The propagation of radio waves through With this definition of basic transmission space (and the atmosphere) is the essential loss, a power budget can be developed. This phenomenon exploited by a radio is shown graphically in figure 34, which communications system. As described shows the effects of the major elements in earlier (sec. 3), this phenomenon has been the radio link. studied extensively using theoretical and empirical methods. The simplest mode of propagation occurs between two point- sources in free space—the ideal situation.
Radio-wave propagation, in realistic situations, is affected by reflections from the earth, scattering by particles, diffraction over hills, and bending due to atmospheric refractivity. The study of propagation has led to models that can be used to predict the field strength (and/or power density) expected at a specific receiver location of a radio wave radiated from a distant Figure 34. Gains and losses as described transmitter location. in the power-budget equation
7.1 Transmission Loss and the Power It begins with the power output of the
Budget transmitter Pt in decibels relative to some reference (if referenced to 1 W, this is The concept of transmission loss is used to expressed as dBW). The loss, in decibels, quantify the effects of radio wave due to the transmitter transmission line L is propagation in the analysis and engineering t subtracted from P . Then the transmitter of radio communications systems. It is t antenna gain G is added. The basic defined as the ratio of power delivered to the t transmission loss L is subtracted. Then at terminals of the transmitter antenna to the b the receive end of the link, the receive gain power available at the terminals of the G is added; and the receiver transmission receiver antenna. The transmitter and r line loss L is subtracted to arrive at the receiver antenna gains are implicitly r power delivered to the receiver input P . included in this definition. In practice, this is r not useful, so the concept of basic Expressed as an equation, the power budget transmission loss is used. It does not include is the antenna gains. Basic transmission loss Lb is defined as the ratio of the power delivered to a lossless isotropic antenna at the transmit (11) location to the power available at the The received power is in the same units as terminals of a lossless isotropic antenna at those used to express the transmitted power. the receive location.
33 For example, a transmit power of 25 W, (13) expressed in decibels, would be 10 log10 (25) or 14 dBW. The result is that the power available at the receiver input Pr is expressed Using equation 10, the effective area, in in the same units (dBW in this example). square meters, of a lossless isotropic antenna (g = 1) is 7.2 Free-Space Basic Transmission Loss The transmission loss between two lossless (14) isotropic antennas in free space4 is a hypothetical, but very useful, propagation Substituting equations 12 and 14 into model. It can be used as a “first estimate” in equation 13, the result is radio link design or a “best case” value for transmission loss over any real, terrestrial path. (15)
The free-space basic transmission loss L is bf Rearranging this equation to form the very easy to calculate. Since the transmit definition of L as the ratio of P to P (see antenna is considered to be a lossless bf t r sec. 7.1), isotrope (and the transmission line is considered to be lossless as well), all of the transmitter power is radiated equally in all (16) directions. At a distance d selected to be much greater than the wavelength 8, the In decibels, this equation becomes, radiated power density (expressed in watts per square meter) is simply the transmitter power divided by the area of a sphere with (17) radius d, as follows:
(12) By converting wavelength to frequency using equation 1 and taking the logarithm of the terms in parentheses, a very practical Now, looking at the receive side of the link, version of this equation results. the signal power output Pr in watts, of a lossless, isotropic receive antenna can be computed using equation 9. It is the product (18) of that antenna’s effective area Ae times the power density p of the incident wave as where d is expressed in kilometers and f is follows, expressed in megahertz.
This last equation is the propagation model for free-space, basic transmission loss. It 4 Free space is a theoretical concept of space devoid of all matter. predicts a value of basic transmission loss In practice, free space implies remoteness from material objects that could influence the propagation of electromagnetic waves. under a set of assumptions (i.e., lossless
34 isotropic antennas located in free space). As receiver. Some of these effects are described an example of how to use this model and the in the following subsections. Although it is power-budget equation, consider a radio link important for the reader to be familiar with 10 km long operating at 400 MHz. For this these concepts, it is unnecessary for the link, assume that the transmit antenna has a reader to determine the exact extent that gain of 10 dB, the receive antenna has a gain each effect affects the antenna system. of 3 dB, and that both transmit and receive Several computer models are available that transmission line losses are 1 dB. The provide relatively accurate propagation transmitter power is assumed to be 20 W predictions. Such models are described in (13 dBW). section 7.4.
The basic free-space transmission loss is 7.3.1 Effects of Earth first computed as The Earth acts as a reflecting surface for those waves that radiate from an antenna at angles lower than the horizon. Waves that strike the Earth are reflected along the same (19) direction of travel, at the same angle as the angle of incidence. The illustration in figure 35 shows how waves propagate over both direct and reflected paths to reach a Then, using the power budget equation 11, distant location. we find that
(20)
This result shows a receive power of about Figure 35. A propagation path illustrating one one-hundredth of a microwatt, or about direct and reflected rays nine orders of magnitude less than the transmit power. At this level, it is a strong The reflected waves combine with the direct receive signal. An actual radio link, over waves with a variety of results (i.e., they varied terrain, may have a larger measured enhance or cancel each other to some transmission loss by 5 dB to 20 dB. The degree). Some of the factors that cause this transmission loss over a realistic link will variety are the height of the antenna, the also vary with time. orientation of the antenna, the length of the antenna, and the characteristics of the 7.3 Terrestrial Propagation ground reflecting the wave. As part of the A variety of natural and man-made objects wave strikes the ground and reflects and phenomena will affect the radio signal forward, that part of the wave will take a as it propagates from the transmitter to the slightly longer time to arrive at the receiver.
35 At some reflection angles, the direct wave 7.3.2 Coverage and reflected wave will arrive almost in The area over which the signal can be phase (i.e., the amplitude of each wave will detected is called the coverage area for the be at its maximum at the same time). When antenna. The coverage area is often this happens, the power of the received wave displayed as contours on a two-dimensional is approximately twice that of the direct drawing or on a map. wave. At some angles, the reflected wave is exactly out of phase with the direct wave, A radiation pattern is not the same thing as a essentially nullifying the wave. This is coverage area, although they are related. The known as cancellation. At other angles, the radiation pattern for an antenna is a gain resultant wave will be somewhere in factor, in every direction away from the between. antenna, and is a function only of the antenna design. As radio waves strike a radio-opaque object, some of the signal will be reflected in The coverage area for an antenna is the area directions away from the receiver. Some of over which a signal, of predetermined the signal will be absorbed by the object. strength (or greater), can be received. The coverage area is a function of the transmit The waves that strike the edges of the object, power, antenna gain, radiation pattern, noise, however, will be diffracted into the shadow and propagation factors related to the of the object. To a receiver positioned within environment. such a shadow, the object’s edge will seem like another source. This can cause 7.3.3 Noise and Interference interference with the original signal or provide signals to areas that should not All things emit some radiation at all receive them. This phenomenon is shown in frequencies. In most cases, for most objects, figure 36. the level of this continuous radiation, at any given frequency, is small and of little concern. This radiation is called noise. The most common sources of noise for a radio receiver 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 away from a transmitter (or in an antenna pattern Figure 36. An example of diffraction null where reception is difficult), the strength of the transmitted signal is low enough that ambient radiation noise from other objects or transmissions in the area
36 can obscure the desired signal. When the account for the time and location variability strength of a received signal is less than the of that loss, over defined terrain profiles. strength of the ambient noise for that These terrain profiles are compiled from frequency, the signal is said to be “lost in the terrain-elevation data tabulated by agencies noise.” such as the Defense Mapping Agency and the U.S. Geological Survey. One such Ambient noise for a specific transmission is computer program, written and maintained usually measured at the transmission by the U.S. Department of Commerce’s frequency. Institute for Telecommunication Sciences (ITS), is the Communication Systems Interference is the term for unwanted Performance Model (CSPM) [17]. This signals, generated from other transmitters, program is based on the ITS Irregular that interfere with clear reception of the Terrain Model [18]. intended signal. Interference is not technically included under the definition of Usually, manufacturers and vendors of radio noise, although slang usage of the term and antenna systems and components have “noise” includes any unwanted signals. computer programs similar to CSPM to assist customers in defining radio-coverage 7.3.4 Terrestrial Propagation Models areas. Private-industry radio-engineering consultants also have computer programs Radio-wave propagation in the terrestrial like CSPM to perform radio-coverage environment is an enigmatic phenomenon analysis for customers. Alternatively, whose properties are difficult to predict. agencies can access CSPM on a This is particularly true for LMR fee-reimbursable basis through the ITS applications where terrain features (hills, Internet site5. trees, buildings, etc.) and the ever-changing atmosphere provide scattering, reflection, 7.4 Co-Site Analysis refraction, and diffraction obstacles with dimensions of the same order of magnitude Intermodulation (IM) interference (i.e., as the wavelengths. “intermod”) and receiver desensitization are detrimental to the performance of co-sited Some models are general and some are more repeaters and base stations. There are several specialized. An example of the former is a different ways intermod interference can be model that would predict radio coverage generated. One way is when sufficiently areas in “generic” urban areas or “generic” large power from a transmitter enters into rural areas, without regard to specific terrain the final output power stage of another profiles. One of these generalized models is transmitter. This may occur when, for the Okumura-Hata model [16]. Models such example, the transmitters are connected to a as the Okumura-Hata model are based on combiner junction and the combining extensive collections of empirical cavity/isolators of the affected transmitter do measurements.
Other more sophisticated computer 5 http://flattop.its.bldrdoc.gov/tas.html. For additional information, contact the Institute for programs predict transmission loss, and Telecommunication Sciences at 325 Broadway (ITS.E), Boulder, Colorado 80305–3328, Telephone 303–497–5301.
37 not provide sufficient rejection between Other ways that intermod can occur are by transmitters. the mixing of signals from two or more transmitters in the front-end of a receiver. Intermodulation may also arise when several Off-frequency signals strong enough to transmitting antennas are located in very overcome the suppression of bandpass close proximity to each other, such as cavities and preselector filters may saturate multiple omni-directional antennas on a the nonlinear first or second intermediate building rooftop. In these situations, mixing frequency (IF) mixers of the victim receiver, of signals may occur between the offending creating intermod products which adversely transmitter(s) and the desired signal, thereby affect receiver performance. generating new signals in the victim transmitter, at frequencies determined by the External intermod can also be created in intermod products. These intermod signals elements such as corroded antenna guy will be emitted by the victim transmitter. wires, anchor rods, and even chain link fences. As strong signals impinge upon these items, the corroded objects act as diodes and detect the signals, mix them, and passively reradiate the energy at the intermod product frequencies.
38 8. ANTENNA SYSTEM REQUIREMENTS AND DESIGN
The selection of a particular antenna system typically inductively or resistively loaded (to for use in a radio communications system is simulate, electrically, a longer antenna one of many interrelated decisions that must and/or to provide a better impedance match). be made to meet the system-level These features and attributes are requirements for a complete LMR system. accomplished at the cost of decreased Other system-level decisions include the antenna gain. number and locations of base and repeater stations, the antenna heights, and the Selecting appropriate antennas for fixed transmitter powers. stations is more critical than selecting the antennas for mobile and hand-held units. Selecting the appropriate antennas for This is because antennas for fixed stations vehicular and hand-held units is a much must be chosen to adequately receive signals simpler process than selecting the from the least-capable mobile/hand-held appropriate antennas for fixed stations. units (those with low transmitter power, low Although there are exceptions, the whip antenna gain, low antenna height, etc.). antenna is, essentially, the only practical antenna design for vehicular and hand-held When designing a radio system for LMR radios. The most important aspect for use, a systematic development plan must be vehicular antennas has to do with where it is created and followed: mounted on the vehicle. The best location for vehicular antennas is in the middle of the • Define System Requirements. roof. This is the highest point and presents • Design System. the flattest and most symmetric ground plane • Select Appropriate Components. to the antenna, both important factors to • Procure and Install Components. optimize communications range and • Perform Routine Maintenance. performance. Vehicular obstructions, such as the light bar on law enforcement vehicles, Each of these steps is dependent on the will, however, distort the radiation pattern previous step in the chain. and/or alter the antenna’s terminal impedance slightly. 8.1 Define System Requirements The design and deployment of an LMR Bumper mount installations for low-band communications system, or the upgrade or VHF, for example, may be selected based expansion of an existing system, generally more on the antenna installation structural begins with some knowledge of what rigidity requirements than on ground plane geographic regions need to be “covered” by symmetry, radiation pattern distortion, and the radio communications system. This is a other RF performance parameters. fundamental requirement that is determined by the nature of an agency’s jurisdiction, Choices for hand-held radio antennas are, in where population and transportation routes practice, limited to short whip antennas. are located, and so on. These requirements These antennas are physically very short, are must be developed by the agency itself.
39 Several other system-level requirements can Having a working knowledge of these also be developed by the agency before system-level requirements will help ensure obtaining the services of a radio system efficiency in the system design process that vendor or consultant. Larger law follows, and will help ensure the reliable enforcement agencies will have a performance of the system when it is fully communications department that will deployed. perform these services. The number of mobile and handheld units, for example, is Begin by developing some initial system usually determined by the number of users requirements. Consider, for example, the that must be supported. following:
Next, some decisions must be made • At what frequencies will you be regarding how many channels will be transmitting? Lower frequencies needed. For example, one or more channels require larger antennas. Higher may be needed for dispatch functions. frequencies have more limited range. Additional channels may be needed for A wide range of frequencies will mobile-to-mobile communications. Other require multiple antennas, multiband channels that might be needed could antennas, or broadband antennas. include: • What is the maximum distance over which your users must communicate? • A local command channel for Systems to support distant itinerant operations at an event site. users will require a combination of • A channel to interoperate with other higher-power transmitters and agencies in mutual-aid situations. antennas with greater gain. • A channel dedicated to tactical forces. • What is your coverage area? Required • Other channels to support the antenna radiation patterns will be operational and administrative dictated by the location of the fixed activities of the agency. site relative to the required coverage area. Other aspects that may affect system design • Are there other nearby radio systems include: operating in neighboring frequency bands or on the same or adjacent • The typical length of messages for the frequency channels? Your system various channels and talk groups. might cause interference to these other • Maximum level of activity for the users, or conversely, these systems dispatch channel. might cause interference to your new • The nature of operations that use the system. local command channel. • What physical limits are there on your • Geographic coverage requirements. design? Is there sufficient property to • The specific level of performance (i.e., construct a site, or is there sufficient speech intelligibility and/or data- space on shared antenna towers for transmission throughput) required. your system? Will building code and zoning ordinances impact the construction of your system?
40 • What about standby/back-up systems First, coverage predictions are made. Values at this site or at another antenna site? for transmitter power, operating frequency, • What level of speech intelligibility is antenna system losses, antenna gain, antenna required? How much noise and height, antenna pattern, and minimum distortion can be tolerated? For data acceptable received signal strength for some transmissions, what data-throughput specified level of speech intelligibility are rates are required (how large is the required in order to make these coverage data file and how much time is predictions. available for transmission)? Intermodulation interference analysis must 8.2 Design System also be considered during the design phase, particularly where multiple transmitters, The decisions that must be made during the receivers, and/or repeaters are collocated. design of a new system or the expansion of Antenna system manufacturers and vendors an existing system include: can assist agencies in predicting the likelihood of IM interference or receiver • The number of base and repeater desensitization, either caused by or inflicted stations and their locations. upon the proposed new system. To mitigate • Some initial choices of antennas and IM interference at repeater and base station antenna heights for those fixed sites. antenna sites, their designs will include isolators, cavity filters, duplexers, The design process will require the combiners, and multicouplers. The number identification of: of collocated systems and the frequency separations between them also influences the • The availability of channels in the choice of combiner and duplexer various LMR frequency bands. components. • Potential/alternative base and repeater station locations. System design and component selection • The potential for sharing fixed-station continue by interactively trying different infrastructure among multiple antenna gains, patterns, and heights at each agencies. potential site location until the desired • The estimated cost of the system. coverage is attained. Then, antenna system installation at each fixed site needs to be 8.3 Select Appropriate Components considered. Points to consider include: Once the system performance and system design requirements have been ascertained, • Antenna tower height—Will the the antenna characteristics that must be proposed antenna tower conform to considered are the antenna system gain and local building codes and zoning radiation pattern. The process of identifying ordinances? the needed gain and pattern is usually an • Environmental considerations—Will iterative one. The process might proceed as the antenna system and supporting follows: structure survive expected wind loading, ice loading, and other anticipated environmental
41 performance factors? EIA/TIA • Co-site analysis and IM Standard 329-B [19], EIA/TIA interference—Will retrofitting new Standard 329-B(1) [20], NIJ Standard- systems into existing infrastructures 0204.02 [21], and introduce adjacent-channel NIJ Standard-0205.02 [22] all provide interference, co-channel interference, guidance regarding the minimum IM interference, or receiver environmental, as well as RF, desensitization upon other existing performance criteria required of all systems or upon the new system? Will antennas used in the law enforcement other nearby users and systems and corrections arenas. detrimentally affect the performance • Security—Is the site secure against of the new system because of these unauthorized intrusions, yet accessible problems? How will the vendor to maintenance personnel? mitigate predicted interference • Accessibility—Could inclement problems? Does the vendor have a weather (i.e., deep snow drifts, plan to mitigate unforeseen washed-out dirt access roads) prevent interference problems? maintenance personnel from reaching • Power source—Is commercial the site? electrical power available? Will solar/battery power be required as primary/backup power? • Wireline/wireless link—Is telephone service available? Is fiber optics service available? Will microwave radio be required?
42 9. INSTALLATION, MAINTENANCE, AND SAFETY
9.1 Vehicular Antenna Systems bonding from the trunk lid to the main vehicle body. One way to accomplish this is The procurement and installation of by using a short length of low-impedance vehicular antenna systems are relatively copper grounding strap affixed to bare areas straightforward. If the design requirements of (interior) sheet metal on the underside of have been well thought out and adequately the trunk deck and the main vehicle body, described in the procurement documents, a using noncorrosive bolts, star washers, and competitive procurement will deliver an lock nuts. A poor ground connection will acceptable product. detrimentally affect antenna operation, resulting in erratic or unacceptable When installing a vehicular antenna system, performance. care must be given to routing the coaxial cable between the radio and antenna. The 9.2 Fixed-Site Antenna Systems cable should not be exposed to the elements (wind, road salt and sand, rain, intense Most new installations of fixed-repeater and sunlight, extreme heat, etc.) nor should it be base-station antenna systems will likely be in a location where it could be severed or performed by contracted installers or pinched (by opening and closing vehicle equipment suppliers. In many cases, doors, for example) or where a vehicle’s retrofitting new or upgraded components occupants might become entangled with it. into existing facilities will similarly be One preferred routing for a roof-mounted performed by contracted installers or antenna might be between the roof and equipment suppliers. Procuring agencies interior headliner of the vehicle, down should, nevertheless, ensure that the installer through a windshield pillar, and behind the observes sound installation practices. As dashboard panel to the radio. Other with vehicular grounding, proper ground equipment, such as duplexers (to combine protection of fixed station antenna facilities multiple radios operating in different is important. frequency bands onto one multiband antenna), should likewise be installed in 9.2.1 Fixed-Site Antenna System locations not readily accessible to vehicle Grounding and Bonding Practices occupants. For example, they should be An effective grounding system is necessary installed under seats or in trunks where they for every antenna tower. In addition to the are “out of the way,” yet reasonably protection a grounding system offers from accessible to maintenance personnel. lightning strikes, grounding also: RF cable connections should be torqued to • Reduces the hazards of electrical the proper force recommended for the shock resulting from ground/neutral particular connectors used, and the outer power faults. grounded conductor of the antenna base • Protects wiring and circuitry by mount must be RF-bonded to the metal of limiting extraneous over-voltages. the roof or trunk deck. Trunk deck installations also require excellent RF
43 • Facilitates rapid discharge of faulted be at the same entry point as RF cable, power circuits. telephone, and power connections. • Reduces noise voltages. • Provides a path to dissipate any stray 9.3 Lightning Protection RF current present inside the The National Fire Protection Association transmitter station; ungrounded RF (NFPA) publishes a guideline related to currents can contribute to equipment lightning protection [23]. This guideline malfunction, or create interference details many additional practices for with other receivers. protecting radio equipment from lightning strikes. 9.2.2 Fixed-Station RF Bonding RF bonding is another important aspect of Metal antennas and towers should be fixed-station antenna systems. Simply connected to the building’s lightning connecting each element of a transmitter protection system. Wires and metallic facility to a metal pipe stuck in the ground is elements comprising an antenna tower’s barely adequate to act as a grounding lightning protection system should be system. The components of such a electrically attached to the Earth. Towers grounding system are not perfect conductors and guy wires anchored to concrete forms in and each will have different, finite values for the ground are often assumed to be well resistance. The resistance and physical grounded, but concrete is a poor electrical design of the grounding system adds to the conductor. Tower legs should be electrically overall resistance and reactance of the attached to the Earth with a copper ground antenna system and transmission line, stake approximately 3 m long. Lightning- causing the system to have different voltage ground connecting leads connecting the potentials at different points within the tower to the ground stake should be at least system, inducing stray currents to flow AWG #10 copper, AWG #8 aluminum, or between equipment chassis. These stray 3/4 in copper braid. currents can affect internal circuits of the equipment and cause erratic operation and The transmission lines must be protected by unpredictable behavior. lightning arresters, protectors, and discharge units. Arresters can be placed at both ends of A bonding system ensures that all equipment the transmission line for added protection. grounding points are at the same electrical potential. A good dc and RF bonding system 9.4 During Installation will use high-quality, low-impedance copper Be sure the installer knows exactly where on strap or braid and attach all equipment the vehicle or on the antenna tower the chassis to a low-impedance copper bus strap antenna components are to be installed. installed on the walls throughout the station Make sure they are installed in the correct facilities. The copper bus leaves the station orientation and positioned correctly. and is attached to the Earth using a copper ground rod approximately 3 m long. The Make sure that the ends of the transmission point of egress for the facility ground should line cable have been prepared properly before affixing RF connectors to the
44 transmission line. Make sure that the RF visual line-of-sight to the repeater or base connectors are properly installed on the station-antenna tower. A portable transmitter transmission line. Loose connector could serve as the signal source. A similar assemblies will result in poor ground method could be used to measure the power connections between the outer connector received by a portable radio service monitor shell and the outer shield of the transmission (such as an IFR1500 or Motorola R-2670) line. Make sure that the center pin is located at the survey-marker point. securely affixed to the center conductor of the coaxial transmission line and that the Make a physical inspection of the center pin’s depth, relative to the connector installation. Be sure all connectors and shell, is maintained at the correct distance, transmission lines are secured properly. or an impedance mismatch or connector damage will result. Make sure that the 9.5 Perform Routine Maintenance antenna and transmission line connectors After a vehicular or repeater/base-station will mate properly before connection to antenna system has been installed, the other equipment is attempted. Connectors system will require periodic maintenance to can be easily misaligned or over- or under- ensure optimum performance. torqued, resulting in degraded or erratic overall RF performance. Make sure that the Agencies should practice three tiers of transmission line has not been damaged in maintenance. The first is performed by the any way, such as crushed, severed, or radio operators and consists of simple, pinched. “common-sense” inspection of the equipment. The second level of maintenance Check the VSWR as soon as possible after is performed by site technicians and requires installation, and, if possible, before the the use of land mobile radio test equipment. installer leaves the job site. In addition to The third level of maintenance is performed VSWR measurements, time-domain by factory-authorized technicians. reflectometry (TDR), line-fault measurements are helpful. Use a portable 9.5.1 Local Inspection transmitting unit if the radio transmitter is not yet installed. Record the VSWR values Radio operators themselves can perform a and TDR data for future reference. wide variety of simple aural and visual inspections of their radio equipment and Measure and record the ambient noise power antennas. For instance: levels for future reference. • Isolated problems noted with reception If possible, the installer should conduct or transmission in the field. Direct “over-the-air” RF power sensitivity comparison between two radios measurements immediately after installation, (“I cannot hear the base station when I and document the test configuration and am in this location.” “Oh, really? I can measurement results for future reference. hear the base station okay.”) gives an For example, a portable antenna mast, 7 m excellent indication of a problem with to 10 m high, could be placed at a geological a subscriber unit. survey marker that has unobstructed, clear,
45 • Loose or missing connectors—Over the antenna radiates electromagnetic energy time, temperature variations, shock, as expected. vibration, exposure to the elements, and handling can cause connections Vehicular antenna systems should be and connector flanges to become loose periodically checked according to an or missing altogether. Rubber O-ring established maintenance schedule, typically grommet seals may deteriorate, concurrent with the maintenance schedule of allowing moisture to penetrate into the mobile radio (perhaps once or twice per connectors or the transmission line, year). Performance values such as VSWR altering their performance. should be recorded, compared to the • Cracked or broken whip antenna base- performance values measured just after loading coils—Cracks in the plastic installation, and tracked over time in order housing of vehicular antenna base- to assist in keeping the antenna system loading coils can permit moisture and functioning at an optimal performance level. corrosive materials to penetrate into the loading coil, altering the antenna’s The above statements apply equally to fixed- electrical performance. Weathering of site repeaters and base stations. rubber grommet O-ring seals (where Unfortunately, whereas problems with the loading coil is affixed to the roof mobile and portable radio systems can be or trunk deck of the vehicle) may readily identified by direct performance similarly permit moisture and comparison at the operator level, such is not corrosive materials to penetrate into necessarily the case for fixed sites. Because the connector between the loading coil performance degradation at the fixed site and its attachment to the coaxial cable affects all subscriber units equally, slowly connector, altering the antenna’s occurring degradation in performance, electrical performance. caused by corrosion effects, weathering, etc., may go unnoticed for years until 9.5.2 Site Technician catastrophic failure finally occurs. Therefore, regularly scheduled site visits to The site technician will often have an array conduct maintenance performance of RF test equipment at his or her disposal. inspections must be a part of the site For example, a communications service technician’s routine. Recording a time monitor can determine whether a radio is history of the RF performance, and transmitting on the proper frequency, at the comparing it to the RF performance proper power level, with the proper measured immediately following the frequency bandwidth. A VSWR meter gives repeater or base-station installation, will be an indication of how well RF energy is an invaluable maintenance aid. coupled from the radio system into the antenna. A time-domain reflectometer can Remote automated in-line diagnostic test determine where faults or other equipment can also provide indications of discontinuities exist along the length of a needed maintenance. Several vendors of transmission line. Portable field strength antenna system equipment, such as Bird meters can give an indication as to whether Electronic Corp., Decibel Products, Sinclair Technologies, Telewave, and many others,
46 manufacture remote in-line diagnostic • Safety belt. measurement equipment. This equipment • Safety glasses. can report the health and status of a fixed • Work boots with firm, nonslip soles repeater/base-station site’s RF performance, and well-defined heels. up to the point where the radio wave is • Hard hat. launched into space. For example, conditions of low transmitter power and/or It is recommended that antenna tower excessive VSWR, which might result from installations have a personnel fall-arrester detuned combiner cavities or duplexers, system. These systems permit maintenance failing radios, weathering of components, personnel to attach their safety belts at etc., can be sensed by the automated ground level, and remain attached during diagnostic equipment. Alarm conditions can tower ascent and descent, as well as while automatically be reported to a computer by working at height. Antenna tower preconfigured telephone dial-up to the manufacturers, such as Rohn Tower, offer centralized maintenance facility. fall arrester systems, which comply with OSHA regulations. 9.5.3 Factory-Authorized Technicians In addition to personnel safety while If an antenna system or radio problem is too ascending, working on, or descending complicated for the local site technician to antenna towers, maintenance personnel must repair, the manufacturer should be be cognizant to the danger of RF electrical contacted. Many large public safety burns arising from direct contact with organizations have service contracts with energized antenna elements, and to exposure factory-authorized repair facilities to to high levels of RF radiation. Industry maintain, repair, and replace equipment. standards have been developed that specify Even without a contract, contacting the the levels of electromagnetic exposure to manufacturer about specific problems is which personnel can be “safely” exposed advised if the problems are beyond the [24]. Personnel working on transmitting abilities of the agency to repair. antenna towers must ensure that all antennas that they are working on or near are 9.5.4 Antenna Tower Safety disconnected from their associated Working on antenna towers can be transmitters, and that the transmitters are dangerous and potentially fatal. Serious routed into dummy loads (to prevent damage personal injury and equipment damage can to the transmitter final amplifier in the event result from personnel falling, improperly of inadvertent transmissions), or that some installed equipment, and RF radiation sort of fail-safe mechanism for exposure. disconnecting power to those transmitters has been engaged. When maintenance personnel must work on antenna towers, safety equipment should be Lastly, maintenance personnel should be selected, used, and cared for as if their lives alert to weather conditions. It is ill-advised depend on it—because they do! A list of to work on an antenna tower during an safety equipment should include: electrical storm or in high winds. Remember—safety first!
47 9.5.5 Vehicular Antenna Systems 9.5.6 The Importance of Maintaining Your Radio System The same precautions regarding RF exposure on antenna towers should be It must be stressed that regularly scheduled, observed when operating or performing periodic testing and preventive maintenance maintenance on vehicular antenna systems. of the entire radio system is of paramount For example, personnel should not stand importance. Motor vehicles used by law outside and next to their vehicle (where they enforcement and corrections, fire, do not have the benefit of RF shielding emergency medical services, and other afforded by the vehicle’s roof that they public-safety agencies are maintained by would have if they were inside the passenger complying with strict maintenance compartment of the vehicle) while operating schedules. Weapons are maintained by a 45 W (or greater) mobile radio transmitter, following regular preventive maintenance nor should they touch the antenna when schedules of cleaning, lubrication, and transmitting. inspection for worn or damaged parts. The same must hold true for all parts of a radio communications system—the lives of fire, emergency medical services, and law enforcement and corrections personnel may depend on it.
48 10. ANTENNA SYSTEM RESOURCES
This section describes some of the resources or biannual basis. One such periodical that available to agencies to research information does this is Mobile Radio Technology (URL: about and identify pertinent products, http://mrtmag.com, telephone antenna systems and related components. 1–913–341–1300). Another is APCO’s There are many qualified and conscientious monthly APCO Bulletin (URL: manufacturers and products available; only a http://www.apcointl.org/bulletin/, telephone few are cited herein in order to provide a 1–888–APCO–911). Public libraries may sampling of what is available. maintain subscriptions to this and other LMR-related periodicals, or they may be 10.1 Internet Resources able to obtain issues under interlibrary loan agreements. As the World Wide Web (WWW or Web) has grown in recent years, many antenna 10.3 Manufacturers’ and Vendors’ manufacturers and suppliers have created Catalogs Web pages devoted to their products. Most antenna manufacturers and vendors Most Web-page authors will register their distribute free catalogs that describe page with one or more of the major “search antennas, transmission lines, and related engines” on the Web. A search engine is a components that they offer for sale. Most program that will use a search phrase catalogs provide useful technical provided by a user (usually a single word or information such as antenna patterns, simple phrase) to search hundreds of operating frequencies, physical dimensions, thousands of Web pages looking for and costs. Frequently, these catalogs will occurrences of the phrase. The engine will provide basic explanations about various then return a list of those Web pages that aspects of antenna systems, such as most closely match the search criteria (i.e., collocated transmitter-combining those that contain the search phrase). A few techniques, intermodulation interference, search engines, such as Metacrawler transmission line theory, etc. (http://www.metacrawler.com) conduct searches by simultaneously querying more Many manufacturers have more than one than one search engine. product line, e.g., antennas and duplexers. Information about manufacturers’ offerings 10.2 Periodicals can be identified and researched via the Internet or through the periodicals and Several periodicals are written for the land buyers' guides discussed in the previous two mobile radio industry. Most are free to subsections. “qualified” subscribers. These periodicals contain articles of technical interest related to land mobile radio, and advertisements for land mobile radio systems, components, and services. These periodicals publish, as special issues, buyers’ guides on an annual
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