CHAPTER IIREVIEW OF RELATED LITERATURE

A. Microwave System Overview

1. Microwave Line-of-Sight SystemsMicrowave frequencies range from 300 MHz to 30 GHz, corresponding to wavelengths of 1 meter to 1 cm. These frequencies are useful for terrestrial and satellite communication systems, both fixed and mobile. In the case of point-to-point radio links, antennas are placed on a tower or other tall structure at sufficient height to provide a direct, unobstructed line-of-sight (LOS) path between the transmitter and receiver sites. In the case of mobile radio systems, a single tower provides point-to-multipoint coverage, which may include both LOS and non-LOS paths. LOS microwave is used for both short- and long-haul telecommunications to complement wired media such as optical transmission systems. Applications include local loop, cellular back haul, remote and rugged areas, utility companies, and private carriers. Early applications of LOS microwave were based on analog modulation techniques, but todays microwave systems used digital modulation for increased capacity and performance.Amicrowavesystem is a system of equipment used for microwave data transmission. The typical microwave system includes radios located high atop microwave towers, which are used for the transmission of microwave communications using line of sight microwave radio technology.MicrowaveTowers are the Most Visible Component of the Microwave System. A microwave system is composed of at least two microwave towers. At the top of these towers are microwave antennas. These antennas are what allow the transmitter hardware of the microwave system to transmit data from site to site. The area between the microwave system components must be clear of any major structures, such as tall buildings, mountains, or other objects that could potentially obstruct microwave transmission. Only when this has been achieved can data travel through the microwave system.This is why microwave communication is categorized as a line of sight technology. When planning a microwaveradio system, one must remember the requirements of microwave equipment. Microwave antennas must be placed at the top of tall radio towers to provide a clear line communication path. This allows the microwave system data to travel the long distances required by telecommunications service providers (http://www.dpstele.com)2. History of Microwave CommunicationIn 1864, James Clark Maxwell predicted the existence of electromagnetic waves. He also noted that microwave is part of the electromagnetic spectrum. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna. In 1894 J.C. Bose publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research into the propagation of microwaves. Microwave technology was developed during World War II (19391945) in connection with secret military radar research. Today, microwaves are used primarily in microwave ovens and communications. The technology that was used for microwave communication was developed in the early 1940s by Western Union. In 1945, the first microwave message was sent from towers located in New York and Philadelphia. On August 17, 1951, the first transcontinental microwave radio system began operation. The system was comprised of 107 relay stations spaced an average of 30 miles apart to form a continuous radio link between New York and San Francisco that cost the Bell System approximately $40 million. By 1954, there were over 400 microwave stations scattered across the United States and, by 1958, microwave carriers were the dominant means of long distance communications as they transported the equivalent of 13 million miles of telephone circuits (Tomasi, 2004).Historical Milestones 1950s Analog Microwave Radio Used FDM/FM in 4, 6, and 11 GHz bands for long-haul Introduced into telephone networks by Bell System1970s Digital Microwave Radio Replaced analog microwaves Became bandwidth efficient with introduction of advanced modulation techniques (QAM and TCM) Adaptive equalization and diversity became necessary for high data rates1990s and 2000s Digital microwave used for cellular back-haul Change in MMDS and ITFS spectrum to allow wireless cable and point-to-multipoint broadcasting IEEE 802.16 standard or WiMax introduces new application for microwave radio Wireless local and metro area networks capitalize on benefits of microwave radio3. Microwave Frequency BandMost, if not all, microwave system will be subject to regulation by the government of the country in which the system is to be allocated. In general, each country allocates specific bands of frequencies for specific services or for specific users. Within the United States the Federal Communications Commission (FCC) is the controlling authority for all systems except those operated by agencies of the Federal Government, the latter usually being placed in frequency bands separate from those controlled by FCC. In Canada, the licensing body is the Department of Communications. In many countries it is the Department of Posts and Telegraphs, or some similar entity. Most countries, other than the United States, follow the frequency allocations recommended by the International Radio Consultative Committee (CCIR).The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands are defined in the table 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 of theelectromagnetic spectrumranging roughly from 0.39 to 1.55GHz. It is used by somecommunications satellites, and byterrestrialEureka 147digital audio broadcasting. In theU.S., the L band is held by theU.S. Militaryfortelemetry, thereby forcingdigital radiotoin-band on-channel(IBOC) solutions.DABis typically done in the 14521492-MHzrange as inCanada, but other countries also useVHFandUHFbands. TheGlobal Positioning Systemcarriersare in the L band, centered at 1176.45 MHz (L5), 1227.60 MHz (L2), 1381.05 MHz (L3), and 1575.42 MHz (L1) frequencies.S band, or 10-cmradarshort-band, is the part of the microwave band of theelectromagnetic spectrumranging roughly from 1.55 to 5.2GHz. It is used byweatherradar and somecommunications satellites.C band ("compromise" band) is a portion of electromagnetic spectrum in themicrowaverange of frequencies ranging from 4 to 6GHz. C band is primarily used for satellite communications; normally downlink 3.74.2 GHz horizontalpolarization, uplink 5.96.4 GHz vertical polarization, usually 24 36 MHz transponders on board a satellite. The applications include full-timesatellite TVnetworks or raw satellite feeds, although subscriptionprogrammingalso exists. There are more than 20 C-bandsatelliteshovering over North America, which provide more than 250 video channels and 75 audio services. Typical antenna sizes on C-band capable systems range from 7.5 to 12 feet (2 to 3.5 m). This contrasts withdirect broadcast satellite, which is a completely closed system used to deliver subscription programming to small satellite dishes connected to proprietary receiving equipment. C band is highly associated withTVROsatellite reception systems or "big dish" systems. Larger antennas and more expensive receivers, C band usually provides better video quality and is less affected by rain attenuation than theKu band. Contrary to popular belief, digital C band does in fact exist.X band(3-cm radar spot-band) of the microwave band of theelectromagnetic spectrumroughly ranges from 5.210.9GHz. It is used by somecommunications satellitesandX-band radar.K bandis a portion of theelectromagnetic spectrumin themicrowaverange of frequencies ranging between 12 to 63GHz. K band between 18 and 26.5 GHz is absorbed easily by water vapor (H2O resonance peak at 22.24 GHz, 1.35 cm). TheNATOK-band is defined as frequency band between 2040 GHz (7.5-15 mm).Kaband(kurz-above band) is a portion of theK bandof themicrowaveband of theelectromagnetic spectrum. Kaband roughly ranges from 18 to 40GHz. The 20/30 GHz band is used incommunications satellites,downlink18.318.8 GHz and 19.720.2 GHz. The term Ka band is frequently used to refer to the recommended operating frequencies of WR-28 rectangular waveguide, which is 26.5 to 40.0 GHz.V bandof theelectromagnetic spectrumranges from 50 to 75 GHz.TheKuband("kay-yoo" kurz-under band) is a portion of theelectromagnetic spectrumin themicrowaverange of frequencies ranging from 11 to 18GHz. Kuband is primarily used forsatellite communications, particularly for satellitebackhaulsfrom remote locations back to atelevision networks studio for editing andbroadcasting. Kuband is split into two segments byFCC. The 11.7 to 12.2 GHz band is known as FSS (fixed satellite service,uplink14.0 to 14.5 GHz). There are more than 22 FSS Ku-band satellites orbiting over North America, each carrying 12 to 24 transponders, 20 to 120watts per transponder, and requiring a 3 to 5 ft (1 to 1.5 m) antenna for clear reception. The 12.2 to 12.7 GHz segment is known as BSS (broadcasting satellite service). BSS/DBSdirect broadcast satellitesnormally carry 16 to 32 27MHztransponders at 100 to 240 watts, allowing the use of receiver antennas as small as 18 inches (450 mm). Ku-band signals can be affected by rain attenuation.In the United States, radio channel assignments are controlled by the Federal Communications Commission (FCC) for commercial carriers and by the National Telecommunications and Information Administration (NTIA) for government systems. The FCC's regulations for use of spectrum establish eligibility rules, permissible use rules, and technical specifications. FCC regulatory specifications are intended to protect against interference and to promote spectral efficiency. Equipment type acceptance regulations include transmitter power limits, frequency stability, out-of-channel emission limits, and antenna directivity.

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

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

Types of Microwave Systems1. Intrastate or feeder service microwave systems - generally categorized as short haul since they are used to carry information for relatively short distances, such as between cities within the same state.2. Long haul microwave systems - used to carry information for long distances.

Microwave RepeatersMicrowave communications requires the line-of-sight or space wave propagation method. There are some instances where barriers are inevitable which cause obstructions between the transmitter and receiver. This kind of problem is best resolved by repeaters.Passive RepeaterIt is a device used to re-radiate the intercepted microwave energy without the use of additional electronic power. It also has the ability to redirect intercepted microwave radars to the other direction.

Active RepeaterIt is a receiver and a transmitter placed back to back or in tandem with microwave repeaters. There are two types of active repeater namely: baseband and heterodyne or IF.In baseband repeaters, the received radio frequency (RF) carrier is down-converted to an intermediate frequency (IF), amplified, filtered, and then demodulated to baseband. In a heterodyne repeater, 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 signal is never demodulated below IF.

DiversityThe microwave systems use LOS transmission, thus a direct signal path must exit between the transmit and receive antennas. When the signal path undergoes a sever degradation, a service interruption will occur. The radio path losses vary with atmospheric conditions that can cause corresponding reductions in the received signal strength. This reduction in signal strength is temporary and is referred to asradio fade.The purpose of using diversity is to increase the reliability of the system by increasing its ability. There is more than one transmission path or method of transmission available between a transmitter and a receiver in diversity. Depending on the type of combiner in use, the output signal-to-noise ratio is improved as compared to any single path.

Frequency DiversityFrequency diversity is simply modulating two different RF carrier frequencies with the same IF intelligence, then transmitting both RF signals to a given destination. It utilizes the phenomenon that the period of fading differs for carrier frequencies separated by 2-5%. This system employs two transmitters and two receivers. Frequency diversity arrangements provide simple equipment redundance. Its disadvantage is that it doubles the amount of necessary frequency spectrum and equipment.Space DiversityIn space diversity, the output of a transmitter is fed to two or more antennas that are physically separated by an appreciable number of wavelengths. At the receiving end, there may be more than one antenna providing the input signal to the receiver. It has been observed that multipath fading will not occur simultaneously at both antennas.Polarization DiversityIn polarization diversity, a single RF carrier is propagated with two different electromagnetic polarizations (either vertical or horizontal). Electromagnetic waves of different polarizations do not necessarily experience the same transmission impairments. This type of diversity is used in conjunction with space diversity. One transmit/receive antenna pair is vertically polarized, and the other is horizontally polarized. It is also possible to use frequency, polarization and space diversity simultaneously.Hybrid DiversityIt is specialized form of diversity that consists of a standard frequency-diversity path where the two transmitter/receiver pairs at one end of the path are separated from each other and connected to different antennas that are vertically separated as in space diversity. This arrangement provides a space-diversity effect in both directions: in one direction because the receivers are vertically spaced and in the other direction because the transmitters are vertically spaced.

Figure 2.1 Radiation properties of electromagnetic wavesThe image shows some radiation properties of electromagnetic waves, which includes the microwaves. The approximate wavelengths are also indicated in this photo

5. Microwave System Design.The design of microwave radio systems involves engineering of the path to evaluate the effects of propagation on performance, development of a frequency allocation plan, and proper selection of radio and link components. This design process must ensure that outage requirements are met on a per link and system basis. The frequency allocation plan is based on four elements: the local frequency regulatory authority requirements, selected radio transmitter and receiver characteristics, antenna characteristics, and potential intrasystem and intersystem RFinterference.

6. Microwave Propagation Characteristics.Various phenomena associated with propagation, such as multipath fading and interference, affect microwave radio performance. The modes of propagation between two radio antennas may include a direct, line-of-sight (LOS) path but also a ground or surface wave that parallels the earth's surface, a sky wave from signal components reflected off the troposphere or ionosphere, a ground reflected path, and a path diffracted from an obstacle in the terrain. The presence and utility of these modes depend on the link geometry, both distance and terrain between the two antennas, and the operating frequency. For frequencies in the microwave (~2 30 GHz) band, the LOS propagation mode is the predominant mode available for use; the other modes may cause interference with the stronger LOS path. Line-of-sight links are limited in distance by the curvature of the earth, obstacles along the path, and free-space loss. Average distances for conservatively designed LOS links are 25 to 30 mi, although distances up to 100 mi have been used. For frequencies below 2 GHz, the typical mode of propagation includes non-line-of-sight (NLOS) paths, where refraction, diffraction, and reflection may extend communications coverage beyond LOS distances. The performance of both LOS and NLOS paths is affected by several phenomena, including free-space loss, terrain, atmosphere, and precipitation.

7. Strengths and Weaknesses / Advantages and DisadvantagesStrengths Adapts to difficult terrain Loss versus distance (D) = Log D (not linear) Flexible channelization Relatively short installation time Can be transportable Cost usually less than cable No back-hoe fadingWeaknessesPaths could be blocked by buildings Spectral congestion Interception possible Possible regulatory delays Sites could be difficult to maintain Towers need periodic maintenance Atmospheric fading

The advantages and disadvantages of microwave radio include the following:Advantages are:1. Do not require a right-of-way acquisition between stations.2. Each station requires the purchase or lease of only a small area of land.3. Because of their high operating frequencies, microwave systems can carry large quantities of information.4. High frequencies mean short wavelengths, which require relatively small antennas. 5. Some signals are more easily propagated around physical obstacles such as water and high mountains.6. Fewer repeaters are necessary for amplification.7. Distance between switching centers are less.8. Underground facilities are minimized.9. Minimum delay times are introduced.10. Minimal crosstalk exists between voice channels.11. Increased reliability and less maintenance are important factors.

Disadvantages are:1. It is more difficult to analyze and design circuits at microwave frequencies.2. Measuring techniques are more difficult to perfect and implement at microwave frequencies.3. It is difficult to implement conventional circuit components (resistors, capacitors, inductors, and so on) at microwave frequencies.4. Transient time is more critical at microwave frequencies.5. It is often necessary to use specialized components for microwave frequency.6. Microwave frequencies propagate in a straight line, which limits their use to line-of-sight applications.

8. Microwave ApplicationsThe tremendous growth in wireless services is made possible today through the use of microwaves for backhaul in wireless and mobile networks and for point-to-multipointnetworks. Towers can be used for both mobile, e.g. cellular, and point-to-point applications, enhancing the potential for microwave as wireless systems grow. Increases in spectrum allocations and advances in spectrum efficiency through technology have created business opportunities in the field of microwave radio. Telecommunications carriers, utility companies, and private carriers all use microwave to complement wired and optical networks. (http://www.eogogics.com)Microwave ApplicationsThe microwave frequency spectrum is used for telephone communications. Many long-distance telephone systems use microwave relay links for carrying telephone calls. With multiplexing techniques, thousands of two-way communications are modulated on a single carrier and then relayed from one station to another over long distances.Radar (Radio Detection and Ranging) also operates in the microwave region. It is a method of detecting the presence of a distant object and determining its distance and direction. Radar systems transmit a high-frequency signal which is then deflected from the distant object. The reflected signal is picked up by the radar unit and compared to the transmitted signal. The time difference between the two gives the distance to the object.Television stations and networks use microwave relay links to transmit TV signals over long distances rather than rely on coax cables.A growing application for microwave communications is space communications. 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 reflected or absorbed by the ionosphere as are many lower-frequency signals. (http://jemuelo.hubpages.com/hub/Microwave-Radio-Communications)

5. Fundamental Antenna PatternSimple Antennas The simplest antenna, in terms of its radiation pattern, is the isotropic radiator. It has zero size, is perfectly efficient, and radiates power equally in all directions. Though merely a theoretical construct, the isotropic radiator makes a good reference with which to compare the gain and directionality of other antennas. That is because, even though this antenna cannot be built and tested, its characteristics are simple and easy to derive. The half-wave dipole antenna, on the other hand, is a simple, practical antenna which is in common use. An understanding of the half-wave dipole is important both in its own right and as a basis for the study of more complex antennas. A half-wave dipole is sketched in Figure 2.2. (Blake:2007 Wireless Communication Technology)

Figure 2.2 Half-wave dipoleThe word dipole simply means it has two parts. A dipole antenna does not have to be one-half wavelength in length like the one shown in the figure, but this length is handy for impedance matching. Actually, in practice its length should be slightly less than one-half the free-space wavelength to allow for capacitive effects. A half-wave dipole is sometimes called a Hertz antenna, though strictly speaking the term Hertzian dipole refers to a dipole of infinitesimal length. This, like the isotropic radiator, is a theoretical construct; it is used in the calculation of antenna radiation patterns. Typically the length of a half-wave dipole, assuming that the conductor diameter is much less than the length of the antenna, is 95% of one-half the wavelength measured in free space.Antenna CharacteristicsRadiation Pattern

The xy plane is horizontal, and the angle is measured from the x axis in the direction of the y axis. The z axis is vertical and the angle is usually measured from the horizontal plane toward the zenith. This vertical angle, measured upward from the ground, is called the angle of elevation. Most work with antennas uses positive angles of elevation, but sometimes (as when the transmitting antenna is on a tall tower and the receiving antenna is close to it and much lower) we are interested in angles below the horizon. Different manufacturers handle below-horizon angles differently 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 to have gain can be seen from Figure 2.3. This sketch shows the pattern of a dipole, from Figure 2.2, superimposed on that of an isotropic radiator. It can be seen that while the dipole has a gain of 2.14 dBi in certain directions, in others its gain is negative. If the antennas were to be enclosed by a sphere that would absorb all the radiated power, the total radiated power would be found to be the same for both antennas. Remember that for antennas, power gain in one direction is at the expense of losses in others.

Figure 2.4. Isotropic and Dipole Antennas

Beamwidth

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

Front-to-Back RatioAs you might expect, the direction of maximum radiation in the horizontal plane is considered to be the front of the antenna, and the back is the direction 180 from the front. For a dipole, the front and back have the same radiation, but this is not always the case. Consider the unidirectional antenna shown in Figure 2.5: there is a good deal more radiation from the front of this antenna than from the back. The ratio between the gains to the front and back is the front-to-back ratio. It is generally expressed in dB, in which case it can be found by subtracting the gains in dBi or dBd.

Figure 2.5 Unidirectional antenna

Effective Isotropic Radiated Power and Effective Radiated Power

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

EIRP = PtGt

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

EIRP = ERP + 2.14dB

Where,

EIRP = effective isotropic radiated power for a given transmitter and antennaERP = effective radiated power for the same transmitter and antenna

The path loss equations require EIRP, but they can easily be used with ERP values. Simply add 2.14 dB to any ERP value to convert it to EIRP. Convert the power to dBm or dBW first, if it is not already expressed in such units.

Impedance The radiation resistance of a half-wave dipole situated in free space and fed at the center is approximately 70 ohms. The impedance is completely resistive at resonance, which occurs when the length of the antenna is about 95% of the calculated free-space half-wavelength value. The exact length depends on the diameter of the antenna conductor relative to the wavelength. If the frequency is above resonance, the feedpoint impedance has an inductive component; if the frequency is lower than resonance, the antenna impedance is capacitive. Another way of saying the same thing is that an antenna that is too short appears capacitive, while one that is too long is inductive. Figure 2.6 shows graphically how reactance varies with frequency.

Figure 2.6 Variation of dipole reactance with frequency

Polarization The polarization of a radio wave is the orientation of its electric field vector. The polarization of the radiation from a half-wave dipole is easy to determine: it is the same as the axis of the wire. That is, a horizontal antenna produces horizontally polarized waves, and a vertical antenna gives vertical polarization. It is important that the polarization be the same at both ends of a communication path. Wireless communication systems usually use vertical polarization because this is more convenient for use with portable and mobile antennas.

Monopole Antenna

Many wireless applications require antennas on vehicles. The directional effects of a horizontal dipole would be undesirable. A vertical dipole is possible, but awkward to feed in the center and rather long at some frequencies. Similar results can be obtained by using a vertical quarter-wave monopole antenna. It is mounted on a ground plane, which can be the actual ground or an artificial ground such as the body of a vehicle. The monopole is fed at the lower end with coaxial cable. The ground conductor of the feedline is connected to the ground plane. See Figure 2.7. The radiation pattern of a quarter-wave monopole in the vertical plane has the same shape as that of a vertical half-wave dipole in free space. Only half the pattern is present, however, since there is no underground radiation. In the horizontal plane, of course, a vertical 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 in free space. The input impedance at the base of a quarter-wave monopole is one-half that of a dipole. This can be explained as follows: with the same current, the antenna produces one half the radiation pattern of a dipole, and therefore one-half the radiated power. The radiated power is given by

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

If the radiated power decreases by a factor of two for a given current, then so must the feedpoint radiation resistance. In some mobile and portable applications a quarter wavelength is too long to be convenient. In that case, the electrical length of the antenna can be increased by adding inductance to the antenna. This can be done at the base or at the center, or the whole antenna can be coiled. The rubber duckie antennas on many handheld transceivers use this technique. Inductors used to increase the effective length of 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 base antenna in VHF and UHF systems. Like the quarter-wave monopole, it has omnidirectional response in the horizontal plane. However, the radiation has a higher feedpoint impedance and therefore does not require as good a ground, because the current at the feedpoint is less. The impedance is typically lowered to match that of a 50 ohms feedline by the use of an impedance-matching section. The circular section 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 circumference of each turn is about one wavelength and the turns are about a one-quarter wavelength apart.Helical antennas produce circularly polarized waves whose sense is the same as that of the helix. A helical antenna can be used to receive circularly polarized waves with the same sense and can also receive plane polarized waves with the polarization in any direction. Helical antennas are often used with VHF and UHF satellite transmissions. Since they respond to any polarization angle, they avoid the problem of Faraday rotation, which makes the polarization of waves received from a satellite impossible to predict. The gain of a helical antenna is proportional to the number of turns.

Figure 2.9 Helical Antenna

Antenna Arrays

The simple elements described above can be combined to build a more elaborate antenna. The radiation from the individual elements will combine, resulting in reinforcement in some directions and cancellation in others to give greater gain and better directional characteristics. For instance, it is often desirable to have high gain in only one direction, something that is not possible with the simple antennas previously described. In mobile and portable applications, often an omnidirectional pattern in the vertical plane is wanted, but radiation upward and downward from the antenna is of no use. In both of these situations, a properly designed array could redirect the unwanted radiation in more useful directions.Arrays can be classified as broadside or end-fire, according to their direction of maximum radiation. If the maximum radiation is along the main axis of the antenna (which may or may not coincide with the axis of its individual elements), the antenna is an end-fire array. If the maximum radiation is at right angles to this axis, the array has a broadside configuration. Antenna arrays can also be classified according to the way in which the elements are connected. A phased array has all its elements connected to the feedline. There may be phase-shifting, power-splitting, and impedance matching arrangements for individual elements, but all receive power from the feedline (assuming a transmitting antenna). Since the transmitter can be said to drive each element by supplying power, these are also called driven arrays. On the other hand, in some arrays only one element is connected to the feedline. The others work by absorbing and reradiating power radiated from the driven element. These are called parasitic elements, and the antennas are known as parasitic arrays.

ReflectorsThe antennas and arrays described in the preceding section can often be used with reflecting surfaces to improve their performance. A reflector may consist of one or more planes, or it may be parabolic in shape. In order to reduce wind and snow loads, reflectors are often constructed of mesh or closely-spaced rods. As long as the spacing is small compared with a wavelength, the effect on the antenna pattern, compared with a solid reflector, is negligible.

Parabolic ReflectorParabolic reflectors have the useful property that any ray originating at a point called the focus and striking the reflecting surface will be reflected parallel to the axis of the parabola. That is, a collimated beam of radiation will be produced. The parabolic dish antenna, familiar from backyard satellite receiver installations, consists of a small antenna at the focus of a large parabolic reflector, which focuses the signal in the same way as the reflector of a searchlight focuses a light beam. Figure 2.10 shows a typical example. Of course the antenna is reciprocal: radiation entering the dish along its axis will be focused by the reflector.

Figure 2.10 Variations of Parabolic Antenna (Courtesy of Andrew Corporation)

Standard Antenna ConstructionRadomes are used to protect microwave antennas against accumulation of ice, snow, and dirt and to reduce wind loading. All Andrew shielded antennas include a planar radome. Antennas which include a radome are indicated in the antenna specification tables. Optional molded radomes, are available for most other solid reflector, standard unshielded parabolic antennas. Radomes for shielded antennas. All Andrew shielded antennas, except ValuLine include a flexible planar radome. The radome is stretched across the opening of the shield (through tensioning springs) flexing slightly in the wind to shed ice and snow in most environments. Two types of flexible planar radomes are used, TEGLAR and Hypalon. Hypalon is a rubber coated nylon and is pro- vided with HP and HPX series antennas. TEGLAR is a polymer-coated fiberglass material and is provided with HSX, UHX and UMX type antennas. In addition, TEGLAR radomes are extremely durable, and excel in resistance to heat, rain, snow, fungus, ice accumulation, corrosive atmosphere and ultraviolet light. Upgrades to TEGLAR on HP and HPX series is optional. Pre-tensioned radomes. Some high performance antennas are supplied with a pre-tensioned radome. Pre-tensioned radomes are made from TEGLAR material bonded to a support ring. They replace the previously offered spring tensioned design. Radomes for standard antennas. Molded radomes are manufactured of ABS plastic or fiberglass. They help reduce tower wind loading and are optional for most antennas. (Terrestrial Microwave System Products, Andrew Catalogue)

Mounts All microwave antennas are supplied with a vertical tower mount. Roof, vertical tilt and horizontal tilt mounts are available as options. Shields Cylindrical shields, attached to the reflector rim, improve the radiation pattern performance of parabolic antennas. RF absorbing material is placed at critical locations inside the shield to reduce RF energy reflections.Antenna Finish Standard colors for microwave antennas and radomes are listed in the 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 color antennas will be the standard color.