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1 ransmzsszon Systems The basic line transmission theory that we have considered so far, together with the principles of analogue and digital signal transmission over pairs of electrica

1 ransmzsszon

Systems

The basic line transmission theory that we have considered so far, together with the principles of analogue and digital signal transmission over pairs of electrical wires, is quite suitable for short range conveyance of a small number of circuits However, it is not always practical or economic

to use many multiple numbers of physical ‘pairs’ between exchanges, so we have recourse to frequency division and time division multiplexing to reduce the number of physical pairs of wires needed to convey a large number of long haul circuits between common end-points There are, of course, different transmission media to be considered, some of them much better suited to particular applications than plain electric wire They include coaxial cable, radio, satellites and optical fibre In this chapter we discuss the main features of each of these transmission types, to ease the decision about which one is best suited to any particular application

by the circuit are a direct match with those of the audio signal they represent

In other words, the electrical signal frequencies are in the audio range; the signal has not

typical audio circuit configuration, Figure 8.1 shows a four-wire repeatered circuit as

already discussed in Chapter 3 on long haul communication

Audio circuits may be used to carry any of a wide range of bandwidths, although in general the greater the bandwidth to be carried, the shorter the maximum range of the circuit, because the electrical properties o f the line (its impedance, reactance and

capacitance) tend to interfere It is known that high frequency signals are carried only

141

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

Repeater

Figure 8.1 A four-wire repeatered 'audio circuit'

by the outer surface of wire conductors (the skin e f e c t ) , and this makes high frequency

signals prone to interference from electrical and magnetic fields emanating from adjacent wires

telephone junction circuits of 4 kHz bandwidth between exchanges over a range up to about 20 km, and amplification can be used to extend this range A typical use of an

audio circuit is to connect an analogue local (or end ofice or central ofice) exchange to

are used, economics tend to favour other transmission means As an example, an un- amplified pair of wires using 0.63 mm diameter conductors has an insertion loss (i.e a line loss which must be added to other circuit losses) of 3 dB at a range of 7 km The transmission range for speech is increased to 33 km by the use of an amplifier, but this range is usually precluded by the line quality needs of the signalling system

As also explained in Chapter 3, the rate of signal degradation (i.e attenuation and distortion) in tlvisted pair transmission media can be reduced by loading the cable (see Chapter 3) and more simply, by increasing the diameter (gauge) of the wires them-

selves Bigger conductors cause less signal attenuation because they have less resistance However, the relative cheapness of alternative transmission media makes it uneconomic

At this distance the conductors of an unamplified circuit need to be around 1 mm thick,

proportional to the square of the diameter the cost escalates rapidly as the cable gauge is increased On a more cheerful view, the converse also applies, so that at shorter distances

operator has extensive information available to help him choose the right conductor (copper, aluminium, etc.), and the appropriate gauge of wire, thereby cutting costs and still giving the transmission performance required

Because they are susceptible to external electro-magnetic interference, audio circuits

wires Audio circuits are most commonly found in small scale networks, and within the

local area of a telephone exchange A number of twisted pairs are normally packed together into large cables A cable may vary in size from about 3 mm diameter to about

STANDARD TWISTED PAIR CABLE TYPES FOR INDOOR USE 143

80mm, carrying anything between one and several thousand individual pairs Thus, running out from a 10 000 line local exchange, a small number (say 10-20) of main backbone cables of several hundred or a thousand pairs may run out along the street conduits to streetside cabinets where a simply cross-connect frame is used to extend the pairs out in a star fashion on smaller cables (say 25 or 50 pairs) Finally, at the

distribution point individual pairs of wires may be crimped within a cable joint (at the

top of a telegraph pole) and run out from here to individual customer’s premises

8.2 STANDARD TWISTED PAIR CABLE TYPES FOR INDOOR USE

For indoor use in conjunction with structured cabling systems (Chapter 45), there are a

number of standardized unshielded twisted pair ( U T P ) and shielded twisted pair ( S T P )

cabling types Table 8.1 provides a brief overview of them

8.3 TRANSVERSE SCREEN AND COAXIAL CABLE TRANSMISSION

A considerable problem with twistedpairs is signal distortion caused by the skin efSect

The interference may be manifested as noise or ‘hum’ on the line, or in extreme cases a stronger voice or other signal may be induced from adjacent wire pairs, causing an

effect known as crosstalk The likelihood of such interference is increased when higher

the interference The first is to screen the cable with a light metal foil, usually wound

into the cable sheath This is called screened or shielded cable In addition, transverse

screen cable also includes screens between wires within the cable, isolating transmit from receive pairs A second type of cable not prone to electromagnetic interference of

this kind is coaxial cable This is constructed so that most of the signal is carried by an electromagnetic field near the centre of the cable Coaxial cable was the workhorse of

Table 8.1 Standardized UTP and STP (horizontal) cabling types

~~

Category 1 (Cat 1) and Category 2 (Cat 2) voice and low speed data

Category 3 (Cat 3)

Category 4 (Cat 4)

specification allows use up to 16 MHz, typically allowing use for combined voice and ethernet or

4 Mbit/s token ring LANs

specification allows use up to 20 MHz, allowing for 16Mbit/s token ring LAN use as well as voice

specification allows use up to 100 MHz, allowing use for voice or data up to lOOMbit/s

Category 5 (Cat 5 )

frequency division multiplex (FDM), high bandwidth analogue and early digital

transmission systems Now optical fibre is supplanting it for new digital lines

Instead of a twisted pair of wires, a coaxial cable consists of two concentric con-

ductors, made usually of aluminium or copper The central wire, or pole, is separated from

the cylindrical outer conductor by a cylindrical spacing layer of insulation, as shown in

Figure 8.2

The outer conductor is a metal foil or mesh, wound spirally around the insulation

layer, and outside this is a layer of sheathing to provide external insulation and physical

protection for the cable

A single coaxial cable is equivalent to a single pair of twisted wires To achieve the

equivalent of four-wire transmission, two coaxial cables must be used, one to carry the

transmit signal and one to carry the receive signal

Coaxial cables function in much the same way as twisted pair circuits, but there are

some important differences in performance Unlike twisted pair audio circuits, coaxial

cable circuits are said to be unbalanced By this we mean that the two conductors in a

coaxial cable do not act equally in conveying the signals In a balanced circuit such

as a twisted pair, where the signal is carried by the electrical currents in the two wires

passing in opposite directions, the currents generate equal but opposite electromagnetic

fields around them which tend to cancel each other out By contrast, the signal in a

coaxial cable is carried largely by the electromagnetic field surrounding the inner and

outer conductors, which is largely induced by the central conductor The outer

conductor, sometimes called the shield, is usually operated at earth or ground voltage,

and prevents the electromagnetic field from radiating outside the cable sheath The

signal is thereby protected from interference to some extent

Unfortunately the overall rate of signal attenuation is greater on unbalanced circuits

like coaxial cables than o n balanced ones such as twisted pairs, and this needs to be

counteracted by the use of amplifiers On the other hand, unlike twisted pair circuits

which are liable to lose balance as the result of damp or damage, coaxial cables are

reflections which sometimes affect twisted pairs Coaxial cables are more reliable in

service, are easier to install, and they require less maintenance Further, because of their

insensitivity to electromagnetic interference they perform better than twisted pairs when

the cable route passes near metal objects or other electrical cables

\ Protective sheath

Outer conductor (mesh or spiral band)

/ (cylindrical s p o c e r )

Insulation

Central conductor

or pole Figure 8.2 Coaxial cable

FREQUENCY DIVISION MULTILEXING (FDM) 145

8.4 FREQUENCY DIVISION MULTIPLEXING (FDM)

FDM, or frequency division multiplexing is a means of concentrating multiple analogue

circuits over a common physical four-wire circuit It was used predominantly in the ana-

logue era of telecommunication before glass fibres and digital techniques were available

Sophisticated equipment at the two ends of a long distance four-wire transmission line

are used to break up the available signal bandwidth into many different and independent

speech of other analogue signal circuits In effect, the multiplexing equipment multiplies

many times the number of circuits which may be carried, thus saving the expense of

multiple long distance transmission lines

long distance coaxial transmission lines F D M equipment remains in widespread use,

but the advent of cheaper and higher quality digital transmission alternatives (in

particular, TDM or time division multiplexing) is leading to its obsolescence

F D M allows many analogue transmission circuits (or channels) to share the same

physical pair of wires or other transmission medium It requires sophisticated and expen-

sive transmission line terminating equipment and a high quality line, but it has the

potential for overall cost saving because the number of wire pairs between the endpoints

can be reduced The longer the length of the line, the greater the overall saving

Typical transmission systems used in conjuntion with FDM are coaxial cables and

analogue trunk radio systems With analogue satellite systems, a slight variant of the

technique is used, FDMA or frequency division multiple access This uses a similar

technique, but in addition it allows multiple earth stations to communicate using a

single satellite

carriage of multiple telephone channels has a channel bandwidth of 4 kHz This

comprises the 3.1 kHz needed for the speech itself, together with some spare bandwidth

on either side which buffers the channel from interference by its neighbouring channels

The speech normally resides in the bandwidth between 300Hz and 3400Hz

In order to prepare the baseband signal which forms the basic building block of an

F D M system, each of the individual input speech circuits are filtered to remove all the

signals outside of the 300-3400Hz band The resulting signal is represented by the

baseband signal (schematically a triangle) shown in Figure 8.3

amplitude

A

carrier original signal frequency

baseband lower sideband upper sideband

Figure 8.3 The principle of FDM, frequency shifting by carrier modulation

circuit 1

.”

circuit 12

channel translating equipment

group one four-wire circuit circuit 13

channel translating equipment

GTE

group translating equipment

supergroup (of 60 tributary channels) one four-wire

circuit (typically coax)

Figure 8.4 Circuit multiplexing using FDM

The next step is that the signal is frequency shifted by modulating the baseband signal with one of a number of standard carrier signals The frequency of the carrier signal

used will be equal to the value of the frequency shift required In our diagram, a carrier

signal, the lower sideband and the upper sideband All the original information from

the baseband signal is held by each of the sideband images, so it is normal during the

sideband having first suppressed the carrier (suppressed carrier mode; note, however, that suppressed carrier mode, while common for line systems is not common in radio systems as it is often inconvenient to make a carrier signal generator available at the

receiver) Demodulation of the signal into its tributary channels is a similar process to

that of modulation

F D M is usually conducted in a heirarchical manner as shown in Figure 8.4 The heirarchy allows progressively more circuits to be multiplexed onto the same line Of course, for each further stage of multiplexing a further line quality and bandwidth threshold must be exceeded for the system to operate reliably The various different

stages of the F D M hierarchy are achieved by using different translating equipments (the

modulating and demodulating device), as we discuss next

The carrier frequencies used to in a CTE to create a basic group are 64 kHz, 68 kHz,

7 2 kHz, , 108 kHz, and each of the lower sidebands is used The resulting group frequency pattern is as shown in Figure 8.5

This is the basic building block when further multiplexing into supergroup, but when

sending to line it is normal to frequency shift the group into the lowest frequency range

FREQUENCY DIVISION MULTILEXING (FDM) 147

by the use of a group modulating frequency of 120 kHz This brings the signal into the frequency range 12-60 kHz (Figure 8.6)

The further multiplexing steps are listed briefly in Table 8.2

F D M systems formed the backbone of public telephone networks until the mid-1970s when digital transmission and pulse code modulation took over as the cheaper and higher quality alternative FDM was commonly used on the long distance and inter-

national links between trunk ( t o l l ) telephone exchanges The transmission lines were

typically either coaxial cable or analogue radio Although the technique is now falling into disuse for optical fibre and copper line systems, it is likely to remain important

quency division multiple access) is a very effective way of sharing radio bandwidth between multiple communicating endpoints We return to the subject of radio later in the chapter

Table 8.2 FDM hierarchy

CTE channel translating 12 voice channels Group

8.5 HDSL (HIGH BIT-RATE DIGITAL SUBSCRIBER LINE) AND

ADSL (ASYMMETRIC DIGITAL SUBSCRIBER LINE)

Although modern optical fibre cable (discussed later in this chapter) has meant that copper pair and coaxial cabling has to some extent fallen out of fashion, the huge existing base of such cabling has caused recent development effort to be concentrated

noteworthy methods have emerged These are H D S L (high bitrate digital subscriber line) and A D S L (asymmetric digital subscriber line)

H D S L uses two or three copper pairs to provide a full duplex 2Mbit/s access line, enabling high bitrate business applications to operate over existing copper lineplant

A D S L , meanwhile, provides a high bitrate of 4 Mbit/s or 6 Mbit/s in the downstream

channel (e.g for broadcasting a video signal to residential customers) with, say, a

64 kbit/s upstream channel for control of the received signal Likely uses of A D S L are

education) and other interactive multimedia services We return to HDSL and ADSL in

8.6 OPTICAL FIBRES

Most transmission types can be used to support either analogue or digital signal transmission: twisted pair circuits, coaxial cables and radio systems can all be used as the basis of either analogue or digital line transmission systems However, the fact that digital transmission requires only two distinct line states (on/oR mark/space) has opened up a new wider range of digital propagation methods; and the most important new digital transmission medium is optical fibre

The importance of optical fibres and their extensive use stems from their extremely high bit-rate capacity and their low cost Optical fibres are a hair’s breadth in diameter and are made from a cheap raw material: glass They are easy to install because they are small, and because they allow the repeaters to be relatively far apart (well over 100 km spacing is possible) they are easy to maintain

An optical fibre conveys the bits of a digital bit pattern as either an ‘on’ or an ‘off’ state of light The light (of wavelength 1.3 or 1.5 pm) is generated at the transmitting

end of the fibre, either by a laser, or by a cheaper device called a light emitting diode (LED for short) A diode is used for detection The light stays within the fibre (in other words is guided by the fibre) due to the reflective and refractive properties of the outer skin of the fibre, which is fabricated in a ‘tunnel fashion’

Fibre-optic technology is already into its third generation In the first generation,

fibres had two cylindrical layers of glass, called the core and the cladding In addition a

cover or sheath of a different material (say plastic) provided protection The core and the cladding were both glass, but of different refractive index A step in the refractive

index existed at the boundary of core and cladding, causing reflection of the light rays

at this interface, so guiding the rays along the fibre Unfortunately however, because of

the relatively large diameter of the core, a number of different light ray paths could be

OPTICAL FIBRES 149

Figure 8.7 An optical fibre cable, showing clearly the overall construction At the centre, a steel twisted wire provides strength, enabling the cable to be pulled through ducts during installation Around it are ten plastic tubes, each one containing and protecting a hair’s breadth fibre, each capable of carrying many megabits of information per second (Courtesy of British Telecom)

dispersion The different overall ray paths would be of different lengths and therefore take different amounts of time to reach the receiver The received signal is therefore not

as sharp; it is dispersed It is most problematic when the user intends very high bit rate

operation The different wave paths (or modes) explain the name step index multimode fibre which has been given to these fibres Step index multimode fibre is illustrated in

Figure 8.8(a)

A refinement of step index multimode fibre is achieved by using a fibre with a more

gradual grading of the refractive index from core to cladding A graded index multimode

fibre is shown in Figure 8.8(b) This has a slightly improved high bit rate performance

OPTICAL FIBRES 151

over step-index multimode fibres Finally, monomode,fibres (illustrated in Figure 8 8(c)

are the third generation of optical fibre development, and have the best performance In

monomode fibre, more advanced fibre production techniques have produced a very narrow core area in the fibre, surrounded by a cladding area; there is a step change in the refractive index of the glass at the boundary of the core and the cladding The narrow

core of a monomodefihre allows only one of the ray paths (or modes) to exist As a result there is very little light pulse dispersion in monomode fibre, and much higher bit rates

can be carried

The qualities of different types of optical fibre are laid out in ITU-T recommendations

optimized for operation at 1550 nm All the types of cable may be used for both 13 10 nm

For in-building and campus cabling of office, industrial or university sites it is now

commonplace to use optical fibre at least in the building risers (the conduits between floors or buildings) The cable generally used contains multimode fibre, usually suitable

for transmission between relatively cheap, LED transmitting devices over distances up

to 3 km or 15 km Although the multimodejbre itself is nowadays more expensive than monomode fibre, this is usually compensated for by the much cheaper end device costs

In contrast, monomodejbre is the preferred technology of network operators, and this is the cable mainly deployed for metropolitan, national and undersea usage The

despite the more complex manufacture The main benefit is the very high bitrates achievable and the lengthy inter-repeater distances The disadvantage is the need for

expensive laser devices as transmitters to produce the high quality single mode (single

wavelength) light required

An important factor in the design and installation of optical fibre networks is the careful jointing or interconnection of fibre sections The reflections caused at the joint, and the losses caused by slight misalignment of the two fibre cores can lead to major losses of signal strength The latest generation of cabling splicing devices and cable connectors have improved real network performance dramatically in recent years The

and equipment connections, for example, has been designed to ensure correct alignment

of the fibres The SC-connector specially developed for ATM (asynchronous transfer mode, see Chapter 26) also shares the ability for exact alignment

Recent development in optical fibre technology has concentrated on achieving longer operational life of installed cable, without the need to replace ‘active’ components One

of the problems with the original systems was the use of optical signal regenerators specific to a given light wavelength and transmission bitrate This means that the early optical fibres cannot easily be upgraded in bitrate without major re-investment in regenerators and other active components For this reason, considerable effort has been put into optical amplifiers and regenerators which work without reconverting the signal

to electrical form and then re-creating a new light signal for onward transmission Such amplifiers can be built to give greater range of optical wavelength and bitrate Bitrate, wavelength or multiplexing upgrades can thus to some extent be limited to the end

devices, giving scope for new developments like tcavelengtlz division multiplexing ( W D M ) and passive optical networks, which we discuss next

8.6.1 Wavelength Division Multiplexing (WDM)

Wave division multiplexing ( W D M ) is the use of a single fibre to carry multiple optical

signals Thus, for example, two signals at 1300 nm and 1500 nm light wavelength could

carry independent 155 Mbit/s digital signals over the same fibre In this way, transmit

and receive signals could be consolidated to a single fibre, rather than requiring separate

fibres (Figure 8.9) Alternatively, the capacity of two existing fibres can be doubled

8.6.2 Broadband Passive Optical Network (BPON)

Broadband passive optical networks (BPONs) are intended for deployment in customer

and customer premises Here the challenge is to achieve maximum efficiency in the use

of local lineplant, creating the effect of a star-topology (direct connection of each

twisted coaxial cable

ORS = optical repeater station

OSH = optical splitter head

Figure 8.10 An optical fibre access network

customer to the exchange) without needing thousands of fibres and also giving the potential of signal broadcasting (for cable television for example) to each customer over

a common cabling infrastructure Figure 8.10 illustrates a typical optical fibre access

network Optical repeater stations are coupling units for boosting signal strength or for

connecting various fibre rings, meanwhile splitters are passive components which use an optical cavity of the appropriate length to divide the incoming signal between two

fibres The optical nodes of Figure 8.10 are the receiving devices

8.7 RADIO

electricity and light, are forms of electromagnetic radiation; the energy is conveyed by

waves of magnetic and electrical fields In a wire, these waves are induced and guided

by an electrical current passing along an electrical conductor, but that is not the only

way of propagating electromagnetic ( E M ) waves By using a very strong electrical signal as a transmitting source, an electromagnetic wave can be made to spread far and wide through thin air That is the principle of radio The radio waves are produced by

examples of which are

0 troposheric scatter antenna

Radio is a particularly effective means of communication between remote locations and over difficult country, where cable laying and maintenance is not possible or is pro- hibitively expensive It is also effective as a means of broadcasting the same information

to multiple receivers and cost-effective for low volume use-sharing resources between many users

technique of modulation in radio is very similar to that used in FDM, and single sideband ( S S B ) operation, as we discussed earlier in the chapter is common, though the

carrier is rarely suppressed A distinctive feature of a radio carrier signal is its high frequency relative to the frequency bandwidth of the information signal The frequency

of the carrier needs to be high for it to propagate as radio waves

The modulation of the radio carrier signal may follow either an analogue or a digital regime Analogue modulation is carried out in a similar way to FDM Digital modula- tion can be by ‘on/off’ carrier signal modulation (i.e by switching the carrier on and off), or by other methods such as frequency sh$t keying ( F S K ) , phase shift keying

( P S K ) or quadrature amplitude modulation ( Q A M ) , details of which are given in the next chapter

Oscillator R F (carrier Amplifier Antenna

s i g n a l generator)

20-20 k H 2 )

Figure 8.11 A simplified radio transmitter

applied to a radio antenna Amplification boosts the signal strength sufficiently for the antenna to convert the electrical current energy into a radio wave

Figure 8.1 1 illustrates a simple radio transmitter: an audio signal is being filtered (to cut out extraneous signals outside the desired bandwidth) and amplified Next it is modulated onto a radio-frequency ( R F ) carrier signal which is produced by a high

quality oscillator; then the modulated signal is filtered again to prevent possible inter- ference with other radio waves of adjacent frequency Finally the signal is amplified by

a high power amplifier and sent to the antenna where it is converted into radio waves Figure 8.12 illustrates a corresponding radio receiver, the components of which are similar to those of the transmitter shown in Figure 8.1 1 The radiowaves are received by

the antenna and are reconverted into electrical signals A filter removes extraneous and

Antenna Oscillator

Filter Demodulator Amplifier Equalizers Output

s i a n a l Figure 8.12 A simplified radio receiver

RADIO WAVE PROPAGATION 155

interfering signals before demodulation As an alternative to a filter, a tuned circuit

could have been used with the antenna A tuned circuit allows the antenna to select

which radio wave frequencies will be transmitted or received

to the original carrier frequency, leaving only the original audio or information signal

A cruder and cheaper method of demodulation, not requiring an oscillator, employs a

rectif)ing circuit This serves to have the same subtracting effect on the carrrier signal After demodulation, the information signal is processed to recreate the original audio signal as closely as possible The signal is next amplified using an amplifier with

automatic gain control ( A G C ) to ensure that the output signal volume is constant, even

if the received radio wave signal has been subject to intermittent fading Finally, the

frequency distortion by devices called equalizers We will consider the cause and cure of these distortions in Chapter 33

The simplified transmitter and receiver shown in Figures 8.1 l and 8.12 could be used

together for simplex or one-way radio transmission Such components in use are thus

equivalent to one pair of a four-wire transmission line discussed in Chapter 3 For full

duplex or two-way transmission, equivalent to a four-wire system, the equipment must

be duplicated, with two radio transmitters and two receivers, one of each at each end of the radio link Two slightly different carrier frequencies are normally used for the two directions of transmission, to prevent the two channels interfering with one another Alternatively, by duplicating senders and receivers at each end of the link, but using

only one radio channel, half-dup1e.x operation could be used (communication in both

directions, but in only one direction at a time)

8.8 RADIO WAVE PROPAGATION

When radio waves are transmitted from a point, they spread and propagate as spherical wavefronts The wavefronts travel in a direction perpendicular to the wavefront, as shown in Figure 8.13

Figure 8.13 Radio wave propagation

Slght-llne path

Transmitter Recelver

Figure 8.14 Different modes of radio wave propagation

Radiowaves and lightwaves are both forms of electromagnetic radiation, and they

display similar properties Just as a beam of light may be reflected, refracted (i.e slightly

bent), and diffracted (slightly swayed around obstacles), so may a radio wave, and

Figure 8.14 gives examples of various different wave paths between transmitter and

receiver, resulting from these different wave phenomena

transmission system is normally designed to take advantage of one of these modes The

four modes are

0 line-ofsight propagation

0 surface wave (diffracted) propagation

e tropospheric scatter (reflected and refracted) propagation

0 skywave (refracted) propagation

A line-of-sight ( L O S ) radio system (microwave radio above about 2 GHz) relies on the

fact that waves normally travel in a straight line The range of a line-of-site system is

limited by the effect of the earth’s curvature, as Figure 8.14 shows Line-of-sight

systems therefore can reach beyond the horizon only when they have tall masts Radio

systems can also be used beyond the horizon by utilizing one of the other three radio

propagation effects also shown in Figure 8.14

Surface waves have a good range, dependending on their frequency They propagate

by diffraction using the ground as a waveguide Low frequency radio signals are the best

suited to surface wave propagation, because the amount of bending (the effect properly

called dzflraction) is related to the radio wavelength The longer the wavelength, the

greater the effect of diffraction Therefore the lower the frequency, the greater the

scatter This is a form of radio wave reflection It occurs in a layer of the earth’s

atmosphere called the froposhere and works best on ultra high frequency (UHF) radio

waves The final last example of over-the-horizon propagation given in Figure 8.14 is

RADIO ANTENNAS 157

Refroctlon

ongle 9 >angle +

Figure 8.15 Refraction causing skywaves

radiowaves by the earth’s atmosphere, and it occurs because the different layers of the

earth’s upper atmosphere (the ionosphere) have different densities, with the result that

radiowaves propagate more quickly in some of the layers than in others, giving rise to wave deflection between the layers, as Figure 8.15 shows in more detail However, as Figure 8.15 also demonstrates, not all waves are refracted back to the ground; some escape the atmosphere entirely, particularly if their initial direction of propagation relative to the vertical is too low (angle 4)

8.9 RADIO ANTENNAS

Each individual radio system is required to perform slightly different tasks The

distinguishing qualities of a radio system are

0 its range

0 its transmit and receive signal power

0 its directionality (i.e whether the transmitted signal is concentrated in a particular direction or not)

0 its mode of use (e.g point-to-point or point-to-multipoint)

These qualities are determined by the mode of propagation for which the system is designed (e.g surface wave, tropospheric scatter, line-of-sight, skywave, etc.), by the frequency of the radio carrier frequency, by the power of the system and most particularly by the suitability of the transmitting and receiving antennas Antenna design has a significant effect on radio systems’ range, directionality and signal power