0195136047.pdf

668 COMMUNICATION SYSTEMS

Designation Transmission media Applications

1015 Hz

1015 Hz

10 −^6 m

Ultraviolet

Visible light

Experimental

digital

telecommunication

Optical

fibers

Laser

beams

Infrared

100 GHz

10 GHz

1 GHz

100 MHz

10 MHz

1 MHz

100 kHz

10 kHz

1 kHz

1 cm

10 cm

Millimeter

waves

Superhigh

frequency

(SHF)

1 m

Ultrahigh

frequency

(UHF)

10 m

Very high

frequency

(VHF)

100 m

High

frequency

(HF)

1 km

Medium

frequency

(MF)

10 km

Low

frequency

(LF)

100 km

Very low

frequency

(VLF)

Audio

band

Experimental

Navigation

Satellite to satellite

Microwave relay

Earth to satellite

Radar

UHF TV

VHF TV and FM

Mobile, Aeronautical

Mobile radio

Business

Amateur radio

International

Citizens band

AM broadcasting

Aeronautics

Submarine cable

Navigation

Transoceanic radio

Telephone

Telegraph

Waveguide

Wavelength Coaxial Frequency

cable

Wire pairs

Microwave

radio

Long-wave

radio

Short-wave

radio

Figure 15.0.1Frequency bands of the electromagnetic spectrum. (Source:A. Carlson,

Communication Systems,3rd ed., McGraw-Hill, New York, 1986.)

in direct proportion to the power of light striking on the photodiode. Optical fiber channels are

replacing nearly all wire-line channels in the telephone network.

Wireless electromagnetic channelsare used in wireless communication systems, in which

the electromagnetic energy is coupled to the propagation medium through an antenna that serves

as a radiator. The physical size and configuration of the antenna depend mainly on the frequency

of operation. For example, a radio station transmitting AM frequency band of 1 MHz (with

a corresponding wavelength ofλ = c/fc = 300 m) requires an antenna of at least 30 m

(approximately one-tenth of the wavelength).

The mode of propagation of electromagnetic waves in free space and atmosphere may be

subdivided into three categories: