Chapter II Zar - [PDF Document] (2024)


A. Microwave System Overview

1. Microwave Line-of-Sight SystemsMicrowave frequencies rangefrom 300 MHz to 30 GHz, corresponding to wavelengths of 1 meter to1 cm. These frequencies are useful for terrestrial and satellitecommunication systems, both fixed and mobile. In the case ofpoint-to-point radio links, antennas are placed on a tower or othertall structure at sufficient height to provide a direct,unobstructed line-of-sight (LOS) path between the transmitter andreceiver sites. In the case of mobile radio systems, a single towerprovides point-to-multipoint coverage, which may include both LOSand non-LOS paths. LOS microwave is used for both short- andlong-haul telecommunications to complement wired media such asoptical transmission systems. Applications include local loop,cellular back haul, remote and rugged areas, utility companies, andprivate carriers. Early applications of LOS microwave were based onanalog modulation techniques, but todays microwave systems useddigital modulation for increased capacity andperformance.Amicrowavesystem is a system of equipment used formicrowave data transmission. The typical microwave system includesradios located high atop microwave towers, which are used for thetransmission of microwave communications using line of sightmicrowave radio technology.MicrowaveTowers are the Most VisibleComponent of the Microwave System. A microwave system is composedof at least two microwave towers. At the top of these towers aremicrowave antennas. These antennas are what allow the transmitterhardware of the microwave system to transmit data from site tosite. The area between the microwave system components must beclear of any major structures, such as tall buildings, mountains,or other objects that could potentially obstruct microwavetransmission. Only when this has been achieved can data travelthrough the microwave system.This is why microwave communication iscategorized as a line of sight technology. When planning amicrowaveradio system, one must remember the requirements ofmicrowave equipment. Microwave antennas must be placed at the topof tall radio towers to provide a clear line communication path.This allows the microwave system data to travel the long distancesrequired by telecommunications service providers( History of Microwave CommunicationIn1864, James Clark Maxwell predicted the existence ofelectromagnetic waves. He also noted that microwave is part of theelectromagnetic spectrum. In 1888, Heinrich Hertz was the first todemonstrate the existence of electromagnetic waves by building anapparatus that produced and detected microwaves in the UHF region.The design necessarily used horse-and-buggy materials, including ahorse trough, a wrought iron point spark, Leyden jars, and a lengthof zinc gutter whose parabolic cross-section worked as a reflectionantenna. In 1894 J.C. Bose publicly demonstrated radio control of abell using millimetre wavelengths, and conducted research into thepropagation of microwaves. Microwave technology was developedduring World War II (19391945) in connection with secret militaryradar research. Today, microwaves are used primarily in microwaveovens and communications. The technology that was used formicrowave communication was developed in the early 1940s by WesternUnion. In 1945, the first microwave message was sent from towerslocated in New York and Philadelphia. On August 17, 1951, the firsttranscontinental microwave radio system began operation. The systemwas comprised of 107 relay stations spaced an average of 30 milesapart to form a continuous radio link between New York and SanFrancisco that cost the Bell System approximately $40 million. By1954, there were over 400 microwave stations scattered across theUnited States and, by 1958, microwave carriers were the dominantmeans of long distance communications as they transported theequivalent of 13 million miles of telephone circuits (Tomasi,2004).Historical Milestones 1950s Analog Microwave Radio UsedFDM/FM in 4, 6, and 11 GHz bands for long-haul Introduced intotelephone networks by Bell System1970s Digital Microwave RadioReplaced analog microwaves Became bandwidth efficient withintroduction of advanced modulation techniques (QAM and TCM)Adaptive equalization and diversity became necessary for high datarates1990s and 2000s Digital microwave used for cellular back-haulChange in MMDS and ITFS spectrum to allow wireless cable andpoint-to-multipoint broadcasting IEEE 802.16 standard or WiMaxintroduces new application for microwave radio Wireless local andmetro area networks capitalize on benefits of microwave radio3.Microwave Frequency BandMost, if not all, microwave system will besubject to regulation by the government of the country in which thesystem is to be allocated. In general, each country allocatesspecific bands of frequencies for specific services or for specificusers. Within the United States the Federal CommunicationsCommission (FCC) is the controlling authority for all systemsexcept those operated by agencies of the Federal Government, thelatter usually being placed in frequency bands separate from thosecontrolled by FCC. In Canada, the licensing body is the Departmentof Communications. In many countries it is the Department of Postsand Telegraphs, or some similar entity. Most countries, other thanthe United States, follow the frequency allocations recommended bythe International Radio Consultative Committee (CCIR).The microwavespectrum is usually defined as electromagnetic energy ranging fromapproximately 1 GHz to 1000 GHz in frequency, but older usageincludes lower frequencies. Most common applications are within the1 to 40 GHz range. Microwave Frequency Bands are defined in thetable below:Table 2.1 Microwave Frequency Bands

DesignationFrequency range

L band1 to 2 GHz

S band2 to 4 GHz

C band4 to 8 GHz

X band8 to 12 GHz

Kuband12 to 18 GHz

K band18 to 26 GHz

Kaband26 to 40 GHz

Q band30 to 50 GHz

U band40 to 60 GHz

V band50 to 75 GHz

E band60 to 90 GHz

W band75 to 110 GHz

F band90 to 140 GHz

D band110 to 170 GHz

L band(20-cmradarlong-band) is a portion of themicrowaveband oftheelectromagnetic spectrumranging roughly from 0.39 to 1.55GHz. Itis used by somecommunications satellites, and byterrestrialEureka147digital audio broadcasting. In theU.S., the L band is held bytheU.S. Militaryfortelemetry, thereby forcingdigital radiotoin-bandon-channel(IBOC) solutions.DABis typically done in the14521492-MHzrange as inCanada, but other countries alsouseVHFandUHFbands. TheGlobal Positioning Systemcarriersare in the Lband, centered at 1176.45 MHz (L5), 1227.60 MHz (L2), 1381.05 MHz(L3), and 1575.42 MHz (L1) frequencies.S band, or10-cmradarshort-band, is the part of the microwave band oftheelectromagnetic spectrumranging roughly from 1.55 to 5.2GHz. Itis used byweatherradar and somecommunications satellites.C band("compromise" band) is a portion of electromagnetic spectrum inthemicrowaverange of frequencies ranging from 4 to 6GHz. C band isprimarily used for satellite communications; normally downlink3.74.2 GHz horizontalpolarization, uplink 5.96.4 GHz verticalpolarization, usually 24 36 MHz transponders on board a satellite.The applications include full-timesatellite TVnetworks or rawsatellite feeds, although subscriptionprogrammingalso exists. Thereare more than 20 C-bandsatelliteshovering over North America, whichprovide more than 250 video channels and 75 audio services. Typicalantenna sizes on C-band capable systems range from 7.5 to 12 feet(2 to 3.5 m). This contrasts withdirect broadcast satellite, whichis a completely closed system used to deliver subscriptionprogramming to small satellite dishes connected to proprietaryreceiving equipment. C band is highly associated withTVROsatellitereception systems or "big dish" systems. Larger antennas and moreexpensive receivers, C band usually provides better video qualityand is less affected by rain attenuation than theKu band. Contraryto popular belief, digital C band does in fact exist.X band(3-cmradar spot-band) of the microwave band of theelectromagneticspectrumroughly ranges from 5.210.9GHz. It is used bysomecommunications satellitesandX-band radar.K bandis a portion oftheelectromagnetic spectrumin themicrowaverange of frequenciesranging between 12 to 63GHz. K band between 18 and 26.5 GHz isabsorbed easily by water vapor (H2O resonance peak at 22.24 GHz,1.35 cm). TheNATOK-band is defined as frequency band between 2040GHz (7.5-15 mm).Kaband(kurz-above band) is a portion of theK bandofthemicrowaveband of theelectromagnetic spectrum. Kaband roughlyranges from 18 to 40GHz. The 20/30 GHz band is usedincommunications satellites,downlink18.318.8 GHz and 19.720.2 GHz.The term Ka band is frequently used to refer to the recommendedoperating frequencies of WR-28 rectangular waveguide, which is 26.5to 40.0 GHz.V bandof theelectromagnetic spectrumranges from 50 to75 GHz.TheKuband("kay-yoo" kurz-under band) is a portion oftheelectromagnetic spectrumin themicrowaverange of frequenciesranging from 11 to 18GHz. Kuband is primarily used forsatellitecommunications, particularly for satellitebackhaulsfrom remotelocations back to atelevision networks studio for editingandbroadcasting. Kuband is split into two segments byFCC. The 11.7to 12.2 GHz band is known as FSS (fixed satelliteservice,uplink14.0 to 14.5 GHz). There are more than 22 FSS Ku-bandsatellites orbiting over North America, each carrying 12 to 24transponders, 20 to 120watts per transponder, and requiring a 3 to5 ft (1 to 1.5 m) antenna for clear reception. The 12.2 to 12.7 GHzsegment is known as BSS (broadcasting satellite service).BSS/DBSdirect broadcast satellitesnormally carry 16 to 3227MHztransponders at 100 to 240 watts, allowing the use of receiverantennas as small as 18 inches (450 mm). Ku-band signals can beaffected by rain attenuation.In the United States, radio channelassignments are controlled by the Federal Communications Commission(FCC) for commercial carriers and by the NationalTelecommunications and Information Administration (NTIA) forgovernment systems. The FCC's regulations for use of spectrumestablish eligibility rules, permissible use rules, and technicalspecifications. FCC regulatory specifications are intended toprotect against interference and to promote spectral efficiency.Equipment type acceptance regulations include transmitter powerlimits, frequency stability, out-of-channel emission limits, andantenna directivity.

The International Telecommunications Union Radio Committee(ITU-R) issues recommendations on radio channel assignments for useby national frequency allocation agencies. Although the ITU-Ritself has no regulatory power, it is important to realize thatITU-R recommendations are usually adopted on a worldwide basis.

4. Principles and OperationMicrowave Link StructureThe basiccomponents required for operating a radio link are the transmitter,towers, antennas, and receiver. Transmitter functions typicallyinclude multiplexing, encoding, modulation, up-conversion frombaseband or intermediate frequency (IF) to radio frequency (RF),power amplification, and filtering for spectrum control. Receiverfunctions include RF filtering, down-conversion from RF to IF,amplification at IF, equalization, demodulation, decoding, anddemultiplexing. To achieve point-to-point radio links, antennas areplaced on a tower or other tall structure at sufficient height toprovide a direct, unobstructed line-of-sight (LOS) path between thetransmitter and receiver sites.

Types of Microwave Systems1. Intrastate or feeder servicemicrowave systems - generally categorized as short haul since theyare used to carry information for relatively short distances, suchas between cities within the same state.2. Long haul microwavesystems - used to carry information for long distances.

Microwave RepeatersMicrowave communications requires theline-of-sight or space wave propagation method. There are someinstances where barriers are inevitable which cause obstructionsbetween the transmitter and receiver. This kind of problem is bestresolved by repeaters.Passive RepeaterIt is a device used tore-radiate the intercepted microwave energy without the use ofadditional electronic power. It also has the ability to redirectintercepted microwave radars to the other direction.

Active RepeaterIt is a receiver and a transmitter placed back toback or in tandem with microwave repeaters. There are two types ofactive repeater namely: baseband and heterodyne or IF.In basebandrepeaters, the received radio frequency (RF) carrier isdown-converted to an intermediate frequency (IF), amplified,filtered, and then demodulated to baseband. In a heterodynerepeater, the received RF carrier is down-converted to an IF,amplified, reshaped, up-converted to RF, and then retransmitted.The baseband signal is unaltered by the repeater because the signalis never demodulated below IF.

DiversityThe microwave systems use LOS transmission, thus adirect signal path must exit between the transmit and receiveantennas. When the signal path undergoes a sever degradation, aservice interruption will occur. The radio path losses vary withatmospheric conditions that can cause corresponding reductions inthe received signal strength. This reduction in signal strength istemporary and is referred to asradio fade.The purpose of usingdiversity is to increase the reliability of the system byincreasing its ability. There is more than one transmission path ormethod of transmission available between a transmitter and areceiver in diversity. Depending on the type of combiner in use,the output signal-to-noise ratio is improved as compared to anysingle path.

Frequency DiversityFrequency diversity is simply modulating twodifferent RF carrier frequencies with the same IF intelligence,then transmitting both RF signals to a given destination. Itutilizes the phenomenon that the period of fading differs forcarrier frequencies separated by 2-5%. This system employs twotransmitters and two receivers. Frequency diversity arrangementsprovide simple equipment redundance. Its disadvantage is that itdoubles the amount of necessary frequency spectrum andequipment.Space DiversityIn space diversity, the output of atransmitter is fed to two or more antennas that are physicallyseparated by an appreciable number of wavelengths. At the receivingend, there may be more than one antenna providing the input signalto the receiver. It has been observed that multipath fading willnot occur simultaneously at both antennas.Polarization DiversityInpolarization diversity, a single RF carrier is propagated with twodifferent electromagnetic polarizations (either vertical orhorizontal). Electromagnetic waves of different polarizations donot necessarily experience the same transmission impairments. Thistype of diversity is used in conjunction with space diversity. Onetransmit/receive antenna pair is vertically polarized, and theother is horizontally polarized. It is also possible to usefrequency, polarization and space diversity simultaneously.HybridDiversityIt is specialized form of diversity that consists of astandard frequency-diversity path where the twotransmitter/receiver pairs at one end of the path are separatedfrom each other and connected to different antennas that arevertically separated as in space diversity. This arrangementprovides a space-diversity effect in both directions: in onedirection because the receivers are vertically spaced and in theother direction because the transmitters are vertically spaced.

Figure 2.1 Radiation properties of electromagnetic wavesTheimage shows some radiation properties of electromagnetic waves,which includes the microwaves. The approximate wavelengths are alsoindicated in this photo

5. Microwave System Design.The design of microwave radio systemsinvolves engineering of the path to evaluate the effects ofpropagation on performance, development of a frequency allocationplan, and proper selection of radio and link components. Thisdesign process must ensure that outage requirements are met on aper link and system basis. The frequency allocation plan is basedon four elements: the local frequency regulatory authorityrequirements, selected radio transmitter and receivercharacteristics, antenna characteristics, and potential intrasystemand intersystem RFinterference.

6. Microwave Propagation Characteristics.Various phenomenaassociated with propagation, such as multipath fading andinterference, affect microwave radio performance. The modes ofpropagation between two radio antennas may include a direct,line-of-sight (LOS) path but also a ground or surface wave thatparallels the earth's surface, a sky wave from signal componentsreflected off the troposphere or ionosphere, a ground reflectedpath, and a path diffracted from an obstacle in the terrain. Thepresence and utility of these modes depend on the link geometry,both distance and terrain between the two antennas, and theoperating frequency. For frequencies in the microwave (~2 30 GHz)band, the LOS propagation mode is the predominant mode availablefor use; the other modes may cause interference with the strongerLOS path. Line-of-sight links are limited in distance by thecurvature of the earth, obstacles along the path, and free-spaceloss. Average distances for conservatively designed LOS links are25 to 30 mi, although distances up to 100 mi have been used. Forfrequencies below 2 GHz, the typical mode of propagation includesnon-line-of-sight (NLOS) paths, where refraction, diffraction, andreflection may extend communications coverage beyond LOS distances.The performance of both LOS and NLOS paths is affected by severalphenomena, including free-space loss, terrain, atmosphere, andprecipitation.

7. Strengths and Weaknesses / Advantages andDisadvantagesStrengths Adapts to difficult terrain Loss versusdistance (D) = Log D (not linear) Flexible channelizationRelatively short installation time Can be transportable Costusually less than cable No back-hoe fadingWeaknessesPaths could beblocked by buildings Spectral congestion Interception possiblePossible regulatory delays Sites could be difficult to maintainTowers need periodic maintenance Atmospheric fading

The advantages and disadvantages of microwave radio include thefollowing:Advantages are:1. Do not require a right-of-wayacquisition between stations.2. Each station requires the purchaseor lease of only a small area of land.3. Because of their highoperating frequencies, microwave systems can carry large quantitiesof information.4. High frequencies mean short wavelengths, whichrequire relatively small antennas. 5. Some signals are more easilypropagated around physical obstacles such as water and highmountains.6. Fewer repeaters are necessary for amplification.7.Distance between switching centers are less.8. Undergroundfacilities are minimized.9. Minimum delay times are introduced.10.Minimal crosstalk exists between voice channels.11. Increasedreliability and less maintenance are important factors.

Disadvantages are:1. It is more difficult to analyze and designcircuits at microwave frequencies.2. Measuring techniques are moredifficult to perfect and implement at microwave frequencies.3. Itis difficult to implement conventional circuit components(resistors, capacitors, inductors, and so on) at microwavefrequencies.4. Transient time is more critical at microwavefrequencies.5. It is often necessary to use specialized componentsfor microwave frequency.6. Microwave frequencies propagate in astraight line, which limits their use to line-of-sightapplications.

8. Microwave ApplicationsThe tremendous growth in wirelessservices is made possible today through the use of microwaves forbackhaul in wireless and mobile networks and forpoint-to-multipointnetworks. Towers can be used for both mobile,e.g. cellular, and point-to-point applications, enhancing thepotential for microwave as wireless systems grow. Increases inspectrum allocations and advances in spectrum efficiency throughtechnology have created business opportunities in the field ofmicrowave radio. Telecommunications carriers, utility companies,and private carriers all use microwave to complement wired andoptical networks. ( microwave frequency spectrum is used for telephonecommunications. Many long-distance telephone systems use microwaverelay links for carrying telephone calls. With multiplexingtechniques, thousands of two-way communications are modulated on asingle carrier and then relayed from one station to another overlong distances.Radar (Radio Detection and Ranging) also operates inthe microwave region. It is a method of detecting the presence of adistant object and determining its distance and direction. Radarsystems transmit a high-frequency signal which is then deflectedfrom the distant object. The reflected signal is picked up by theradar unit and compared to the transmitted signal. The timedifference between the two gives the distance to theobject.Television stations and networks use microwave relay linksto transmit TV signals over long distances rather than rely on coaxcables.A growing application for microwave communications is spacecommunications. Communications with satellites, deep-space probes,and other spacecraft is usually done by microwave transmission.This is due to the reason that microwave signals are not reflectedor absorbed by the ionosphere as are many lower-frequency signals.(

5. Fundamental Antenna PatternSimple Antennas The simplestantenna, in terms of its radiation pattern, is the isotropicradiator. It has zero size, is perfectly efficient, and radiatespower equally in all directions. Though merely a theoreticalconstruct, the isotropic radiator makes a good reference with whichto compare the gain and directionality of other antennas. That isbecause, even though this antenna cannot be built and tested, itscharacteristics are simple and easy to derive. The half-wave dipoleantenna, on the other hand, is a simple, practical antenna which isin common use. An understanding of the half-wave dipole isimportant both in its own right and as a basis for the study ofmore complex antennas. A half-wave dipole is sketched in Figure2.2. (Blake:2007 Wireless Communication Technology)

Figure 2.2 Half-wave dipoleThe word dipole simply means it hastwo parts. A dipole antenna does not have to be one-half wavelengthin length like the one shown in the figure, but this length ishandy for impedance matching. Actually, in practice its lengthshould be slightly less than one-half the free-space wavelength toallow for capacitive effects. A half-wave dipole is sometimescalled a Hertz antenna, though strictly speaking the term Hertziandipole refers to a dipole of infinitesimal length. This, like theisotropic radiator, is a theoretical construct; it is used in thecalculation of antenna radiation patterns. Typically the length ofa half-wave dipole, assuming that the conductor diameter is muchless than the length of the antenna, is 95% of one-half thewavelength measured in free space.Antenna CharacteristicsRadiationPattern

The xy plane is horizontal, and the angle is measured from the xaxis in the direction of the y axis. The z axis is vertical and theangle is usually measured from the horizontal plane toward thezenith. This vertical angle, measured upward from the ground, iscalled the angle of elevation. Most work with antennas usespositive angles of elevation, but sometimes (as when thetransmitting antenna is on a tall tower and the receiving antennais close to it and much lower) we are interested in angles belowthe horizon. Different manufacturers handle below-horizon anglesdifferently as shown in Figure 2.3.

Figure 2.3 Radiation pattern of half-wave dipole

Gain and Directivity

The sense in which a half-wave dipole antenna can be said tohave gain can be seen from Figure 2.3. This sketch shows thepattern of a dipole, from Figure 2.2, superimposed on that of anisotropic radiator. It can be seen that while the dipole has a gainof 2.14 dBi in certain directions, in others its gain is negative.If the antennas were to be enclosed by a sphere that would absorball the radiated power, the total radiated power would be found tobe the same for both antennas. Remember that for antennas, powergain in one direction is at the expense of losses in others.

Figure 2.4. Isotropic and Dipole Antennas


Just as a flashlight emits a beam of light, a directionalantenna can be said to emit a beam of radiation in one or moredirections. The width of this beam is defined as the angle betweenits half-power points. These are also the points at which the powerdensity is 3 dB less than it is at its maximum point. An inspectionwe will show that the half-wave dipole has a beamwidth of about 78in one plane and 360 in the other. Many antennas are much moredirectional than this, with a narrow beam in both planes.

Front-to-Back RatioAs you might expect, the direction of maximumradiation in the horizontal plane is considered to be the front ofthe antenna, and the back is the direction 180 from the front. Fora dipole, the front and back have the same radiation, but this isnot always the case. Consider the unidirectional antenna shown inFigure 2.5: there is a good deal more radiation from the front ofthis antenna than from the back. The ratio between the gains to thefront and back is the front-to-back ratio. It is generallyexpressed in dB, in which case it can be found by subtracting thegains in dBi or dBd.

Figure 2.5 Unidirectional antenna

Effective Isotropic Radiated Power and Effective RadiatedPower

In a practical situation we are usually more interested in thepower emitted in a particular direction than in the total radiatedpower. Looking from a distance, it is impossible to tell thedifference between a high-powered transmitter using an isotropicantenna and a transmitter of lower power working into an antennawith gain. Effective isotropic radiated power (EIRP), which issimply the actual power going into the antenna multiplied by itsgain with respect to an isotropic radiator.


Another similar term that is in common use is effective radiatedpower (ERP), which represents the power input multiplied by theantenna gain measured with respect to a half-wave dipole. Since anideal half-wave dipole has a gain of 2.14 dBi, the EIRP is 2.14 dBgreater than the ERP for the same antenna-transmitter combination.That is,

EIRP = ERP + 2.14dB


EIRP = effective isotropic radiated power for a giventransmitter and antennaERP = effective radiated power for the sametransmitter and antenna

The path loss equations require EIRP, but they can easily beused with ERP values. Simply add 2.14 dB to any ERP value toconvert it to EIRP. Convert the power to dBm or dBW first, if it isnot already expressed in such units.

Impedance The radiation resistance of a half-wave dipolesituated in free space and fed at the center is approximately 70ohms. The impedance is completely resistive at resonance, whichoccurs when the length of the antenna is about 95% of thecalculated free-space half-wavelength value. The exact lengthdepends on the diameter of the antenna conductor relative to thewavelength. If the frequency is above resonance, the feedpointimpedance has an inductive component; if the frequency is lowerthan resonance, the antenna impedance is capacitive. Another way ofsaying the same thing is that an antenna that is too short appearscapacitive, while one that is too long is inductive. Figure 2.6shows graphically how reactance varies with frequency.

Figure 2.6 Variation of dipole reactance with frequency

Polarization The polarization of a radio wave is the orientationof its electric field vector. The polarization of the radiationfrom a half-wave dipole is easy to determine: it is the same as theaxis of the wire. That is, a horizontal antenna produceshorizontally polarized waves, and a vertical antenna gives verticalpolarization. It is important that the polarization be the same atboth ends of a communication path. Wireless communication systemsusually use vertical polarization because this is more convenientfor use with portable and mobile antennas.

Monopole Antenna

Many wireless applications require antennas on vehicles. Thedirectional effects of a horizontal dipole would be undesirable. Avertical dipole is possible, but awkward to feed in the center andrather long at some frequencies. Similar results can be obtained byusing a vertical quarter-wave monopole antenna. It is mounted on aground plane, which can be the actual ground or an artificialground such as the body of a vehicle. The monopole is fed at thelower end with coaxial cable. The ground conductor of the feedlineis connected to the ground plane. See Figure 2.7. The radiationpattern of a quarter-wave monopole in the vertical plane has thesame shape as that of a vertical half-wave dipole in free space.Only half the pattern is present, however, since there is nounderground radiation. In the horizontal plane, of course, avertical monopole is omnidirectional. Since, assuming no losses,all of the power is radiated into one-half the pattern of a dipole,this antenna has a power gain of two (or 3 dB) over a dipole infree space. The input impedance at the base of a quarter-wavemonopole is one-half that of a dipole. This can be explained asfollows: with the same current, the antenna produces one half theradiation pattern of a dipole, and therefore one-half the radiatedpower. The radiated power is given by

Pr = I 2RrwherePr = radiated powerI = antenna current at thefeedpointRr = radiation resistance measured at the feedpoint

If the radiated power decreases by a factor of two for a givencurrent, then so must the feedpoint radiation resistance. In somemobile and portable applications a quarter wavelength is too longto be convenient. In that case, the electrical length of theantenna can be increased by adding inductance to the antenna. Thiscan be done at the base or at the center, or the whole antenna canbe coiled. The rubber duckie antennas on many handheld transceiversuse this technique. Inductors used to increase the effective lengthof antennas are called loading coils

Figure 2.7 Monopole Antenna

The Five-Eighths Wavelength Antenna

This antenna is often used vertically as either a mobile or baseantenna in VHF and UHF systems. Like the quarter-wave monopole, ithas omnidirectional response in the horizontal plane. However, theradiation has a higher feedpoint impedance and therefore does notrequire as good a ground, because the current at the feedpoint isless. The impedance is typically lowered to match that of a 50 ohmsfeedline by the use of an impedance-matching section. The circularsection at the base of the antenna is an impedance-matching device.Figure 2.8 shows a 5/8 wavelength antenna.

Figure 2.8 Photograph of a 5/8 wavelength antenna

Helical Antennas

A helical antenna is a spiral, usually several wavelengths long.Such an antenna is shown in Figure 2.9 Typically the circumferenceof each turn is about one wavelength and the turns are about aone-quarter wavelength apart.Helical antennas produce circularlypolarized waves whose sense is the same as that of the helix. Ahelical antenna can be used to receive circularly polarized waveswith the same sense and can also receive plane polarized waves withthe polarization in any direction. Helical antennas are often usedwith VHF and UHF satellite transmissions. Since they respond to anypolarization angle, they avoid the problem of Faraday rotation,which makes the polarization of waves received from a satelliteimpossible to predict. The gain of a helical antenna isproportional to the number of turns.

Figure 2.9 Helical Antenna

Antenna Arrays

The simple elements described above can be combined to build amore elaborate antenna. The radiation from the individual elementswill combine, resulting in reinforcement in some directions andcancellation in others to give greater gain and better directionalcharacteristics. For instance, it is often desirable to have highgain in only one direction, something that is not possible with thesimple antennas previously described. In mobile and portableapplications, often an omnidirectional pattern in the verticalplane is wanted, but radiation upward and downward from the antennais of no use. In both of these situations, a properly designedarray could redirect the unwanted radiation in more usefuldirections.Arrays can be classified as broadside or end-fire,according to their direction of maximum radiation. If the maximumradiation is along the main axis of the antenna (which may or maynot coincide with the axis of its individual elements), the antennais an end-fire array. If the maximum radiation is at right anglesto this axis, the array has a broadside configuration. Antennaarrays can also be classified according to the way in which theelements are connected. A phased array has all its elementsconnected to the feedline. There may be phase-shifting,power-splitting, and impedance matching arrangements for individualelements, but all receive power from the feedline (assuming atransmitting antenna). Since the transmitter can be said to driveeach element by supplying power, these are also called drivenarrays. On the other hand, in some arrays only one element isconnected to the feedline. The others work by absorbing andreradiating power radiated from the driven element. These arecalled parasitic elements, and the antennas are known as parasiticarrays.

ReflectorsThe antennas and arrays described in the precedingsection can often be used with reflecting surfaces to improve theirperformance. A reflector may consist of one or more planes, or itmay be parabolic in shape. In order to reduce wind and snow loads,reflectors are often constructed of mesh or closely-spaced rods. Aslong as the spacing is small compared with a wavelength, the effecton the antenna pattern, compared with a solid reflector, isnegligible.

Parabolic ReflectorParabolic reflectors have the useful propertythat any ray originating at a point called the focus and strikingthe reflecting surface will be reflected parallel to the axis ofthe parabola. That is, a collimated beam of radiation will beproduced. The parabolic dish antenna, familiar from backyardsatellite receiver installations, consists of a small antenna atthe focus of a large parabolic reflector, which focuses the signalin the same way as the reflector of a searchlight focuses a lightbeam. Figure 2.10 shows a typical example. Of course the antenna isreciprocal: radiation entering the dish along its axis will befocused by the reflector.

Figure 2.10 Variations of Parabolic Antenna (Courtesy of AndrewCorporation)

Standard Antenna ConstructionRadomes are used to protectmicrowave antennas against accumulation of ice, snow, and dirt andto reduce wind loading. All Andrew shielded antennas include aplanar radome. Antennas which include a radome are indicated in theantenna specification tables. Optional molded radomes, areavailable for most other solid reflector, standard unshieldedparabolic antennas. Radomes for shielded antennas. All Andrewshielded antennas, except ValuLine include a flexible planarradome. The radome is stretched across the opening of the shield(through tensioning springs) flexing slightly in the wind to shedice and snow in most environments. Two types of flexible planarradomes are used, TEGLAR and Hypalon. Hypalon is a rubber coatednylon and is pro- vided with HP and HPX series antennas. TEGLAR isa polymer-coated fiberglass material and is provided with HSX, UHXand UMX type antennas. In addition, TEGLAR radomes are extremelydurable, and excel in resistance to heat, rain, snow, fungus, iceaccumulation, corrosive atmosphere and ultraviolet light. Upgradesto TEGLAR on HP and HPX series is optional. Pre-tensioned radomes.Some high performance antennas are supplied with a pre-tensionedradome. Pre-tensioned radomes are made from TEGLAR material bondedto a support ring. They replace the previously offered springtensioned design. Radomes for standard antennas. Molded radomes aremanufactured of ABS plastic or fiberglass. They help reduce towerwind loading and are optional for most antennas. (TerrestrialMicrowave System Products, Andrew Catalogue)

Mounts All microwave antennas are supplied with a vertical towermount. Roof, vertical tilt and horizontal tilt mounts are availableas options. Shields Cylindrical shields, attached to the reflectorrim, improve the radiation pattern performance of parabolicantennas. RF absorbing material is placed at critical locationsinside the shield to reduce RF energy reflections.Antenna FinishStandard colors for microwave antennas and radomes are listed inthe table below. Other colors in compliance with U.S. FCC and U.S.FAA regulations or special applications are available on request.Unless otherwise specified, radomes supplied with special colorantennas will be the standard color.

Chapter II Zar - [PDF Document] (2024)
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