RF Safety

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RF Safety

Jul 1, 2011 1:00 AM, By Jeremy Ruck, PE

As the FCC becomes more environmentally minded, RF safety takes on a greater importance. Radiation seems to be a taboo word in America. We tend to give any mention of radiation a wide berth. While it is true that radiation can be harmful, it is important to remember that the stuff we are talking about in radio is not the same as that from power plants or bombs.

Radio radiation is non-ionizing. This means that when it strikes or passes through a material it has insufficient energy to ionize atoms. Rather, we see that radio waves typically cause changes in the vibrational and rotational configurations. In other words, radio frequency energy tends to heat a medium. Intuitively this should make sense, as this is the concept behind a microwave oven.

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An example of improper sign placement.

Our goal, then, is to reduce the potential for a similar effect in body tissues. To that end, standards have been developed over the years to describe and provide guidance toward minimizing the potentially harmful effects on humans. The latest revision of the venerable IEEE C95.1 standard is the 2005 revision, although the Commission still utilizes portions of the 1992 revision. The IEEE standard is not necessarily ubiquitous on a global scale, as other countries may utilize their own standards. Some of these are tighter than the IEEE standard, and some go the other way.

Other portions of the FCC's adopted procedures, including the exposure limits, are based on recommendations from the National Council on Radiation Protection and Measurements (NCRP). These requirements, available from the Office of Engineering and Technology section of the FCC home page, are the result of the National Environmental Policy Act of 1969. While some of the other portions of this legislation may be of dubious importance, the protection of personnel and the public from the generally undetectable hazards of RF radiation makes sense.

Make a plan

To promote awareness of RF hazards and mitigate their effects at your facility it is necessary to implement an RF safety plan. Under such a plan you are definitively recognizing and defining the potential harmful effects, establishing procedures that employees and contractors must follow, and identifying physical areas that may be problematic for exposure. While a boiler-plate approach is a good first iteration for a facility, it is crucial to recognize that each facility is unique, and therefore will likely require a customized solution. These suggestions are not meant to be an exhaustive list, and it is recommended that you consult with a competent engineer when you build your plan.

Typically hazards can be split into two general categories: contact hazards and non-contact hazards. Contact hazards are situations, as the name suggests, where the human body comes into direct physical contact with the hazard. The archetypal example here is coming into contact with a hot AM tower. Exposure to physical contact hazards can be minimized through a combination of barriers, reduction in transmitter power and cessation of operation. The general result of a contact hazard is the RF burn (a.k.a. the rite of passage).

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RF Safety

Jul 1, 2011 1:00 AM, By Jeremy Ruck, PE

The broader category is the non-contact hazard. Here all areas where hazards may exist that do not require physical contact with the tower are examined. Non-contact hazards do not typically in themselves result in burns, although excitation of metallic clothing, jewelry and tools by high-intensity fields can result in material heating. The non-contact hazards tend to relate more to tissue heating and their effects. Some of these effects are not fully understood yet, and may not significantly manifest themselves for many years after exposure.

The exposure limits under this second category are divided into two conditions: uncontrolled environment condition and controlled environment condition. The uncontrolled condition relates to exposure by the general public, or to those areas where you have no control over the access or exposure of persons residing, working or traversing. The maximum permissible power density in such areas is lower by a factor of five than in the controlled environment. As part of the application process, the FCC requires all facilities to demonstrate compliance with the uncontrolled environment condition at ground level, unless it can be satisfactorily demonstrated that areas where the predicted power density exceeds this limit qualify as a controlled environment.

In the controlled environment, the facility has the ability to control the exposure of personnel who are known to be cognizant of the hazards of RF and the implementation of safety procedures. Because of these factors, a higher power density limit is permitted. If work is necessary in areas where these limits are generally exceeded, additional action must be taken, such as the use of RF suits, reduction in power or cessation of operation.

RF suits, while useful in some instances, should never be considered a panacea. While they can attenuate the RF inside to certain levels, tears, rips or other integrity problems with the suit render them unusable. Their initial cost can be prohibitive, and for most stations it usually is more cost effective to build in some downtime rather than make the investment.

Personal radiation monitors, while useful, can be costly to keep calibrated. They can also provide a false sense of security that can simply be eliminated with lock-out/tag-out. Rendering an antenna inoperable through this methodology leaves absolutely no question as to whether it is causing a problem in its vicinity. This ultimately is the best solution to any RF safety concern, and should be part of the implementation of any program.

What's your sign?

Any RF safety plan must include appropriate signage. There are three different types of signs with differing color schemes approved for FCC compliance. Blue signs represent areas where the uncontrolled environment condition may be exceeded, yellow where the controlled environment condition may be exceeded and orange where the controlled environment condition is exceeded. The signs demarcate boundaries between the varying levels. Posting multiple different colored signs at one location is inappropriate and confusing. A sufficient number of signs should be posted so as to be visible from multiple different vantage points at the site.

There are multiple ways to establish the appropriate boundary locations. For FM-only sites, the FCC's FM Model software can predict where the peak power density will occur. The equations in OET Bulletin 65 along with simple trigonometric identities can also be used. For more conservative results, assume the relative field in the vertical plane is 1.0 at all elevations when using the equations.

Finally a survey of the site could be performed. A survey, with a high density of measurement locations properly mapped will provide the best graphical illustration of the situation. Prior to the survey, however, ensure your consultant has fully studied the specific antennas and geometries to ensure appropriate measurement locations are not missed.

While an RF safety plan can be as simple as a de-energize/lock-out/tag-out plan or as complex as various combinations of antenna changes, the importance of crafting, using, and maintaining such a plan cannot be emphasized enough. Although accidents may still occur, those accidents will tend to be viewed as such, and not as willful negligence.

Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.

July 2011

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