Tuesday, June 21, 2011

RF Exposure Calculations For License Renewals

When I am asked to perform an RF calculation for an FM or television station, I gather some key pieces of information before I start. I need to know the effective radiated power of the station, the polarization of the signal (horizontal, elliptical or circular), the make and model of the transmitting antenna, its orientation (if directional) and its height above ground (or roof level, if mounted to a structure on a building). If I know the make and model of the antenna, I can usually find its elevation pattern either in my files or on the manufacturer’s web site. The elevation pattern allows me to determine the intensity of each side-lobe of the antenna pattern as well as its distance from the base of the tower.

The other very important fact that plays a part in the situation is whether or not the area surrounding the transmitter site can be considered a “controlled” environment. The Commission has separate RF exposure guidelines for controlled sites and uncontrolled (usually public) areas. The exposure guidelines are five times more strict for uncontrolled areas than they are for controlled environments.

The FCC considers a site to be controlled if two requirements can be met. First, the site must be secure from public access. This usually means a fence with a locked gate surrounding the transmitter building/tower compound or locked doors to all entrances to a building’s roof if your antenna is so mounted. As I mentioned in my previous blog, a locked gate across an access road to a site is not sufficient to consider the site as controlled, if hikers, bikers or ATV users can go around the gate or access the site from another path. The second requirement for a controlled site is that every person that enters the site must be trained in RF safety awareness. This means everyone, from station personnel to contractors to equipment/site maintenance people. If even one person accesses the site without such training, the site will be considered to be “uncontrolled” by the FCC and the stricter exposure standard will apply.

Once I am convinced as to which standard to apply in the case at hand, I conduct a study to see if there are other FM and television broadcast facilities operating at the same site as my client’s facility. Usually, stations within a few hundred feet of my client’s are considered. Although there can be RF contributions from other types of antennas (cell phone, two-way, microwave, etc.) at the site, their individual contributions are usually less than 1% of the FCC’s public exposure guidelines and can therefore be ignored.

I then perform my RF calculations and find the maximum power density value that would occur two meters (six feet) above ground from my client’s facility. I also compute the distance from the base of the tower where this maximum value is calculated to occur (and the direction, if a directional antenna is used). I divide the result by the applicable exposure standard for that frequency and type of environment in order to get a percentage of the maximum allowable value (MPE). There are three basic factors that result in high RF numbers. They are: high effective radiated power, low antenna height above ground (or roof), and frequency. With respect to frequency, FM and VHF television stations have the strictest exposure standard, because their wavelength is closest to the height of an average human being. Therefore, these frequencies are absorbed by the body much more efficiently than is the RF from a UHF station, which has a much smaller wavelength.

If there are no other co-located or nearby FM or television station antennas in operation, I am unconcerned as long as my client’s contribution to the RF environment is within 80 to 90 percent of the applicable maximum guideline value. If there other FM or television stations operate from the site, I am finished with my study if my client’s maximum calculated contribution to the RF environment is less than 5% of its guideline value. That’s because the FCC considers a station to be liable for fines and employment of RF mitigation techniques only if its RF contribution in an area of concern is greater than five percent. Since calculations are based on some very conservative assumptions and since I tend to find lower RF numbers in the field when I make measurements, I feel comfortable in claiming that my client’s RF contribution can be excluded from consideration if I calculate it to be less than five percent of the applicable standard.

If I calculate a number greater than 5 percent and there are other stations at my client’s site, I have to perform a similar calculation for each of the other stations. If I assume that each station’s RF maximum value occurs at the same point (which they don’t) and the total calculated RF is significantly less than 100 percent of the MPE, then everything is peachy.

If the number is close to or greater than the MPE in any of the above situations, I usually suggest that a power density survey of the site be conducted. In my next blog, I will talk about surveys and some things I have learned from doing more than 100 of them.

If you have any questions or would like us to help with the RF portion of your renewal, please call or send an e-mail to Kevin@SmithandFisher.com

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