Probing public safety antenna standards
A technical standard, separate from commercial ones and specific to public safety, is available to aid law enforcement agencies in procuring and using communications antennas. What does this standard require, and how useful is it as a purchasing and operations guide?
A fable: An analyst comes out to a tower site to advise the manager on antenna spacing. He shimmies up the tower, unravels an old ball of string from his pocket and, like a tailor, matches the distance between the sticks with his string. When he comes back down, the manager asks “What next?” The analyst says “Well, I guess the length of the string, which gives us an estimation of the separation, and then we change the spacing accordingly. “Good grief, man,” the manager cries, “In that case, why not just estimate the spacing to begin with?” The analyst shakes his head and replies, “You don’t understand analysis; there has to be a standard.”
Many standards apply to public safety radio. The most celebrated example in recent years is Project 25. Standards aid procurement, facilitate replacement, promote interoperability and act as quality control. The commercial minimum standard for communication antennas is TIA/EIA 329-B. One of its counterparts in the public safety realm is NIJ Standard-0204.02: Fixed and Base Station Antennas.
The standard1 is one of many projects of a government program created by Congress 21 years ago to determine technology needs for justice agencies, to set technology performance standards and to distribute those results to the federal, state and local law enforcement communities. This program, the National Institute of Justice (NIJ), essentially provides impartial and free resources that a user, e.g., the communications division head of a local police department, can turn to for technical standards, reports and user guidelines.
The NIJ Standard 0204.02 for antennas, released about a year and a half ago, replaces version 0204.01, created in 1981. Its stated purpose is to “establish minimum performance requirements and methods of test for antennas that are used at base stations or other fixed sites by law enforcement agencies.” Land mobile antennas covered by the standard are those for use at VHF lowband, VHF highband, UHF and 800MHz/900MHz.
Equipment requirements * Minimum performance – The minimum performance standards in 0204.02, meeting or exceeding TIA/EIA 329-B, include: 1. Operating at the rated power output with no physical damage. 2. A radiation pattern 61.5dB of the specified gain for the main lobe (or full range for omnis) and 65dB of the gain in the minor lobes. 3. A VSWR of 1.5 or less on all frequencies (referenced to a 50V system). 4. A wind load rating times 1.65 the values specified in Chapter 16 of the TIA/EIA-222F tower/support structure standard; or the local building code, whichever is more stringent.
*User information – To meet the standard, OEMs, system integrators or distributors must provide the antenna purchaser with information on: * operating frequency range. * power rating. * relative antenna gain, in standard gain unit, over the frequency range to be used. * polarization. * vertical radiation pattern. * horizontal radiation pattern. * nominal impedance, * VSWR over the frequency range. * connector type. * wind load rating and ice load rating, if applicable. * physical dimensions. * weight. * material composition. * installation, operation and maintenance instructions. *RF radiation hazard zone at full power. *certification of compliance with the standard.
* Operating environment – Many environmental factors can degrade the structural or electrical performance of an antenna, including its radome, insulating materials and connector. For environmental and climatic endurance, the 0204.02 standard references military standard MIL STD 210C and requires the testing methods specified in MIL STD 810E as the means for manufacturers to demonstrate compliance. It does recognize that some environmental specifications not applicable to the user’s climate may be waived by the procuring agency. Other specific requirements include: * the ability to operate under sustained temperatures as high as 43 degrees C (110 degrees F) and solar radiation as high as 1,120W/m2, except in the Southwestern United States, where a more stringent temperature standard of 49 degrees C (120 degrees F) is applied. * the ability to operate under sustained temperatures as low as -32 degrees C (-25 degrees F), except for Alaska, where the requirements range from -46 degrees C (-50 degrees F) to -51 degrees C (-60 degrees F) * the ability to resist moisture penetration from blowing rain at a rate of 5.9 inches/hour at 24 degrees C (75 degrees F) relative humidity of 100% at the high and low temperature ranges. * the ability to resist sustained exposure to salt fog, for installations intended in coastal areas. * the ability to withstand exposure to blowing sand, dust and snow. * the ability, for installations in ice-prone states, to withstand a 0.75-inch-thick ice glaze at the wind load.
* Radiation hazard – Radiation hazard zones are the volume of space surrounding an antenna where its power density or field strength when operating at full power exceeds recognized specified limits. The standard points out that this is not an environmental condition, nor can it be waived in a request for proposal. The standard references ANSI/IEEE standard C95.3 (1991). (However, sincethe issuance of 0204.02, the OSHA and the FCC have revised minimum permissible exposure limits, and these standards should be taken into account when specifying.)
Test methodology The detailed test methods specified in 0204.02 are mostly of interest to the OEMs, system integrators and distributors, who must certify compliance with the standard. However, a brief review of the procedures will give the procuring agent an idea of how these certifications are achieved.
* Test frequencies and sites – Three frequencies are chosen from the operating range of the antenna under test, occurring roughly at the lower, middle and upper portions of that range. For multiband antennas, each frequency band must be tested in this fashion. Outdoor test sites, or ranges, have to be open, level ground. Above ground, they have to be free of any obstruction (trees, poles overhead wires, buildings, etc.) that would interfere with an EMF for at least 50l, or 100m, whichever is greater. Below ground, the soil has to be free of geologic peculiarities, and any buried utility lines or control cables have to be at least one foot below the surface.
Elevated test ranges, to simulate mountainous conditions, have additional range design requirements. For the higher land mobile frequencies (800MHz/900MHz), indoor ranges can be used, as shown in the opening photo on page 62.
* Instrumentation – Test equipment standards include specifications for frequency stability, phase noise limits and transmitter power minimums to exceed ambient RF noise. Receivers have to have frequency stability equivalent to transmitters and must be phase-locked to them. Pattern recorders have to be accurate to +/-0.2dB. Power meters must be able to measure forward and reflected power in a 50 ohms to a tolerance of 5% or less. The reference antenna for measuring gain is specified as either an adjustable, lambda/2 standard dipole, or an EIA standard gain antenna.
* Lowband modeling – Scale model techniques are allowed for testing VHF lowband antennas because their physical size can make getting accurate radiation pattern and gain measurements difficult on a range. Equivalents for scaling down the antennas are provided in the standard.
Procedures are also outlined for polarizing the test and source antennas through various planes of rotation and for assessing the environmental ratings previously mentioned, as shown in Figure 1 on page 65.
Practical applications of standards Standards are generally derived in three ways. The first situation is when an OEM that has an advantage in the manufacturing process or a proprietary technology imposes a de facto standard on the industry for a component or system. If there is widespread adoption of the technology, then there is improved interoperability for all customers. The disadvantage is that customers have a sole source for replacement parts, service and resupply. The customer also may be forced to commit to an entire product line to maintain compatibility. If the line is dropped or superseded for any reason, the customer is stuck.
The second situation is when an industry or professional association acts as a clearinghouse or sponsor for standardizing the technology. Advantages are broad adoption, the creation of standard definitions, procedures and advertising guidelines, and the discouragement of substandard manufacturers. Disadvantages include exclusion of some ideas and technical approaches, and the onus of being non-compliant and shut out of the RFP process if you are a manufacturer with a different idea.
The third situation is where an independent or government “think tank” designs an ideal standard, which, if adhered to, would produce the best component for durability, redundancy and performance. Advantages in this case are relative freedom from commercial bias and a focus on the consumer. These standards are also usually freely distributed, as opposed to those that form a reprint profit center for an association. The disadvantage is that such standards, like isotropic antennas, are theoretical concepts. They often ignore physical-or commercial-realities.
Like the analyst in the opening fable, we can agree that the worst standard is none at all. All standards, including NIJ 0204-02, are useful tools for obtaining quality, durable equipment. However, there are other practical considerations when procuring equipment for a public safety agency: budget and cost, warranties, maintenance costs and specific operating conditions in the agency’s environment. Encompassing all of these is the reality that communications systems generally require a holistic approach. That is, they are purchased as a system.
In the public safety arena, only the largest city, state and federal agencies buy directly from manufacturers and have the luxury of dictating a tight set of standards. Most systems for police, fire and EMS at the local level are purchased from a trusted or familiar radio dealership with which there is an ongoing relationship. Superseding product quality, service and support, the main factor in this relationship is usually cost.
The NIJ standard draws heavily on other standards created by the Institute of Electrical and Electronics Engineers (IEEE), the Electronics Industry Association/Telecommunications Industry Association (EIA/TIA), the American National Standards Institute (ANSI) and the U.S. military, acting separately or cooperatively. In addition to these bodies, there are test procedures and test ranges maintained by individual manufacturers, the National Institute of Standards (NIST), the FCC and the National Telecommunications Information Administration (NTIA). (The NTIA just added an advanced test bed for smart antennas at its Institute for Telecommunications Sciences [ITS] in Boulder, CO, last November.)
With all these sources of information, is there a demonstrated need for separate standards specific to the public safety market? Antennas are “transparent equipment” to most radio systems; that is, one manufacturer’s antenna will work with 50 different manufacturers’ base stations. However, part of the NIJ mission has been to raise the level of technical awareness in the public safety communications arena. Many radio managers and technicians come into the job with little experience in land mobile. Concepts like wind loading and radiation patterns may be new to those with only dispatch, computer or digital backgrounds. Distribution of this type of information can help them make more informed equipment choices. There are also mission-related differences between public safety systems and commercial or private radio.
To explore this issue of separate public safety communications standards further, an expanded version of this article is available on MRT’s Web site, www.mrtmag.com. It includes assessments from public safety procurement officers, manufacturing engineers and those who create technical standards.