Choose your weapon
In 1997, the FCC instituted new rules — designed to protect workers and the general public from exposure to RF energy — that set many people back on their heels.
Instead of electromagnetic field measurement procedures and compliance rules that were simple (and largely unenforced), owners and operators of broadcast, land-mobile, cellular, paging and public-safety systems were faced with two sets of maximum permissible exposure limits instead of one. More importantly, the commission intended to put teeth into its enforcement efforts. The task of demonstrating compliance was further compounded by the proliferation of multi-emitter sites, which made measurements in some cases virtually impossible.
As a result, 10 years after the rules went into effect, there are hundreds of sites throughout the United States that are not fully in compliance. However, it is now possible to make the required measurements, regardless of how many services are crammed into a specific location, thanks to the addition of narrowband instruments to the standard broadband types that had been — until recently — the only choice. Still, narrowband instruments are not required in all cases, so it is extremely important to understand the attributes of each one in order to determine which is best-suited for a particular situation.
Until the early 1980s, electromagnetic (EM) field measurements were comparatively simple to execute. Standards at that time specified a single maximum permissible exposure (MPE) level for all frequencies, so antennas (probes) employed by EM measurement systems were equally sensitive at all frequencies and not too complex. The technician or engineer simply measured the total field strength at various places, and assuming the total was below that mandated by the current applicable standard, all was well.
Where the measured levels exceeded the standard, the rule was essentially “last on, first off.” The most recent company to add its transmitter to the mix was deemed the problem, and the company had to remedy the situation, which potentially could mean finding somewhere else from which to transmit.
In the early 1980s, standards became frequency-dependent because the human body absorbs radiation more readily at some frequencies than others. This required the use of more complex “shaped” probes with sensitivity that mirrors the requirements of a particular standard. For example, many standards and guidances then (as now) set E-field MPE limits at 614 V/m (100 mW/cm2) below 1 MHz, and 61.4 V/m (1.0 mW/cm2) from 30-300 MHz. Consequently, a shaped probe is 100 times more sensitive in the 100 MHz region than at 1 MHz.
The Telecommunications Act of 1996 mandated, among many other things, that the FCC create new rules regulating RF emissions, culminating in the issuance of the current standard, which was modified in 2003 to better reflect the potential harm from specific types of devices, eliminating some altogether from the need to make routine measurements.
The new FCC rules, based on the IEEE (formerly ANSI) standard and National Council on Radiation Protection and Measurements guidelines, make compliance the responsibility of every service operating at a site, effectively ending the “last on, first off” solution. Today, if the total measured EM field anywhere at the site exceeds the level specified by the FCC rules, all emitters contributing 5% or more to the total must be part of the remedy. In addition, there are now two tiers of exposure limits: a “controlled/occupational” limit and a “general population/uncontrolled” limit, which is five times more stringent.
To evaluate compliance, broadband measurement equipment gathers energy over a broad range of spectrum and computes a single value — the aggregate field strength generated by the site — along with the percentage of an applicable standard that this level represents. Although it cannot itself identify the contribution of each emitter at a co-located site, this result can be obtained by selectively turning off all transmitters at the site and then turning them back on one by one and measuring their emissions. However, this process is impractical at co-located sites because the authority for turning each service on or off rests with its operator, and — as experience has shown — few are willing to shut down for hours while measurements are made. If public-safety agencies transmit from the site, this is not even an option.
The introduction of narrowband measurement equipment in 2004 solved this problem by capturing signals over a much narrower spectral region with much greater sensitivity, measuring their field strength and recording and displaying the strength for each one along with its corresponding percentage of the total allowable by the FCC rules. Such equipment also allows all emitters at the site to be identified, which often is more difficult than it might seem because radomes may contain multiple antennas, each transmitting at a different frequency.
In addition, this type of instrument makes it possible for system operators (and the FCC) to evaluate a site’s compliance without turning off a single transmitter (and without the operators knowing that the test is being conducted). These instruments provide information in seconds that previously required hours, if it could be obtained at all. Finally, interference from extremely low frequency (ELF) emitters often is a concern when broadband instruments are used to evaluate systems co-located on power distribution poles and towers, because broadband equipment employs high-impedance probes that are susceptible to the intense E-field energy generated by power distribution lines. Narrowband instruments use low-impedance probes that are immune to ELF energy.
However, narrowband instruments also are more expensive than broadband types, and in some cases provide more information than is necessary to establish compliance. When only one — or even a few — emitters operate at a site, a broadband instrument can provide a viable solution from both performance and cost perspectives. Thanks largely to advances in digital semiconductor technologies, the latest broadband instruments also have far greater capabilities than their predecessors. They can perform spatial measurements as well as time averaging, output the results to Excel or other spreadsheet programs, and they are supported — as are their narrowband counterparts — by software that allows RF safety officers or anyone responsible for FCC compliance to easily provide a complete measurement report. Some broadband models even include an instrument-mounted GPS receiver that pinpoints and verifies where each measurement was made, and the software then integrates each location into the report. With two types of instruments available, the question is: Which should be used in a particular situation?
From an EM measurement perspective, every site is unique. If there is a single emitter at the site, a broadband instrument is the best choice because control of the transmitter rests with a single organization and its frequency is known.
A broadband instrument may be acceptable even when there are several emitters at a site. For example, a site may have five emitters owned or controlled by a single organization, so their specifications — especially service types and operating frequencies — are known, and the authority to selectively turn each one on and off probably resides with a single person or group. With the ability to control the operation of all emitters, the owner might well be able to employ a broadband instrument, unless public-safety systems are part of the mix. However, this approach is time-consuming and annoying (or worse) to customers, as the transmitters must still be turned off and on and measured one by one.
Conversely, a five-emitter site where each emitter is owned and operated by a different organization presents several important unknowns, including service type, operation frequency and the identity of the owner/operator of the system. While this information can be obtained, the authority to turn all transmitters on and off for measurement purposes probably cannot. Nevertheless, the FCC’s 5% rule effectively requires that all organizations operating at a site know not only their contribution to total field strength, but that of the site’s other occupants as well, because the commission will fine equally all operators generating 5% or more of the total should a site be deemed non-compliant.
Where the specifications of each emitter are known by the organization that wishes to perform the measurements and all operating frequencies are spectrally near each other, a broadband instrument can be used. The issue remains, however, that if the total exceeds 100% of the standard, all operators at the site ultimately will have to determine whether they are contributing 5% or more. Other than resorting to turning the emitters on and off, a narrowband instrument still will be required. If a narrowband instrument is employed at the outset, any organization wishing to know its contribution (as well as that of the others) can quickly evaluate compliance without knowing anything about its counterparts (or for that matter, securing their permission).
In short, while narrowband instruments make it much easier for system operators at densely populated sites to determine compliance with FCC rules, single-transmitter sites and those with several emitters each controlled by a single entity may still be well-served by broadband instruments. The ultimate decision always is site-specific, which makes it important to fully understand the compliance and measurement process and — when necessary — to employ the services of one of the many organizations throughout the United States that specializes in RF safety.
Marv Wessel is chief executive officer of Global RF Solutions, a consulting firm that specializes in the measurement, analysis and mitigation of RF hazards.
