the behavior of electromagnetic

waves in the presence of obstacles.

Electromagnetic waves in this frequency range are scattered by the atmosphere

in such a way that they do not penetrate the upper atmosphere.

The best-known example of ground wave communication is AM radio.

Sky Wave Propagation

Sky wave propagation is used for amateur radio, CB radio, and international broadcasts

such as BBC and Voice of America. With sky wave propagation, a signal from

an earth-based antenna is reflected from the ionized layer of the upper atmosphere

(ionosphere) back down to earth. Although it appears the wave is reflected from the

ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact

caused by refraction. Refraction is described subsequently.

A sky wave signal can travel through a number of hops, bouncing back and

forth between the ionosphere and the earth's surface (Figure 5.5b). With this propagation

mode, a signal can be picked up thousands of kilometers from the transmitter.

Line-of-Sight Propagation

Above 30 MHz, neither ground wave nor sky wave propagation modes operate, and

communication must be by line of sight (Figure 5.5c). For satellite communication, a

signal above 30 MHz is not reflected by the ionosphere and therefore can be transmitted

between an earth station and a satellite overhead that is not beyond the horizon.

For ground-based communication, the transmitting and receiving antennas must be

within an effective line of sight of each other. The term effective is used because

microwaves are bent or refracted by the atmosphere. The amount and even the direction

of the bend depends on conditions, but generally microwaves are bent with the

curvature of the earth and will therefore propagate farther than the optical line of sight.

Refraction Before proceeding, a brief discussion of refraction is warranted. Refraction

occurs because the velocity of an electromagnetic wave is a function of the density

of the medium through which it travels. In a vacuum, an electromagnetic wave (such as

light or a radio wave) travels at approximately 3 X 108 m/s. This is the constant, c,

commonly referred to as the speed of light, but actually referring to the speed of light

in a vacuum. In air, water, glass, and other transparent or partially transparent media,

electromagnetic waves travel at speeds less than c.

When an electromagnetic wave moves from a medium of one density to a

medium of another density, its speed changes. The effect is to cause a one-time

bending of the direction of the wave at the boundary between the two media. This is

illustrated in Figure 5.6. If moving from a less dense to a more dense medium, the

wave will bend toward the more dense medium. This phenomenon is easily

observed by partially immersing a stick in water. The result will look much like

Figure 5.6, with the stick appearing shorter and bent.

The index of refraction of one medium relative to another is the sine of the

angle of incidence divided by the sine of the angle of refraction. The index of refraction

is also equal to the ratio of the respective velocities in the two media. The

absolute index of refraction of a medium is calculated in comparison with that of a

vacuum. Refractive index varies with wavelength, so that refractive effects differ for

signals with different wavelengths.

Although Figure 5.6 shows an abrupt, one-time change in direction as a signal

moves from one medium to another, a continuous, gradual bending of a signal will

occur if it is moving through a medium in which the index of refraction gradually

changes. Under normal propagation conditions, the refractive index of the atmosphere

decreases with height so that radio waves travel more slowly near the ground than at