Wireless Systems Design

Tools Aid Compliance With RF Safety Program

If they’re armed with the proper guidelines and a combined battery-powered analyzer and isotropic probe, FCC licensees can institute an effective radio-frequency safety program.

Since the construction of the earliest television and radio antenna towers, health concerns over "overexposure" to non-ionizing radiation have surrounded wireless transmissions. With the growth of wireless communications services, these concerns have only increased. The Federal Communications Commission (FCC) has therefore adopted guidelines that govern the exposure to RF emissions for those operating FCC-licensed wireless transmitters. Compliance by FCC licensees isn't just recommended. It is mandatory.

Meanwhile, the U.S. Occupational Safety and Health Administration (OSHA) has issued its own recommendations. OSHA's recommendations cover a far broader range of organizations than the ones that are licensed by the FCC. The implications of these guidelines are significant for manufacturers of RF and microwave equipment, providers of wireless services, users of RF and microwave-based industrial and medical equipment, and tower owners and operators. Thankfully, the latest generation of test gear from Narda Safety Test Solutions can simplify the measurement process that's needed to achieve such compliance.

The IEEE and National Council on Radiation Protection and Measurements (NCRP) standards, which were referenced by the FCC, cover exposure limits, RF measurements, and warning signs and labels. Yet two key elements are missing from those standards: guidelines for implementing an RF safety program (RFSP) and the measurement tools with which measurements at collocated sites could easily and reliably be made. These two missing links have recently been inserted, however. Available instruments can now remove the obstacles to performing measurements at collocated sites. When the IEEE completes its C95.7 recommended practice, guidelines for constructing an RF safety program also will be available.

When the FCC announced that it would enforce the guidelines governing non-ionizing radiation exposure, broadcasters were faced with a challenge. In practical terms, it's nearly impossible to make RF measurements of sufficient quality at sites where many services are operating from a single or multiple towers (or other structures) in close proximity. For example, take a typical transmitter location like the roof of a commercial office building, water tower, or transmission tower. That location may have multiple operating licensees, which range from paging services to local fire and police as well as various commercial communications systems. If emissions at a specific site produce a radiated emissions level of more than 100% of the limit, FCC guidelines demand that any single emitter that generates more than 5% of the total must be part of the solution for bringing the emissions level into compliance.

The problem is that the only way to make effective RF measurements at such sites has been to evaluate individual emitters with all of the other emitters shut down. Once the transmitters have been turned off, they are turned on one by one. Their individual radiated emissions are then measured. Next, the percentage of standard is computed for these emissions. At a location where numerous independent companies are continuously generating RF emissions (for either profit or government or law-enforcement operations), gaining cooperation for these measurements is almost impossible. To radio and television broadcasters, for example, the loss of revenue due to a shutdown would be unacceptable. For wireless service providers, shutting down a base station interrupts service and causes them to lose customers. It's no surprise that compliance with FCC guidelines has reportedly been minimal.

Note that all emitters must be turned off except the one under test. The broadband test equipment, which is employed to measure radiated emissions, only allows the site's overall percentage of the standard (i.e., the total percentage contributed by all emitters) to be measured. The equipment cannot easily identify a specific emitter at the site, however. Only a frequency-selective instrument, such as a spectrum analyzer, can perform such identification.

Yet radiated emission measurements have unique characteristics, which make spectrum analyzers a poor choice in this application. As general-purpose instruments, spectrum analyzers don't compute and display an emitter's received signal strength as a percentage of a specific standard. Nor do they provide either tabular or graphical data about all of the emitters at the site. Measurements must be made as quickly as possible with antennas that are optimized for azimuth, elevation, and direction. With such antennas, a change in the field won't occur between measurements. Spectrum analyzers aren't optimized to make such application-specific tests.

Fortunately, the model SRM-3000 selective radiation meter from Narda Safety Test Solutions addresses these issues. It allows all transmitters at the site to remain operating while the measurements are made. The SRM-3000 is a battery-powered, handheld instrument that combines the measurement and analysis functions of a spectrum analyzer with a calibrated probe, Windows CE-based personal computer (PC), and dedicated analysis software (SEE FIGURE). The probe uses three orthogonally mounted, isotropic-dipole antennas that are automatically switched at high speed. The instrument analyzes the received signals, thereby ensuring that measurements on all three axes are performed rapidly before conditions change. The probe's 75-MHz-to-3-GHz range enables the instrument to cover all major broadcast and cellular radio bands.

The algorithms in the instrument calculate the field-strength values. The results are displayed on its liquid-crystal-display (LCD) screen. The SRM-3000 has three main operating modes: safety evaluation, spectrum analysis, and time analysis. The safety-evaluation mode provides a listing of signals by name (such as a call sign or service type). It also shows the field strength of each signal as a percentage of a specific standard as well as the percentage of the standard that is reached when signals at the site are combined. With this mode, signals at a site can be sorted out by frequency, type of service, and measured field strength along with their relation to standard-imposed limits.

In spectrum-analysis mode, the SRM-3000 displays the measured spectrum with resolution bandwidth and other parameters that were selected by the user. In the time-analysis mode, the user chooses a center frequency and the resolution bandwidth. That bandwidth corresponds to the bandwidth of the channel that will be monitored.

Given a capable instrument like the SRM-3000, licensees were still faced with the need for RF safety guidelines. The IEEE went to work on a recommended practice to provide this information. The result is IEEE C95.7, which is working its way through the review process. It should be completed within the year.

IEEE C95.7 leaves little to the imagination about the elements of an RF safety program. Yet it does leave room for interpretation. RF field measurements require a high level of both technical expertise and experience. As a result, licensees will still probably need the expertise of consultants who specialize in RF radiation safety. Nevertheless, the document provides basic guidelines that haven't been previously included in the FCC bulletin or an IEEE document.

In brief, this practice separates environments in which RF is present into four categories (SEE TABLE). The first step in creating the RF safety program is to identify the category into which a particular site falls. Category 1 locations contain only RF sources, which cannot produce fields that exceed the maximum permitted exposure levels (MPEs). As a result, those sources don't require an RF safety program. Categories 2, 3, and 4 locations require an RF safety program and an RF safety officer to administrate it. Selecting the category is arguably the most technically difficult part of the process. In all but the most obvious cases, this choice cannot be made without getting comprehensive RF field measurements and interpreting the results.

The duties of the RF safety officer aren't trivial. After all, he or she is responsible for administration of the entire program. The officer's duties include conducting the initial safety analyses, constructing and documenting the program, implementing the controls, and monitoring the use of safety procedures. Obviously, these duties require comprehensive training in RF safety awareness and a reasonable understanding of all elements of RF exposure. This level of training is available from various consulting organizations as well as Narda Safety Test Solutions. Once the RF safety officer has been trained, technicians and other employees must be trained as well.

When IEEE C95.7 is complete, it will complement the existing standards that the FCC and OSHA use as "guidelines" for their licensees (in the case of the FCC) and the workplace (OSHA). Together with instruments like the SRM-3000, basic tools will become available. Such tools will allow organizations to create and administrate RF safety programs. The organizations can then be confident that they are protecting their employees, other workers, and the public while limiting the likelihood of litigation.

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