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(11)

7. RADIO WAVE PROPAGATION

The propagation of radio waves throughspace (and the atmosphere) is the essentialphenomenon exploited by a radiocommunications system. As describedearlier (sec. 3), this phenomenon has beenstudied extensively using theoretical andempirical methods. The simplest mode ofpropagation occurs between two point-sources in free space—the ideal situation.

Radio-wave propagation, in realisticsituations, is affected by reflections from theearth, scattering by particles, diffraction overhills, and bending due to atmosphericrefractivity. The study of propagation has ledto models that can be used to predict thefield strength (and/or power density)expected at a specific receiver location of aradio wave radiated from a distanttransmitter location.

7.1 Transmission Loss and the PowerBudget

The concept of transmission loss is used toquantify the effects of radio wavepropagation in the analysis and engineeringof radio communications systems. It isdefined as the ratio of power delivered to theterminals of the transmitter antenna to thepower available at the terminals of thereceiver antenna. The transmitter andreceiver antenna gains are implicitlyincluded in this definition. In practice, this isnot useful, so the concept of basictransmission loss is used. It does not includethe antenna gains. Basic transmission loss Lb

is defined as the ratio of the power deliveredto a lossless isotropic antenna at the transmitlocation to the power available at theterminals of a lossless isotropic antenna atthe receive location.

With this definition of basic transmissionloss, a power budget can be developed. Thisis shown graphically in figure 34, whichshows the effects of the major elements inthe radio link.

It begins with the power output of thetransmitter Pt in decibels relative to somereference (if referenced to 1 W, this isexpressed as dBW). The loss, in decibels,due to the transmitter transmission line Lt issubtracted from Pt. Then the transmitterantenna gain Gt is added. The basictransmission loss Lb is subtracted. Then atthe receive end of the link, the receive gainGr is added; and the receiver transmissionline loss Lr is subtracted to arrive at thepower delivered to the receiver input Pr.

Expressed as an equation, the power budgetis

The received power is in the same units asthose used to express the transmitted power.

Figure 34. Gains and losses as describedin the power-budget equation

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(12)

(13)

(14)

(15)

(16)

(17)

(18)

For example, a transmit power of 25 W,expressed in decibels, would be 10 log10 (25)or 14 dBW. The result is that the poweravailable at the receiver input Pr is expressedin the same units (dBW in this example).

7.2 Free-Space Basic Transmission Loss

The transmission loss between two losslessisotropic antennas in free space4 is ahypothetical, but very useful, propagationmodel. It can be used as a “first estimate” inradio link design or a “best case” value fortransmission loss over any real, terrestrialpath.

The free-space basic transmission loss Lbf isvery easy to calculate. Since the transmitantenna is considered to be a losslessisotrope (and the transmission line isconsidered to be lossless as well), all of thetransmitter power is radiated equally in alldirections. At a distance d selected to bemuch greater than the wavelength 8, theradiated power density (expressed in wattsper square meter) is simply the transmitterpower divided by the area of a sphere withradius d, as follows:

Now, looking at the receive side of the link,the signal power output Pr in watts, of alossless, isotropic receive antenna can becomputed using equation 9. It is the productof that antenna’s effective area Ae times thepower density p of the incident wave asfollows,

Using equation 10, the effective area, insquare meters, of a lossless isotropic antenna(g = 1) is

Substituting equations 12 and 14 intoequation 13, the result is

Rearranging this equation to form thedefinition of Lbf as the ratio of Pt to Pr (seesec. 7.1),

In decibels, this equation becomes,

By converting wavelength to frequencyusing equation 1 and taking the logarithm ofthe terms in parentheses, a very practicalversion of this equation results.

where d is expressed in kilometers and f isexpressed in megahertz.

This last equation is the propagation modelfor free-space, basic transmission loss. Itpredicts a value of basic transmission lossunder a set of assumptions (i.e., lossless

4 Free space is a theoretical concept of space devoid of all matter.In practice, free space implies remoteness from material objectsthat could influence the propagation of electromagnetic waves.

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(19)

(20)

isotropic antennas located in free space). Asan example of how to use this model and thepower-budget equation, consider a radio link10 km long operating at 400 MHz. For thislink, assume that the transmit antenna has again of 10 dB, the receive antenna has a gainof 3 dB, and that both transmit and receivetransmission line losses are 1 dB. Thetransmitter power is assumed to be 20 W(13 dBW).

The basic free-space transmission loss isfirst computed as

Then, using the power budget equation 11,we find that

This result shows a receive power of aboutone one-hundredth of a microwatt, or aboutnine orders of magnitude less than thetransmit power. At this level, it is a strongreceive signal. An actual radio link, overvaried terrain, may have a larger measuredtransmission loss by 5 dB to 20 dB. Thetransmission loss over a realistic link willalso vary with time.

7.3 Terrestrial Propagation

A variety of natural and man-made objectsand phenomena will affect the radio signalas it propagates from the transmitter to the

receiver. Some of these effects are describedin the following subsections. Although it isimportant for the reader to be familiar withthese concepts, it is unnecessary for thereader to determine the exact extent thateach effect affects the antenna system.Several computer models are available thatprovide relatively accurate propagationpredictions. Such models are described insection 7.4.

7.3.1 Effects of Earth

The Earth acts as a reflecting surface forthose waves that radiate from an antenna atangles lower than the horizon. Waves thatstrike the Earth are reflected along the samedirection of travel, at the same angle as theangle of incidence. The illustration infigure 35 shows how waves propagate overboth direct and reflected paths to reach adistant location.

The reflected waves combine with the directwaves with a variety of results (i.e., theyenhance or cancel each other to somedegree). Some of the factors that cause thisvariety are the height of the antenna, theorientation of the antenna, the length of theantenna, and the characteristics of theground reflecting the wave. As part of thewave strikes the ground and reflectsforward, that part of the wave will take aslightly longer time to arrive at the receiver.

Figure 35. A propagation path illustratingdirect and reflected rays

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At some reflection angles, the direct waveand reflected wave will arrive almost inphase (i.e., the amplitude of each wave willbe at its maximum at the same time). Whenthis happens, the power of the received waveis approximately twice that of the directwave. At some angles, the reflected wave isexactly out of phase with the direct wave,essentially nullifying the wave. This isknown as cancellation. At other angles, theresultant wave will be somewhere inbetween.

As radio waves strike a radio-opaque object,some of the signal will be reflected indirections away from the receiver. Some ofthe signal will be absorbed by the object.

The waves that strike the edges of the object,however, will be diffracted into the shadowof the object. To a receiver positioned withinsuch a shadow, the object’s edge will seemlike another source. This can causeinterference with the original signal orprovide signals to areas that should notreceive them. This phenomenon is shown infigure 36.

7.3.2 Coverage

The area over which the signal can bedetected is called the coverage area for theantenna. The coverage area is oftendisplayed as contours on a two-dimensionaldrawing or on a map.

A radiation pattern is not the same thing as acoverage area, although they are related. Theradiation pattern for an antenna is a gainfactor, in every direction away from theantenna, and is a function only of theantenna design.

The coverage area for an antenna is the areaover which a signal, of predeterminedstrength (or greater), can be received. Thecoverage area is a function of the transmitpower, antenna gain, radiation pattern, noise,and propagation factors related to theenvironment.

7.3.3 Noise and Interference

All things emit some radiation at allfrequencies. In most cases, for most objects,the level of this continuous radiation, at anygiven frequency, is small and of littleconcern. This radiation is called noise. Themost common sources of noise for a radioreceiver are:

• Atmospheric and galactic noise.• Noise from the first amplifier in the

receiver.• Man-made noise (motors, flourescent

lights, etc.).

When a radio receiver is far enough awayfrom a transmitter (or in an antenna patternnull where reception is difficult), thestrength of the transmitted signal is lowenough that ambient radiation noise fromother objects or transmissions in the area

Figure 36. An example of diffraction

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can obscure the desired signal. When thestrength of a received signal is less than thestrength of the ambient noise for thatfrequency, the signal is said to be “lost in thenoise.”

Ambient noise for a specific transmission isusually measured at the transmissionfrequency.

Interference is the term for unwantedsignals, generated from other transmitters,that interfere with clear reception of theintended signal. Interference is nottechnically included under the definition ofnoise, although slang usage of the term“noise” includes any unwanted signals.

7.3.4 Terrestrial Propagation Models

Radio-wave propagation in the terrestrialenvironment is an enigmatic phenomenonwhose properties are difficult to predict.This is particularly true for LMRapplications where terrain features (hills,trees, buildings, etc.) and the ever-changingatmosphere provide scattering, reflection,refraction, and diffraction obstacles withdimensions of the same order of magnitudeas the wavelengths.

Some models are general and some are morespecialized. An example of the former is amodel that would predict radio coverageareas in “generic” urban areas or “generic”rural areas, without regard to specific terrainprofiles. One of these generalized models isthe Okumura-Hata model [16]. Models suchas the Okumura-Hata model are based onextensive collections of empiricalmeasurements.

Other more sophisticated computerprograms predict transmission loss, and

account for the time and location variabilityof that loss, over defined terrain profiles.These terrain profiles are compiled fromterrain-elevation data tabulated by agenciessuch as the Defense Mapping Agency andthe U.S. Geological Survey. One suchcomputer program, written and maintainedby the U.S. Department of Commerce’sInstitute for Telecommunication Sciences(ITS), is the Communication SystemsPerformance Model (CSPM) [17]. Thisprogram is based on the ITS IrregularTerrain Model [18].

Usually, manufacturers and vendors of radioand antenna systems and components havecomputer programs similar to CSPM toassist customers in defining radio-coverageareas. Private-industry radio-engineeringconsultants also have computer programslike CSPM to perform radio-coverageanalysis for customers. Alternatively,agencies can access CSPM on afee-reimbursable basis through the ITSInternet site5.

7.4 Co-Site Analysis

Intermodulation (IM) interference (i.e.,“intermod”) and receiver desensitization aredetrimental to the performance of co-sitedrepeaters and base stations. There are severaldifferent ways intermod interference can begenerated. One way is when sufficientlylarge power from a transmitter enters intothe final output power stage of anothertransmitter. This may occur when, forexample, the transmitters are connected to acombiner junction and the combiningcavity/isolators of the affected transmitter do

5 http://flattop.its.bldrdoc.gov/tas.html.For additional information, contact the Institute forTelecommunication Sciences at 325 Broadway (ITS.E), Boulder,Colorado 80305–3328, Telephone 303–497–5301.

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not provide sufficient rejection betweentransmitters.

Intermodulation may also arise when severaltransmitting antennas are located in veryclose proximity to each other, such asmultiple omni-directional antennas on abuilding rooftop. In these situations, mixingof signals may occur between the offendingtransmitter(s) and the desired signal, therebygenerating new signals in the victimtransmitter, at frequencies determined by theintermod products. These intermod signalswill be emitted by the victim transmitter.

Other ways that intermod can occur are bythe mixing of signals from two or moretransmitters in the front-end of a receiver.Off-frequency signals strong enough toovercome the suppression of bandpasscavities and preselector filters may saturatethe nonlinear first or second intermediatefrequency (IF) mixers of the victim receiver,creating intermod products which adverselyaffect receiver performance.

External intermod can also be created inelements such as corroded antenna guywires, anchor rods, and even chain linkfences. As strong signals impinge upon theseitems, the corroded objects act as diodes anddetect the signals, mix them, and passivelyreradiate the energy at the intermod productfrequencies.

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8. ANTENNA SYSTEM REQUIREMENTS AND DESIGN

The selection of a particular antenna systemfor use in a radio communications system isone of many interrelated decisions that mustbe made to meet the system-levelrequirements for a complete LMR system.Other system-level decisions include thenumber and locations of base and repeaterstations, the antenna heights, and thetransmitter powers.

Selecting the appropriate antennas forvehicular and hand-held units is a muchsimpler process than selecting theappropriate antennas for fixed stations.Although there are exceptions, the whipantenna is, essentially, the only practicalantenna design for vehicular and hand-heldradios. The most important aspect forvehicular antennas has to do with where it ismounted on the vehicle. The best locationfor vehicular antennas is in the middle of theroof. This is the highest point and presentsthe flattest and most symmetric ground planeto the antenna, both important factors tooptimize communications range andperformance. Vehicular obstructions, suchas the light bar on law enforcement vehicles,will, however, distort the radiation patternand/or alter the antenna’s terminalimpedance slightly.

Bumper mount installations for low-bandVHF, for example, may be selected basedmore on the antenna installation structuralrigidity requirements than on ground planesymmetry, radiation pattern distortion, andother RF performance parameters.

Choices for hand-held radio antennas are, inpractice, limited to short whip antennas.These antennas are physically very short, are

typically inductively or resistively loaded (tosimulate, electrically, a longer antennaand/or to provide a better impedance match).These features and attributes areaccomplished at the cost of decreasedantenna gain.

Selecting appropriate antennas for fixedstations is more critical than selecting theantennas for mobile and hand-held units.This is because antennas for fixed stationsmust be chosen to adequately receive signalsfrom the least-capable mobile/hand-heldunits (those with low transmitter power, lowantenna gain, low antenna height, etc.).

When designing a radio system for LMRuse, a systematic development plan must becreated and followed:

• Define System Requirements.• Design System.• Select Appropriate Components.• Procure and Install Components.• Perform Routine Maintenance.

Each of these steps is dependent on theprevious step in the chain.

8.1 Define System Requirements

The design and deployment of an LMRcommunications system, or the upgrade orexpansion of an existing system, generallybegins with some knowledge of whatgeographic regions need to be “covered” bythe radio communications system. This is afundamental requirement that is determinedby the nature of an agency’s jurisdiction,where population and transportation routesare located, and so on. These requirementsmust be developed by the agency itself.

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Several other system-level requirements canalso be developed by the agency beforeobtaining the services of a radio systemvendor or consultant. Larger lawenforcement agencies will have acommunications department that willperform these services. The number ofmobile and handheld units, for example, isusually determined by the number of usersthat must be supported.

Next, some decisions must be maderegarding how many channels will beneeded. For example, one or more channelsmay be needed for dispatch functions.Additional channels may be needed formobile-to-mobile communications. Otherchannels that might be needed couldinclude:

• A local command channel foroperations at an event site.

• A channel to interoperate with otheragencies in mutual-aid situations.

• A channel dedicated to tactical forces.• Other channels to support the

operational and administrativeactivities of the agency.

Other aspects that may affect system designinclude:

• The typical length of messages for thevarious channels and talk groups.

• Maximum level of activity for thedispatch channel.

• The nature of operations that use thelocal command channel.

• Geographic coverage requirements.• The specific level of performance (i.e.,

speech intelligibility and/or data-transmission throughput) required.

Having a working knowledge of thesesystem-level requirements will help ensureefficiency in the system design process thatfollows, and will help ensure the reliableperformance of the system when it is fullydeployed.

Begin by developing some initial system requirements. Consider, for example, thefollowing:

• At what frequencies will you betransmitting? Lower frequenciesrequire larger antennas. Higherfrequencies have more limited range.A wide range of frequencies willrequire multiple antennas, multibandantennas, or broadband antennas.

• What is the maximum distance overwhich your users must communicate?Systems to support distant itinerantusers will require a combination ofhigher-power transmitters andantennas with greater gain.

• What is your coverage area? Requiredantenna radiation patterns will bedictated by the location of the fixedsite relative to the required coveragearea.

• Are there other nearby radio systemsoperating in neighboring frequencybands or on the same or adjacentfrequency channels? Your systemmight cause interference to these otherusers, or conversely, these systemsmight cause interference to your newsystem.

• What physical limits are there on yourdesign? Is there sufficient property toconstruct a site, or is there sufficientspace on shared antenna towers foryour system? Will building code andzoning ordinances impact theconstruction of your system?

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• What about standby/back-up systemsat this site or at another antenna site?

• What level of speech intelligibility isrequired? How much noise anddistortion can be tolerated? For datatransmissions, what data-throughputrates are required (how large is thedata file and how much time isavailable for transmission)?

8.2 Design System

The decisions that must be made during thedesign of a new system or the expansion ofan existing system include:

• The number of base and repeaterstations and their locations.

• Some initial choices of antennas andantenna heights for those fixed sites.

The design process will require theidentification of:

• The availability of channels in thevarious LMR frequency bands.

• Potential/alternative base and repeaterstation locations.

• The potential for sharing fixed-stationinfrastructure among multipleagencies.

• The estimated cost of the system.

8.3 Select Appropriate Components

Once the system performance and systemdesign requirements have been ascertained,the antenna characteristics that must beconsidered are the antenna system gain andradiation pattern. The process of identifyingthe needed gain and pattern is usually aniterative one. The process might proceed asfollows:

First, coverage predictions are made. Valuesfor transmitter power, operating frequency,antenna system losses, antenna gain, antennaheight, antenna pattern, and minimumacceptable received signal strength for somespecified level of speech intelligibility arerequired in order to make these coveragepredictions.

Intermodulation interference analysis mustalso be considered during the design phase,particularly where multiple transmitters,receivers, and/or repeaters are collocated.Antenna system manufacturers and vendorscan assist agencies in predicting thelikelihood of IM interference or receiverdesensitization, either caused by or inflictedupon the proposed new system. To mitigateIM interference at repeater and base stationantenna sites, their designs will includeisolators, cavity filters, duplexers,combiners, and multicouplers. The numberof collocated systems and the frequencyseparations between them also influences thechoice of combiner and duplexercomponents.

System design and component selectioncontinue by interactively trying differentantenna gains, patterns, and heights at eachpotential site location until the desiredcoverage is attained. Then, antenna systeminstallation at each fixed site needs to beconsidered. Points to consider include:

• Antenna tower height—Will theproposed antenna tower conform tolocal building codes and zoningordinances?

• Environmental considerations—Willthe antenna system and supportingstructure survive expected windloading, ice loading, and otheranticipated environmental

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performance factors? EIA/TIAStandard 329-B [19], EIA/TIAStandard 329-B(1) [20], NIJ Standard-0204.02 [21], andNIJ Standard-0205.02 [22] all provideguidance regarding the minimumenvironmental, as well as RF,performance criteria required of allantennas used in the law enforcementand corrections arenas.

• Security—Is the site secure againstunauthorized intrusions, yet accessibleto maintenance personnel?

• Accessibility—Could inclementweather (i.e., deep snow drifts,washed-out dirt access roads) preventmaintenance personnel from reachingthe site?

• Co-site analysis and IMinterference—Will retrofitting newsystems into existing infrastructuresintroduce adjacent-channelinterference, co-channel interference,IM interference, or receiverdesensitization upon other existingsystems or upon the new system? Willother nearby users and systemsdetrimentally affect the performanceof the new system because of theseproblems? How will the vendormitigate predicted interferenceproblems? Does the vendor have aplan to mitigate unforeseeninterference problems?

• Power source—Is commercialelectrical power available? Willsolar/battery power be required asprimary/backup power?

• Wireline/wireless link—Is telephoneservice available? Is fiber opticsservice available? Will microwaveradio be required?

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9. INSTALLATION, MAINTENANCE, AND SAFETY

9.1 Vehicular Antenna Systems

The procurement and installation ofvehicular antenna systems are relativelystraightforward. If the design requirementshave been well thought out and adequatelydescribed in the procurement documents, acompetitive procurement will deliver anacceptable product.

When installing a vehicular antenna system,care must be given to routing the coaxialcable between the radio and antenna. Thecable should not be exposed to the elements(wind, road salt and sand, rain, intensesunlight, extreme heat, etc.) nor should it bein a location where it could be severed orpinched (by opening and closing vehicledoors, for example) or where a vehicle’soccupants might become entangled with it.One preferred routing for a roof-mountedantenna might be between the roof andinterior headliner of the vehicle, downthrough a windshield pillar, and behind thedashboard panel to the radio. Otherequipment, such as duplexers (to combinemultiple radios operating in differentfrequency bands onto one multibandantenna), should likewise be installed inlocations not readily accessible to vehicleoccupants. For example, they should beinstalled under seats or in trunks where theyare “out of the way,” yet reasonablyaccessible to maintenance personnel.

RF cable connections should be torqued tothe proper force recommended for theparticular connectors used, and the outergrounded conductor of the antenna basemount must be RF-bonded to the metal ofthe roof or trunk deck. Trunk deckinstallations also require excellent RF

bonding from the trunk lid to the mainvehicle body. One way to accomplish this isby using a short length of low-impedancecopper grounding strap affixed to bare areasof (interior) sheet metal on the underside ofthe trunk deck and the main vehicle body,using noncorrosive bolts, star washers, andlock nuts. A poor ground connection willdetrimentally affect antenna operation,resulting in erratic or unacceptableperformance.

9.2 Fixed-Site Antenna Systems

Most new installations of fixed-repeater andbase-station antenna systems will likely beperformed by contracted installers orequipment suppliers. In many cases,retrofitting new or upgraded componentsinto existing facilities will similarly beperformed by contracted installers orequipment suppliers. Procuring agenciesshould, nevertheless, ensure that the installerobserves sound installation practices. Aswith vehicular grounding, proper groundprotection of fixed station antenna facilitiesis important.

9.2.1 Fixed-Site Antenna SystemGrounding and Bonding Practices

An effective grounding system is necessaryfor every antenna tower. In addition to theprotection a grounding system offers fromlightning strikes, grounding also:

• Reduces the hazards of electricalshock resulting from ground/neutralpower faults.

• Protects wiring and circuitry bylimiting extraneous over-voltages.

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• Facilitates rapid discharge of faultedpower circuits.

• Reduces noise voltages.• Provides a path to dissipate any stray

RF current present inside thetransmitter station; ungrounded RFcurrents can contribute to equipmentmalfunction, or create interferencewith other receivers.

9.2.2 Fixed-Station RF Bonding

RF bonding is another important aspect offixed-station antenna systems. Simplyconnecting each element of a transmitterfacility to a metal pipe stuck in the ground isbarely adequate to act as a groundingsystem. The components of such agrounding system are not perfect conductorsand each will have different, finite values forresistance. The resistance and physicaldesign of the grounding system adds to theoverall resistance and reactance of theantenna system and transmission line,causing the system to have different voltagepotentials at different points within thesystem, inducing stray currents to flowbetween equipment chassis. These straycurrents can affect internal circuits of theequipment and cause erratic operation andunpredictable behavior.

A bonding system ensures that all equipmentgrounding points are at the same electricalpotential. A good dc and RF bonding systemwill use high-quality, low-impedance copperstrap or braid and attach all equipmentchassis to a low-impedance copper bus strapinstalled on the walls throughout the stationfacilities. The copper bus leaves the stationand is attached to the Earth using a copperground rod approximately 3 m long. Thepoint of egress for the facility ground should

be at the same entry point as RF cable,telephone, and power connections.

9.3 Lightning Protection

The National Fire Protection Association(NFPA) publishes a guideline related tolightning protection [23]. This guidelinedetails many additional practices forprotecting radio equipment from lightningstrikes.

Metal antennas and towers should beconnected to the building’s lightningprotection system. Wires and metallicelements comprising an antenna tower’slightning protection system should beelectrically attached to the Earth. Towersand guy wires anchored to concrete forms inthe ground are often assumed to be wellgrounded, but concrete is a poor electricalconductor. Tower legs should be electricallyattached to the Earth with a copper groundstake approximately 3 m long. Lightning-ground connecting leads connecting thetower to the ground stake should be at leastAWG #10 copper, AWG #8 aluminum, or3/4 in copper braid. The transmission lines must be protected bylightning arresters, protectors, and dischargeunits. Arresters can be placed at both ends ofthe transmission line for added protection.

9.4 During Installation

Be sure the installer knows exactly where onthe vehicle or on the antenna tower theantenna components are to be installed.Make sure they are installed in the correctorientation and positioned correctly.

Make sure that the ends of the transmissionline cable have been prepared properlybefore affixing RF connectors to the

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transmission line. Make sure that the RFconnectors are properly installed on thetransmission line. Loose connectorassemblies will result in poor groundconnections between the outer connectorshell and the outer shield of the transmissionline. Make sure that the center pin issecurely affixed to the center conductor ofthe coaxial transmission line and that thecenter pin’s depth, relative to the connectorshell, is maintained at the correct distance,or an impedance mismatch or connectordamage will result. Make sure that theantenna and transmission line connectorswill mate properly before connection toother equipment is attempted. Connectorscan be easily misaligned or over- or under-torqued, resulting in degraded or erraticoverall RF performance. Make sure that thetransmission line has not been damaged inany way, such as crushed, severed, orpinched.

Check the VSWR as soon as possible afterinstallation, and, if possible, before theinstaller leaves the job site. In addition toVSWR measurements, time-domainreflectometry (TDR), line-faultmeasurements are helpful. Use a portabletransmitting unit if the radio transmitter isnot yet installed. Record the VSWR valuesand TDR data for future reference.

Measure and record the ambient noise powerlevels for future reference.

If possible, the installer should conduct“over-the-air” RF power sensitivitymeasurements immediately after installation,and document the test configuration andmeasurement results for future reference.For example, a portable antenna mast, 7 mto 10 m high, could be placed at a geologicalsurvey marker that has unobstructed, clear,

visual line-of-sight to the repeater or basestation-antenna tower. A portable transmittercould serve as the signal source. A similarmethod could be used to measure the powerreceived by a portable radio service monitor(such as an IFR1500 or Motorola R-2670)located at the survey-marker point.

Make a physical inspection of theinstallation. Be sure all connectors andtransmission lines are secured properly.

9.5 Perform Routine Maintenance

After a vehicular or repeater/base-stationantenna system has been installed, thesystem will require periodic maintenance toensure optimum performance.

Agencies should practice three tiers ofmaintenance. The first is performed by theradio operators and consists of simple,“common-sense” inspection of theequipment. The second level of maintenanceis performed by site technicians and requiresthe use of land mobile radio test equipment.The third level of maintenance is performedby factory-authorized technicians.

9.5.1 Local Inspection

Radio operators themselves can perform awide variety of simple aural and visualinspections of their radio equipment andantennas. For instance:

• Isolated problems noted with receptionor transmission in the field. Directcomparison between two radios(“I cannot hear the base station when Iam in this location.” “Oh, really? I canhear the base station okay.”) gives anexcellent indication of a problem witha subscriber unit.

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• Loose or missing connectors—Overtime, temperature variations, shock,vibration, exposure to the elements,and handling can cause connectionsand connector flanges to become looseor missing altogether. Rubber O-ringgrommet seals may deteriorate,allowing moisture to penetrate intoconnectors or the transmission line,altering their performance.

• Cracked or broken whip antenna base-loading coils—Cracks in the plastichousing of vehicular antenna base-loading coils can permit moisture andcorrosive materials to penetrate intothe loading coil, altering the antenna’selectrical performance. Weathering ofrubber grommet O-ring seals (wherethe loading coil is affixed to the roofor trunk deck of the vehicle) maysimilarly permit moisture andcorrosive materials to penetrate intothe connector between the loading coiland its attachment to the coaxial cableconnector, altering the antenna’selectrical performance.

9.5.2 Site Technician

The site technician will often have an arrayof RF test equipment at his or her disposal.For example, a communications servicemonitor can determine whether a radio istransmitting on the proper frequency, at theproper power level, with the properfrequency bandwidth. A VSWR meter givesan indication of how well RF energy iscoupled from the radio system into theantenna. A time-domain reflectometer candetermine where faults or otherdiscontinuities exist along the length of atransmission line. Portable field strengthmeters can give an indication as to whether

the antenna radiates electromagnetic energyas expected.

Vehicular antenna systems should beperiodically checked according to anestablished maintenance schedule, typicallyconcurrent with the maintenance schedule ofthe mobile radio (perhaps once or twice peryear). Performance values such as VSWRshould be recorded, compared to theperformance values measured just afterinstallation, and tracked over time in orderto assist in keeping the antenna systemfunctioning at an optimal performance level.

The above statements apply equally to fixed-site repeaters and base stations.Unfortunately, whereas problems withmobile and portable radio systems can bereadily identified by direct performancecomparison at the operator level, such is notnecessarily the case for fixed sites. Becauseperformance degradation at the fixed siteaffects all subscriber units equally, slowlyoccurring degradation in performance,caused by corrosion effects, weathering, etc.,may go unnoticed for years untilcatastrophic failure finally occurs.Therefore, regularly scheduled site visits toconduct maintenance performanceinspections must be a part of the sitetechnician’s routine. Recording a timehistory of the RF performance, andcomparing it to the RF performancemeasured immediately following therepeater or base-station installation, will bean invaluable maintenance aid.

Remote automated in-line diagnostic testequipment can also provide indications ofneeded maintenance. Several vendors ofantenna system equipment, such as BirdElectronic Corp., Decibel Products, SinclairTechnologies, Telewave, and many others,

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manufacture remote in-line diagnosticmeasurement equipment. This equipmentcan report the health and status of a fixedrepeater/base-station site’s RF performance,up to the point where the radio wave islaunched into space. For example,conditions of low transmitter power and/orexcessive VSWR, which might result fromdetuned combiner cavities or duplexers,failing radios, weathering of components,etc., can be sensed by the automateddiagnostic equipment. Alarm conditions canautomatically be reported to a computer bypreconfigured telephone dial-up to thecentralized maintenance facility.

9.5.3 Factory-Authorized Technicians

If an antenna system or radio problem is toocomplicated for the local site technician torepair, the manufacturer should becontacted. Many large public safetyorganizations have service contracts withfactory-authorized repair facilities tomaintain, repair, and replace equipment.Even without a contract, contacting themanufacturer about specific problems isadvised if the problems are beyond theabilities of the agency to repair.

9.5.4 Antenna Tower Safety

Working on antenna towers can bedangerous and potentially fatal. Seriouspersonal injury and equipment damage canresult from personnel falling, improperlyinstalled equipment, and RF radiationexposure.

When maintenance personnel must work onantenna towers, safety equipment should beselected, used, and cared for as if their livesdepend on it—because they do! A list ofsafety equipment should include:

• Safety belt.• Safety glasses.• Work boots with firm, nonslip soles

and well-defined heels.• Hard hat.

It is recommended that antenna towerinstallations have a personnel fall-arrestersystem. These systems permit maintenancepersonnel to attach their safety belts atground level, and remain attached duringtower ascent and descent, as well as whileworking at height. Antenna towermanufacturers, such as Rohn Tower, offerfall arrester systems, which comply withOSHA regulations.

In addition to personnel safety whileascending, working on, or descendingantenna towers, maintenance personnel mustbe cognizant to the danger of RF electricalburns arising from direct contact withenergized antenna elements, and to exposureto high levels of RF radiation. Industrystandards have been developed that specifythe levels of electromagnetic exposure towhich personnel can be “safely” exposed[24]. Personnel working on transmittingantenna towers must ensure that all antennasthat they are working on or near aredisconnected from their associatedtransmitters, and that the transmitters arerouted into dummy loads (to prevent damageto the transmitter final amplifier in the eventof inadvertent transmissions), or that somesort of fail-safe mechanism fordisconnecting power to those transmittershas been engaged.

Lastly, maintenance personnel should bealert to weather conditions. It is ill-advisedto work on an antenna tower during anelectrical storm or in high winds.Remember—safety first!

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9.5.5 Vehicular Antenna Systems

The same precautions regarding RFexposure on antenna towers should beobserved when operating or performingmaintenance on vehicular antenna systems.For example, personnel should not standoutside and next to their vehicle (where theydo not have the benefit of RF shieldingafforded by the vehicle’s roof that theywould have if they were inside the passengercompartment of the vehicle) while operatinga 45 W (or greater) mobile radio transmitter,nor should they touch the antenna whentransmitting.

9.5.6 The Importance of MaintainingYour Radio System

It must be stressed that regularly scheduled,periodic testing and preventive maintenanceof the entire radio system is of paramountimportance. Motor vehicles used by lawenforcement and corrections, fire,emergency medical services, and otherpublic-safety agencies are maintained bycomplying with strict maintenanceschedules. Weapons are maintained byfollowing regular preventive maintenanceschedules of cleaning, lubrication, andinspection for worn or damaged parts. Thesame must hold true for all parts of a radiocommunications system—the lives of fire,emergency medical services, and lawenforcement and corrections personnel maydepend on it.

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10. ANTENNA SYSTEM RESOURCES

This section describes some of the resourcesavailable to agencies to research informationabout and identify pertinent products,antenna systems and related components.There are many qualified and conscientiousmanufacturers and products available; only afew are cited herein in order to provide asampling of what is available.

10.1 Internet Resources

As the World Wide Web (WWW or Web)has grown in recent years, many antennamanufacturers and suppliers have createdWeb pages devoted to their products.

Most Web-page authors will register theirpage with one or more of the major “searchengines” on the Web. A search engine is aprogram that will use a search phraseprovided by a user (usually a single word orsimple phrase) to search hundreds ofthousands of Web pages looking foroccurrences of the phrase. The engine willthen return a list of those Web pages thatmost closely match the search criteria (i.e.,those that contain the search phrase). A fewsearch engines, such as Metacrawler(http://www.metacrawler.com) conductsearches by simultaneously querying morethan one search engine.

10.2 Periodicals

Several periodicals are written for the landmobile radio industry. Most are free to“qualified” subscribers. These periodicalscontain articles of technical interest relatedto land mobile radio, and advertisements forland mobile radio systems, components, andservices. These periodicals publish, asspecial issues, buyers’ guides on an annual

or biannual basis. One such periodical thatdoes this is Mobile Radio Technology (URL:http://mrtmag.com, telephone1–913–341–1300). Another is APCO’smonthly APCO Bulletin (URL:http://www.apcointl.org/bulletin/, telephone1–888–APCO–911). Public libraries maymaintain subscriptions to this and otherLMR-related periodicals, or they may beable to obtain issues under interlibrary loanagreements.

10.3 Manufacturers’ and Vendors’Catalogs

Most antenna manufacturers and vendorsdistribute free catalogs that describeantennas, transmission lines, and relatedcomponents that they offer for sale. Mostcatalogs provide useful technicalinformation such as antenna patterns,operating frequencies, physical dimensions,and costs. Frequently, these catalogs willprovide basic explanations about variousaspects of antenna systems, such ascollocated transmitter-combiningtechniques, intermodulation interference,transmission line theory, etc.

Many manufacturers have more than oneproduct line, e.g., antennas and duplexers.Information about manufacturers’ offeringscan be identified and researched via theInternet or through the periodicals andbuyers' guides discussed in the previous twosubsections.