0195136047.pdf

670 COMMUNICATION SYSTEMS

transmitting in the VHF and UHF bands have their antennas mounted on high towers to achieve a

broad coverage area. A television antenna mounted on a tower of 1200 feet in height (=h) provides

a coverage of aboutd=

√

2 h∼=50 miles. Microwave radio relay systems (for telephone and

video transmission at about 1 GHz) also have antennas mounted on tall towers.

In the VHF and UHF bands, thermal noise and cosmic noise, picked up by the antenna, become

predominant. Above 10 GHz in the SHF band, atmospheric conditions (such as precipitation and

heavy rains) play a major role in signal propagation. In the infrared and visible light regions of

the electromagnetic spectrum, LOS optical communication in free space is being experimented

with for satellite-to-satellite links.

A good understanding of a communication system can be achieved by studying electro-

magnetic wave propagation (via transmission lines and antennas), and modulation as well as

demodulation involved in analog and digital communication systems. Toward that end, this chapter

is divided into three sections. Since the wave concepts that apply to transmission lines are easily

understood, the first section deals with waves, transmission lines, and antenna fundamentals.

Then we go on to discuss analog and digital communication systems in Sections 15.2 and 15.3,

respectively.

15.1 Waves, Transmission Lines, Waveguides, and Antenna Fundamentals

In basic circuit theory we neglect the effects of the finite time of transit of changes in current and

voltage and the finite distances over which these changes occur. We assume that changes occur

simultaneously at all points in the circuits. But there are situations in which we must consider the

finite time it takes for an electrical or magnetic wave to travel and the distance it will travel. It is

in these situations that one must employtraveling-wave theory. Traveling-wave concepts must

be used whenever the distance is so great or the frequency so high that it takes an appreciable

portion of a cycle for the wave to travel the distance.

For sinusoidal signals, awavelengthλis defined as the distance that a wave travels in one

cycle or period. Since electric waves in free space travel at the velocity of lightc(∼= 3 × 108 m/s),

the free-space wavelength is given byc/f. Table 15.1.1 shows some free-space wavelengths at

selected frequencies. If the traveling-wave technique is to be employed for distances greater than

1/10 wavelength, a distance of 3 mm at 10 GHz would require the use of this technique, whereas

the same distance at 100 MHz would not. On the other hand, a distance of 1 km is insignificant

at power-line frequencies, but not in the broadcast band.

The connection of the high-power output of a transmitter located on a building to the

transmitting antenna on a tower is often made by special conductors calledtransmission lines,

which guide the waves and usually consist of two or more parallel conductors, which are separated

by insulating (dielectric) materials. While transmission lines are available in many forms, Figure

15.1.1 illustrates cross sections of some common types. The two-wire line of Figure 15.1.1(a) is

TABLE 15.1.1Free-Space Wavelengths at Selected Frequencies

Application Frequency Wavelength

Power transmission 60 Hz 5000 km

Voice 1 kHz 300 km

Broadcast band 1 MHz 300 m

FM, television 100 MHz 3 m

X-band radar 10 GHz 3 cm