Avoiding the RF hazard
Stations are responsible for the safety of personnel working in high-RF environments, as well as for the safety of members of the general public. Photo courtesy Wisconsin Educational Communications Board.
In days of yore, prior to 1982, the amount of non-ionizing RF radiation that was acceptable in the workplace was largely based on educated guesses. If the nails in your shoes didn't get hot or the metal frames on your glasses didn't become uncomfortable, you could assume that you weren't working in a dangerous place. Tower workers routinely climbed through the aperture of FM broadcast antennas while the antennas were radiating. When working near a transmitting TV antenna, a tower worker determined how close he could approach it by the point where he started to sense something strange. We now know that such actions weren't too smart.
After a lot of hand waving and supposition, and after commercial, educational and governmental institutions performed a great deal of good research, the first standards were published. For broadcasters, the thesis hit the fan when the Office of Science and Technology published OST Bulletin No. 65 in 1985. Titled “Evaluating Compliance With FCC-Specified Guidelines For Human Exposure To Radiofrequency Radiation,” it explained the theory concerning the effects of non-ionizing radiation on people, established a method for calculating the extent of such exposure from RF sources and established limits for such exposure. The bulletin established limits based on an average exposure over a six-minute period. Further, it based all calculations on a truly worst-case situation. That is, it assumed that FM and TV antennas radiate equally in all directions with the peak value of power. It also assumed that all towers in an AM array operated with the total value of licensed power. It attempted to be overly conservative or, in other words, to err only in the direction of maximum protection.
Then, the American National Standards Institute (ANSI) published a standard identified as C-95.1-1982. The FCC adopted that standard as a requirement for the operation of broadcast stations. Subsequently, the standard was modified and published as ANSI/IEEE C95.1-1992. The modification contained a two-tier approach regarding exposure limits both for workers who supposedly knew how to work around an RF environment, and members of the public that might be exposed. The Commission adopted that standard in 1996 and it still is in effect.
The FCC adopted ANSI/IEEE C95.1-1992 in 1996 to protect tower workers (like this one installing a transmission line system for four digital television stations in Louisville, KY) by limiting their exposure to dangerous non-ionizing radiation. Photo courtesy Andrew.
Now, what does all that mean to the broadcaster? First, virtually any application for a change in facilities requires that the broadcaster assess the non-ionizing levels involved. Applicants should be advised that simply stating that everything is fine on the applications won't necessarily get the job done. The Commission will check to be sure that something isn't totally out of place. The result can be a construction permit that requires the broadcaster to make measurements to show compliance with the standard. That is particularly likely for multiple-station sites.
There are several systems available for such measurements, most of which are offered by two manufacturers -Holaday Industries and Narda - a division of L3 Communications.
One system that is particularly useful has a probe with a shaped frequency response. The overall system response is essentially the same as the exposure-limit curve contained in C-95.1-1992. This means that no extensive calculations are involved because the instrument indicates the measured levels as a percentage of the allowable limit. The measurement procedure is very simple. The tip of the handheld probe contains the measurement point. The user takes the measurement by holding the probe well clear of himself and moving it through the area of concern. The probe must remain clear of the body or any object that could reradiate signals and cause error in the measurement. The measurement and calculation processes are simple and plainly described in the instrument instruction books.
Personal monitors are available with varying features. These small monitors alert the wearer when entering an area with high field levels. They also alert the user if some idiot turns a transmitter back on when it should stay off. This does happen, despite lockout procedures. When using these personal devices, take care. They don't provide an accurate measurement of the non-ionizing radiation levels like the big meter and probe configurations. Instead, they really are intended as alarms.
The question then becomes one of just how often and in what manner should broadcasters take measurements. First, OSHA requires that the normal safety meetings cover exposure to non-ionizing RF. The station should post a hazard assessment that identifies any area where the non-ionizing radiation levels would be in excess of the standard. To confirm the location and/or the existence of those locations, the station should take measurements and record the results. Unfortunately, a meter that will do that job accurately is expensive and is needed rarely, but the station's consulting engineer should own one. The simplest solution is to have your consulting engineer come in and determine the non-ionizing levels for all parts of the site.
For work on the tower, the simplest thing for AM stations is simply turn the station off. There are other ways of handling the problem, but they require more measurements and they leave the station open to possible problems if a governing agency determines that the workers were exposed to levels in excess of the standard. True, it is averaged over six minutes. But the only sure way of avoiding problems is to turn it off. If necessary, for work such as tower painting, get an STA to operate with an alternate antenna. String a wire or two and get off the tower altogether until the work is done.
Broadcasters building multiple-station sites like this six-station tower in Wausau are more likely to find it necessary to provide measurements to the Commission showing compliance with the standard for non-ionizing radiation levels. Photo courtesy Wisconsin Educational Communications Board. Photo by Rick Bowe.
For FM and TV operations, a qualified technician who climbs can go up the tower with the meter and record levels all the way up until the RF values reach the maximum level the standard permits. This is the way many of the large multiple-station sites do it. For example, technicians measure the non-ionizing radiation levels on the Sears Building at all rooftop locations regularly. In addition, when they encounter excessive levels, they also make measurements on the towers at all levels with the necessary transmitters shut down. That information makes it possible to predetermine just which stations must shut down or switch to other antennas when work is being done at any particular location.
Stations should record all this data and maintain it in their files. In addition, whenever work is done on the tower or antenna systems, keep a full record of who did the work, what work was performed, where the work was done on the tower and what steps were taken to avoid hazardous exposures. In today's litigious society, the tendency is to look for someone to sue if anything, however petty, goes wrong — from athlete's foot to dandruff. It is quite possible that a station may someday face a lawsuit claiming that someone's illness was caused by that old devil RF, and that the station didn't provide a suitable level of protection. In such a situation, the station's complete and well-maintained records could save it a lot of money. Not that any of that money will ever get to the engineering department, but the engineering staff will look like properly clever fellows when they produce the necessary records.
Don Markley is president of D. L. Markley and Associates, Peoria, Il.
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