Electronics · Ch 7 — Wireless Communication

Propagation of radio waves

7.2

Propagation of radio waves

Propagation refers to the forward travel of radio waves from the transmitter to the receiver. After leaving the transmitting antenna, a radio wave can reach the receiver by one of three basic paths:

(a) the ground wave,
(b) the space wave, and
(c) the sky wave.

The complete electromagnetic spectrum is shown in Figure 7.2, and Table 7.2 classifies the various radio-frequency bands together with the services that use each of them. Which path a signal takes depends mainly on its frequency band.

Troposphere

The troposphere is the layer of atmosphere lying immediately on the surface of the earth. It holds roughly 75% of the total gaseous mass of the atmosphere and almost all of its moisture, dust particles and winds. It reaches up to an average altitude of about 15 km, and most ground-wave and space-wave communication takes place within it.

(a) Ground waves

A radio wave that is transmitted along, or reflected from, the surface of the earth is called a ground wave or surface wave. Ground waves use the earth itself as a kind of transmission line. The direction of the electric field — horizontal or vertical — decides the polarization of the e.m. wave; a vertically polarized wave, one whose electric field $\vec{E}$ is perpendicular to the earth's surface, can propagate as a ground wave.

The LF and MF bands travel by ground-wave propagation, covering distances from a few hundred to a few thousand kilometres at these low frequencies. As the wave moves over the earth's surface its strength steadily falls, because energy is absorbed by the earth, until finally the electric field $\vec{E}$ is reduced to zero (Figure 7.3).

The conductivity of the earth's surface increases with dampness or wetness — a water surface conducts better than dry land — so ground-wave propagation is especially useful in marine communication, that is, ship-to-shore and shore-to-ship links.

(b) Space waves

Space waves, also called line-of-sight waves, are very useful in VHF propagation. The direct wave together with the ground-reflected wave are jointly called the space wave (Figure 7.4). …

Figure 1The electromagnetic spectrum from radio waves through infrared, visible light, ultraviolet, X-rays and gamma rays, with the radio bands ELF to EHF and their frequencies.

Fig. 1 — The electromagnetic spectrum from radio waves through infrared, visible light, ultraviolet, X-rays and gamma rays, with the radio bands ELF to EHF and their frequencies.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Corresponds to Figure 7.2: a horizontal frequency axis showing wavelengths from about $10^{7}$ m down to $10^{-5}$ m and beyond, the radio bands (ELF, VLF, LF, MF, HF, VHF, UHF, SHF, EHF) with frequencies from 30 Hz up to 300 GHz, and the higher-frequency regions (infrared, visible light, ultraviolet, X-rays, gamma and cosmic rays). It places …

Table 2Radio frequency bands and their uses (Table 7.2)

Frequency	Name of the frequency band	Uses
3 – 30 KHz	Very Low Frequency (VLF)	Long distance point to point communication
30 KHz – 300 KHz	Low Frequency (LF)	Navigation services
300 KHz – 3 MHz	Medium Frequency (MF)	Radio broadcasting, ship to shore communication, Police (walkie-talkie)
3 MHz – 30 MHz	High Frequency (HF)	International broadcasting, telephone, aviation, all classes of communication
30 MHz – 300 MHz	Very High Frequency (VHF)	RADAR, TV, FM broadcasting, short distance communication
300 MHz – 3 GHz	Ultra High Frequency (UHF)	TV, RADAR, aviation

Figure 3Ground-wave propagation: waves radiating from a vertical antenna and following the curved earth surface until the electric field falls to zero.

Fig. 3 — Ground-wave propagation: waves radiating from a vertical antenna and following the curved earth surface until the electric field falls to zero.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Reproduces Figure 7.3. The upper part shows ground waves spreading from a vertical antenna and bending along the earth's curvature, with the field reduced to $\vec{E} = 0$ at the far ends; the lower part shows successive wavefronts tilting more and more as they lose energy to the earth. It e …

Formula 4Radio horizon in terms of the optical horizon

$RH \approx \frac{4}{3}\,OH$ The radio horizon (RH) is about four-thirds of the optical horizon (OH), because radio waves bend slightly around the earth and so reach a little farther than the naked eye can see; for the earth …

Figure 5Space-wave propagation: a direct wave and a ground-reflected wave travelling together between a transmitter antenna and a receiver antenna.

Fig. 5 — Space-wave propagation: a direct wave and a ground-reflected wave travelling together between a transmitter antenna and a receiver antenna.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Corresponds to Figure 7.4: a direct wave travels straight from the transmitting antenna to the receiving antenna, while a second path reflects off the earth; together they form the space wav …

Figure 6Space-wave propagation showing the optical horizon and the larger radio horizon between a transmitting and a receiving antenna over the curved earth.

Fig. 6 — Space-wave propagation showing the optical horizon and the larger radio horizon between a transmitting and a receiving antenna over the curved earth.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Reproduces Figure 7.5: the straight space-wave path between the Tx and Rx antennas, with two curved arcs marking the optical horizon (OH) and the farther radio horizon (RH). It shows visually that radio wa …

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