Microwave Linking

8/8/2019 Microwave Linking

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CHAPTER 1

MICROWAVE RADIO RELAY AND LINKS

Microwave transmission refers to the technology of transmitting information by the use of

the radio waves whose wavelengths are conveniently measured in small numbers

of centimeters, by using various electronics technologies. These are called microwaves. This

part of the radio spectrum ranges across frequencies of roughly 1.0 GHz to 30 GHz. Also by

using the formula = c/f, these correspond to wavelengths from 30 cm down to 1.0 cm.

1.1 - MICROWAVE RELAYS :

Microwave radio relay is a technology for transmitting digital and analog signals, such

as long-distance telephone calls and the relay of television programs to transmitters, between

two locations on a line of sight radio path. In microwave radio relay, radio waves are

transmitted between the two locations with directional antennas, forming a fixed radio

connection between the two points. Long daisy-chained series of such links form

transcontinental telephone and/or television communication systems.

HOW MICROWAVE LINKS ARE FORMED:

Because a line of sight radio link is made, the radio frequencies used occupy only a narrow

path between stations (with the exception of a certain radius of each station). Antennas used

must have a high directive effect, these antennas are installed in elevated locations such as

large radio towers in order to be able to transmit across long distances. Typical types of

antenna used in radio relay link installations are parabolic reflectors, shell

antennas and horn radiators, which have a diameter of up to 4 meters. Highly directive

antennas permit an economical use of the available frequency spectrum, despite long

transmission distances.

PLANNING CONSIDERATIONS:

Because of the high frequencies used, a quasi-optical line of sight between the stations is

generally required. Additionally, in order to form the line of sight connection between the


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two stations, the first Fresnel zone must be free from obstacles so the radio waves

can propagate across a nearly uninterrupted path. Obstacles in the signal field cause

unwanted attenuation, and are as a result only acceptable in exceptional cases. High mountain

peak or ridge positions are often ideal.

Obstacles, the curvature of the Earth, the geography of the area and reception issues arising

from the use of nearby land (such as in manufacturing and forestry) are important issues to

consider when planning radio links. In the planning process, it is essential that "path profiles"

are produced, which provide information about the terrain and Fresnel Zones affecting the

transmission path. The presence of a water surface, such as a lake or river, in the mid-path

region also must be taken into consideration as it can result in a near-perfect reflection (even

modulated by wave or tide motions), creating multipath distortion as the two received signals

("wanted" and "unwanted") swing in and out of phase. Multipath fades are usually deep only

in a small spot and a narrow frequency band, so space and frequency diversity schemes were

usually applied in the third quarter of the 20th century.

The effects of atmospheric stratification cause the radio path to bend downward in a typical

situation so a major distance is possible as the earth equivalent curvature increases from

6370 km to about 8500 km (a 4/3 equivalent radius effect). Rare events of temperature,

humidity and pressure profile versus height, may produce large deviations and distortion of

the propagation and affect transmission quality. High intensity rain and snow must also be

considered as an impairment factor, especially at frequencies above 10 GHz. All previous

factors, collectively known as path loss, make it necessary to compute suitable power

margins, in order to maintain the link operative for a high percentage of time, like the

standard 99.99% or 99.999% used in 'carrier class' services of most telecommunication

operators.

OVER HORIZON MICROWAVE RADIO RELAY:

In over-horizon, or tropospheric scatter, microwave radio relay, unlike a standard microwave

radio relay link, the sending and receiving antennas do not use a line of sight transmission

path. Instead, the stray signal transmission, known as "tropo - scatter" or simply "scatter,"from the sent signal is picked up by the receiving station. Signal clarity obtained by this

method depends on the weather and other factors, and as a result a high level of technical

difficulty is involved in the creation of a reliable over horizon radio relay link. Over horizon

radio relay links are therefore only used where standard radio relay links are unsuitable (for

example, in providing a microwave link to an island).


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USAGE OF MICROWAVE RADIO RELAY SYSTEMS:

During the 1950s the AT&T Communications system of microwave radio grew to carry the

majority of US Long Distance telephone traffic, as well as intercontinental television

network signals. The prototype was called TDX and was tested with a connection betweenNew York City and Murray Hill, the location ofBell Laboratories in 1946. The TDX system

was set up between New York and Boston in 1947. The TDX was improved to the TD2,

which still used klystrons, and then later to the TD3 that used solid state electronics. The

main motivation in 1946 to use microwave radio instead of cable was that a large capacity

could be installed quickly and at less cost. It was expected at that time that the annual

operating costs for microwave radio would be greater than for cable. There were two main

reasons that a large capacity had to be introduced suddenly: Pent up demand for long distance

telephone service, because of the hiatus during the war years, and the new medium of

television, which needed more bandwidth than radio.

Similar systems were soon built in many countries, until the 1980s when the technology lost

its share of fixed operation to newer technologies such as fiber-optic cable and optical radio

relay links, both of which offer larger data capacities at lower cost per bit. Communication

satellites, which are also microwave radio relays, better retained their market share,

especially for television.

At the turn of the century, microwave radio relay systems are being used increasingly in

portable radio applications. The technology is particularly suited to this application because

of lower operating costs, a more efficient infrastructure, and provision of

direct hardware access to the portable radio operator.

1.2- MICROWAVE LINK:

A microwave linkis a communications system that uses a beam of radio waves in the

microwave frequency range to transmit video, audio, or data between two locations, which

can be from just a few feet or meters to several miles or kilometers apart. Microwave links

are commonly used by television broadcasters to transmit programmes across a country, for

instance, or from an outside broadcast back to a studio.

Mobile units can be camera mounted, allowing cameras the freedom to move around without

trailing cables. These are often seen on the touchlines of sports fields on Steadicam systems.

Microwave links provide invaluable "free-space" networking for the telecommunications

industry. The relative ease and economy of installation has seen them deployed in an

increasing number of point-to-point and point-to-multipoint applicationsfrom


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communications backbones, to branch links and distribution networks, not to mention

applications in the broadcast industry and private enterprise. With the rise of new cellular

operators and new technologies, overall microwave network density is undeniably escalating.

Yet this intensification of microwave communications brings added challenge. The greater

the number of point-to-point links in a given area, the greater the potential for these

microwave systems to interact with one another and cause interference. Since any signal

distortion reduces the quality of service (QoS), controlling interference is now the mandate of

any radio network operator and national authority. A good starting point for consideration is

the design and location of the source of the signalthe microwave antenna (Fig.1),

(Fig.2) shows a typical radiation pattern of a microwave antenna, with the main beam at 0

deg. and sidelobes that are significant to about 90 deg. from the main beam. It is these

sidelobes, which can cause interference with adjacent point-to-point links, that must be

minimized through careful antenna design and installation.


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R are used fort o mai appli ations in radio-link antenna desi n. The firstis for

environmental protection, to coverthe antenna feed system in orderto protectit from dirt,

snow, and ice. In addition, a radome si nificantly reduces the windload of an antenna system.

However, both material selection and thickness need to be carefully considered to optimi e

the powertransmitted through a radome, while atthe same time ensuring that sidelobes are

notincreased.

Fig. 3 shows the reflection characteristics of a plane-wall radome for different materials.

Each of these materials is characteri ed by a relative dielectric constant of r= 2; however,

each material has a differentloss parameter, tan , ranging from 0.0018 (low loss) to 18 (high

loss). Figure 3 shows that forlow-loss materials, there exists two distinct minimum values of

the reflection coefficient, for which a radome wall will allow maximum transmission.

The first case of d/ ~ 0 is practically reali ed as d < /10, and leads to flexible radome

materials with typical thicknesses of 0.4 to 0.6 mmessentially as thin as is practical.

Flexible radomes are commonly used forlarger antennas (greaterthan 4 ft.), to avoid the bulk

and weight of solid radomes.

The second design case of d/ ~ 0.5 is more complex, and leads to the design of solid

radomes, which are more economical to produce at smaller si es (less than 6 ft.). The

practicalimplication of d ~ /2 is thatthe thickness of solid radomes is always dependent on


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the wavelength (hence frequency) ofthe application. Assuming a dielectric constant between

2.5 and 3, typical solid radome thicknesses for different frequencies would be 6 mm (14

GHz), 4 mm (22 GHz), and 2.4 mm (38 GHz).

This effect of frequency on solid radome design is illustrated inFig. 4, which shows return-

loss measurements over a range of frequencies for an antenna designed to operate at 23 GHz.

The red curve is the calculated difference between the separate curves for the antenna with

and without the radome: clearly, the minimum differencecorresponding to the minimum

impact ofthe radome at d ~ /2occurs at a design frequency of 22.6 GHz.

If a radome of incorrect thickness is used, the transmitted power will be reduced, and

consequently the antenna gain also reduced. Greater radio power would then be required to

achieve the desired radiated power, resulting in a corresponding increase of side-lobe

radiation. Correct radome design is critical not only for optimizing a link budget, but also for

interference control.


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Fig. 3 is valid for the "ideal" case, where wave fronts hit the wall perpendicularly. Now

consider the situation for signals not having this ideal orientation. Given the longer effective

wave path through the radome material as they hit the wall obliquely, the optimum thickness

is now also dependent on the angle of incidence (), measured as the deviation from normal.

In practice, however, angles of incidence to 20 deg. have negligible effect on the optimum

radome thickness. This is illustrated for flexible radome materials in Fig. 5, which shows the

relationship between angle of incidence () and d/ o (where o is the free-space wavelength),

for achieving 95 percent power transmission through materials with different dielectric

constants. For values of to 20 deg., the optimum thickness is barely impactedparticularly

for low-loss materials, which should be those considered for radome design purposes.

A similar relationship holds for solid radomes. These relationships have been exploited in

practical fashion by many microwave antenna designers. A small degree of tilt of the main

beamaround 5 deg.actually improvesthe performance of the antenna, by directingspurious reflections within the antenna away from the microwave feed system.


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The influence of typical thin wall or flexible radomes can be seen in Fig. 6, which compares

the radiation patterns of microwave antennas operating with and without radomes at 6.4 and

33.4 GHz. At 6.4 GHz, it is evident the radome has negligible effect on the radiation pattern.

However, at 33.4 GHz the gain of the antenna is decreased by 1 dB due to attenuation by the

radome. Once again, to achieve the same link budget the transmitted power must be increased

by 1 dB, causing a higher interference potential outside the main beam. The presence of the

radome also leads to increased side lobe levels, clearly visible in Fig. 6 at azimuth angles

between 20 and 60 deg.

This effect of the radome on the 33.4-GHz antenna is due to the fact that, at higher

frequencies, flexible radome design becomes more sensitive to the practical constraints on

material thickness and stability. Whereas the design ratio of d/ ~ 0.01 can be achieved for

the 6.4-GHz antenna, the best possible case for the 33.4-GHz antenna is just d/ ~ 0.05,

which is not as close to the ideal zero.

No matter how carefully a microwave antenna radome is designed, the potential increase in

side lobes remains. This must be taken into account during other aspects of design and

installation of the antenna to minimize interference.


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The basic "standard-performance" microwave antenna consists of an open dish and a feed

system. Usually lacking a radome (although a molded radome is an option), standard

performance antennas are economical solutions for specific applications. Aside from the lack

of environmental protection of the feed system, the main drawback is the diffraction of

microwave energy at the rim of the dish; these result in significant backward reflections at

azimuth angles of 100 deg., which can interfere with adjacent point-to-point links.

To block these backward rim reflections, antenna designers place a shield around the

circumference of the antenna, to which a planar radome is usually attached (Fig. 7). These

"high-performance" microwave antennas may be further enhanced by the application of

absorbing foam to the inside of the shield, resulting in "ultra-high-performance" microwave

antennas. The foam absorbs spurious reflections within the antenna and improves

performance through limiting the side lobes.

Radiation pattern envelopes for standard, high, and ultra-high-performance antennas are

compared in Fig. 8. The improvement in side-lobe reflection control of the ultra-high-

performance antenna over both other antennas is evident. Interestingly though, the high-


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performance antenna exhibits poorer performance than the standard performance antenna

between 20 and 60 deg.the result of additional reflections off the shield. It nevertheless

proves significantly better at preventing backward reflections. Selection of the appropriate

microwave antenna depends on the intended application, and the expected interference

potential in a given area.

It is important that, once installed, the performance of a microwave network should not

deteriorate due to environmental impact. While a radome might protect the sensitive feed

system from the elements, only a stable construction can protect the dish from wind.

Mechanical stability of an installed antenna is critical to maintain a point-to-point link, as

well as restricting its potential for interference with adjacent links, if its orientation changes.

Different antenna manufacturers use different methods of rating the antenna resistance to

wind. Radio Frequency Systems defines the "operational wind speed" rating of an installed

antenna as that for which temporary deflection of the main beam is within one-third of the

half-power beam width of the antenna. (The half-power beam width is defined as the angle,

relative to the main beam axis, between the two directions at which the measured co-polar

pattern is 3 dB below the value on the main beam axis.) Within this operational wind speed


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of which typical values are 120 to 140 mphthe point-to-point link will be satisfactorily

maintained.

Other standards consider the operational wind speed as that for which the main beam is notdeviated by more than 0.1 deg. Whatever the method used, it is important to take the

deflection of the mounting structure into consideration during calculation of the beam

deflection.

The positional mounting of antennas must also be considered by operators seeking to

minimize interference. Typical multi-antenna tower installations exhibit interference, and

even the mounting structure may directly impact the performance of the microwave link

through shielding and the generation of reflections from outside of the antenna itself. This is

particularly the case when antennas are mounted on the face of buildings and solid towers,

while Fig. 9 shows the significant reflections that arise when an antenna is mounted too close

to a solid structure to the side. Such structure-generated reflections are likely sources of

interference, and are often not taken into consideration during installation.


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PROPERTIES OF MICROWAVE LINKS:

Involve line of sight (LOS) communication technology

Affected greatly by environmental constraints, including rain fade

Have limited penetration capabilities

Sensitive to high pollen count

Signals can be degraded during Solar proton events

USES OF MICROWAVE LINKS:

In communications between satellites and base stations

As backbone carriers for cellular systems

In short range indoor communications

TUNABLE MICROWAVE DEVICES:

A tunable microwave device is a device that works at radio frequency range with the

dynamic tunable capabilities, especially an electric field. The material systems for such a

device usually have multilayer structure. Usually, magnetic or ferroelectric film on ferrite or

superconducting film is adopted. The former two are used as the property tunable component

to control the working frequency of the whole system. Devices of this type include tunable

varators, tunable microwave filters, tunable phase shifters, and tunable resonators. The main

application of them is re-configurable microwave networks, for example, reconfigurable

wireless communication, wireless network, and reconfigurable phase array antenna.

Microwave Linking

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