Various levels of government have become involved in a new arena over the last decade - the regulation of electromagnetic emissions. Before the concern over radiation emanating from transmission towers began, local communities had similar concerns about high voltage power lines.
However, the role of the federal government increased simultaneously with the growth of concern over the potential health impact of such emissions.
Biological Effects of Ground Radiation
The concerns began when various studies suggested that a variety of illnesses could be associated with prolonged exposure to non-ionizing radiation such as radiofrequency radiation (RFR). Even though the connection has never been conclusively proven, lawsuits have been filed claiming leukemia and other illnesses in this regard.
As a result of a variety of such health-related concerns, regulation of transmitter emissions began. Thus, broadcasters (AM, FM and TV) became focal points of concern in this effort. Decades of research preceded the 1982 American National Standards Institute (ANSI) establishment of RF radiation standards. Three years later the FCC adopted the standards.
“Evaluating Compliance With FCC Specified Guidelines for Human Exposure to Radiofrequency Radiation” is the formal title of the document known better as OST No. 65, published in 1985 by the FCC of Science and Technology, It related the ANSI standards for broadcasters. Clarifications of the document have continued since actual enforcement began in 1986.
The National Association of Broadcasters (NAB) first interpreted OST No. 65 in its Nov. 1985 primer “A Broadcaster’s Guide to FCC Radiation Regulation Compliance,” and later suggested changes to the document.
The Commission itself has clarified aspects stemming from the standard, most recently with regards to translators and boosters. In 1990 the FCC changed its rulings on multiple-use sites and measurement of hot spots (defined as intense, localized fields).
A wide variety of other players have also become involved, including state and local governments, researchers, and antenna manufacturers.
Multi-Use Transmitter Sites
The most significant problem with ANSI compliance remains at multiple-use transmitter sites. Federal Aviation Administration (FAA) efforts to cluster antenna sites coupled with generally higher urban land values have led to the concentration of transmission facilities at antenna farms. While resolving some problems, this approach has increased radiation concentrations.
Those broadcasters most apt not to meet the required guidelines typically have antennas transmitting near populated areas or at antenna farm settings. Multiple-use sites include antennas on building tops with locations near floors on which people work. In addition, hub sites are located on mountain tops where towers exist only to support antennas, not to provide elevation.
Since the biological effects of RFR are related to total energy absorbed by the human body, the ANSI standards require consideration to all RF energy sources at a given location. Due to radiation attenuation characteristics, an antenna mounted just 200 feet above occupied sites is far less likely to meet ANSI standards than the same antenna mounted 600 feet up.
AM transmit locations pose less of a problem than their FM or TV counterparts. At AM installations, potentially excessive fields are typically predictable and fairly localized. However, FM and TV fields tend to vary considerably even over small distances. In addition, they have a theoretical problem with potential multiple reflections in antenna farms due to the vagaries of VHF propagation. In practice, however, the problem for television licensees is reduced by the typical tower-top locations of TV antennas, significantly higher than the often side-mounted FM antennas on the same structures.
On-Site Measurements vs. Calculated Levels
The combination of many FM and/or TV signals at a crowded RF site may require numerous measurements for an accurate analysis. One sophisticated computer program for rooftop RFR analysis inputs over 100 measurements while attending to re-radiation from metallic objects.
While it is generally believed that actual, on-site measurements provide the most accurate information, they are not always necessary. Calculations are adequate for ANSI compliance when radiation levels do not approach allowable limits. The FCC has accepted multiple-use site calculations in which radiation levels are well under ANSI limits. Accurate on-site measurements, done correctly, can be expensive and time consuming.
As broadcast station licenses have come up for renewal since 1986, the ANSI standards became a renewal condition for the time. This was how most broadcasters were formally confronted with the compliance criteria.
One newly proposed station wanted to add yet another transmitter and antenna to an existing, multiple-use site; this was representative of how the FCC would attempt to protect the public.
The station sought a construction permit (CP) to transmit at 19 kilowatts effective radiated power from the multiple-use site of a mountain-top with an existing complement of 13 FMS and VHFs. Six towers, the tallest at just 260 feet, supported all antennas.
At the same time, an existing FM station on the mountain had constructed a fence in order to obtain FCC approval of a modification proposal. The objective, to protect the public from an identified RF hot spot, failed when some of the excess energy was re-radiated to a new location near the metal fence itself. The new RF concentration landed on a narrow mountain path over which several engineers traveled to access their transmitters.
It became clear from this process that the FCC had not established standards on materials for building such fences. Thus, consideration given to any immediate or long-term impact. And, as a result, the new station also proposed to build a fence, a plan which won subsequent FCC approval. No mention was made of the type of material or height of the fence.
When the other mountain top broadcasters and the site manager (National Forest Service) learned of the fence resolution and worried aloud about the resulting fence boondoggle which could be:
- several miles long
- 20-25 feet in height of metal materials
- anchored by dynamite-opened/cement posts
- at a price out of reach of the combined resources of all 18 broadcast organizations and the Forest Service
Meanwhile, the local Forest Service ranger had been in contact with various FCC officials about the problem. Out of these discussions came an agreement:
- RF warning signs would be posted
- a new, locked, gate would close the road to the top of the mountain.
After this agreement, the new station sought the total deltion of the fence requirement from its FCC approval. Such a request was made possible by the Jan. 28, 1986 FCC public notice entitled “Further Guidance for Broadcasters Regarding Radiofrequency Radiation and the Environment.”
The general advice offered in the public notice reiterated previous suggestions: not to create high RF levels where people are or could be reasonably be expected to be present and to prevent people from entering areas in which high RF levels are necessarily present.
However, the list of specific alternative ways to comply included situations in which “high RF levels are produced at ground level in a remote area not likely to be visited by the public.” In response, the station successfully convinced the FCC that the mountain top qualified as a “remote” location.
The general response in the field to occupational exposure requirements has been mixed since OST No. 65.
Some tower crews have worked under full RF conditions, only to exceed ANSI standards. Meanwhile, others have required power reductions before climbing. A few were even mandating complete shutdown for re-lamping and inspections, citing the risk of future litigation from climbers,
Recently, workers have begun using personal RFR monitors to measure levels. One firm even sells RFRprotective clothing for tower climbing work.
On this matter, the FCC wants procedures to be instituted and working in the vicinity of high RF sources, such as restricting the amount of time an individual can be so exposed (or, reducing power to increase the time permitted). Another possibility was the use of an auxiliary transmitter at another location during such maintenance. Also, a transmitter could be completely turned off.
The NAB, in its |985 primer on OST No. advised that FCC radiation regulations clearly applied to both public and occupational exposure. lt argued that allowing workers to be exposed to RF radiation levels in excess of ANSI standards, after certifying compliance with the FCC, will be considered as a misrepresentation to the Commission and acted on such.
In 1988, Richard Tell, former chief of the electromagnetics branch of the EPA, climbed AM towers himself to take actual measurements. Predictions had been made that ANSI absorption standards could be exceeded by a parallel field driving current through a climber's hands and the loop formed by the tower and the climber’s hands. Tell measured the strongest emissions at the tops of AM towers, but found that body currents were not as strong as had been expected. In general, he concluded that good judgement was required to determine if transmitter power would need to be reduced to stay below ANSI limits.
In 1992, the Institute 0f Electrical and Electronics Engineers (IEEE) suggested a new standard to replace the ANSI exposure standard. However, it is not yet clear if broadcasters will endorse the change.
Responsibility and Management
The 1985 NAB primer interpreted OST No. 65 to call for cooperation among broadcasters so that each facility at multiple-use sites could accurately determine its adherence to the standards.
In 1990, the FCC issued adopted new rules on multiple transmitter sites with important exclusions. While reaffirming the shared responsibility of all licensees at multiple-use sites, excluded were licensees whose transmitters contributed less than one percent to the overall exposure limits. This included FM boosters with transmitter power of 10 watts or less.
Once RF current levels are known, a management plan can be established in response. At many stations, long established tower climbing procedures and/or engineer’s access to sites may need to be radically changed. This management plan could include time limits on exposure, how the use of auxiliary transmitters/antennas can improve access, what happens when tower lights are changed, etc. Multiple site users who find it difficult to agree on such a plan might hire an outside consultant not associated with any station, for the purpose of recommending operating procedures.
Even when tower climbers are unconcerned about excessive RF exposure, broadcasters have been advised to remain diligent because of potential future liabilities. The fact that a licensee has complied with established federal guidelines may, in fact, be a persuasive argument in defense of such a suit.
RF exposure above the standards is a concern not limited to tower climbers. Engineers at multiple-use sites should also remain vigilant. Their dilemma is worsened by strong RF signals still present even when their station’s individual transmitter is turned off. An antenna at a multiple-use site can transfer RF energy to the transmitter power amplifier cavity, thus presenting unknown levels of RF to the engineer working in that section of the transmitter.
The cavity should be measured, and if excessive levels are identified, an appropriate response must follow. For example, it may be possible to lower the energy to acceptable levels by either grounding the transmission line center conductor or opening the transmission line leading to the antenna. Care is necessary if touching the center conductor, which may contain considerable energy.
In the mountain top example, an engineer was on duty 24 hours a day. Many of the transmitter buildings and both of the residences were adequately RF-protected by copper screening on the walls and rooftops, bound together and tied to elaborate ground systems. For ANSI compliance, both residences were routinely checked for excess radiation. Likewise, the two associated transmitter buildings were similarly checked.
Most other transmitter buildings on the mountain top remained unprotected, however. This meant that the live-in engineer and others were exposed to RF fields reduced only by buildings walls and ceilings, usually some 10 dB of attenuation at FM frequencies.
The most notable example of occupational safety on the mountain was displayed by telephone company personnel. They were required to follow established exposure guidelines at a particular RF hot spot, restricting their exposure to just a few minutes at a time.
Despite some confusion over the precise roles of ANSI, EPA, OSHA, FCC and local setting of standards and enforcement, broadcasters generally understand and comply with the OST No. 65 guidelines. It is clear that the FCC expects both the public and workers to be protected from excess radiation.
The guidelines have joined the broadcast technician’s list of responsibilitiesg1 technical expertise, financial accountability, effective management, prudent planning and ANSI compliance.
by Tim McGartney