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

Wednesday, June 1, 2011

RF Compliance Certification For FCC License Renewal

According to our friends at CommLawBlog, the FCC has revised its Rules with respect to a broadcaster’s certification of compliance with the Commission’s human exposure guidelines to non-ionizing electromagnetic radiation (RF) in its license renewal (Click here to see their take). In past renewal cycles, the broadcaster was required to submit a technical statement from an engineer, showing calculated or measured compliance with the RF exposure limits. In this round of renewals, a broadcaster can simply check a box on the renewal form that indicates that the RF environment surrounding the transmitter site has not materially changed since the last renewal cycle.

For a stand-alone broadcaster at a particular transmitter site, this is a no-brainer. Unless the station has changed its transmission facility since the last renewal and there has been no construction of residences, commercial buildings or tower structures in the vicinity of the transmitter site, there should be no change in the ground-level RF situation. If changes have been made to the transmission facility or if structures have been constructed in the vicinity of the site, new RF calculations should be made in order to ensure compliance with the FCC’s RF exposure Rules.

Most broadcasters operate at sites where there are other broadcast antennas. Such a complex RF environment requires more careful consideration. If there have been no changes to any of the broadcast facilities that have been operating there since the last renewal and there have been no additional broadcast facilities constructed at the site or an adjacent site, and no construction of new structures within close proximity to the transmitter site, the “No Change” box can be checked on the renewal form regarding RF certification.

Lately, the FCC has been ramping up its on-site inspection of stations that are believed to be exceeding the RF exposure guidelines. If the FCC finds that a station is indeed exceeding the allowable limits, the fines are severe and the Commission requires immediate rectification of the situation (including going off the air until an effective solution can be determined). If an inspector finds an area surrounding a particular transmitter site that exceeds the human exposure guidelines, the FCC considers a station liable for fines and mitigation techniques if its contribution to the RF situation in the area of concern is more than five percent (5%) of its particular guideline value.

Let’s say that you operate a broadcast antenna in an antenna farm. In a particular area of the transmitter site, the total RF from all sources at the time of last renewal cycle was 97 percent of the allowable standard. Let’s also say that the RF contribution from your station was 50 percent at that spot. Suppose that last year, a new FM station went on the air at the site. In the area of concern, the RF level is now 120 percent, well over the FCC’s allowable limit. If an FCC inspector measures the RF level in this spot, the Commission would fine your station and require you to take remedial steps to bring that area into compliance. And you hadn’t made any changes to your operation.

One other point regarding RF exposure needs to be made here. The FCC has two different exposure standards. The standard for public areas is five times more strict than it is for “controlled” sites. The definition of a controlled site has two requirements. First, it must be inaccessible to the public. For broadcaster’s that operate from a building rooftop, all points of entry onto the roof must be locked. For tower sites, this means a chain link fence with a locked gate surrounding the transmitter compound and tower. A gate across an access road is not sufficient if hikers, bikers and/or users of ATV’s can reach the site. The second and most overlooked requirement is that all those that access the transmitter site must be trained in RF safety practices. This applies to HVAC mechanics, window washers (for building sites), electricians, janitors, waste removal personnel, food service personnel, and, of course, all station personnel. The onus is on all of the broadcast users of a particular site to make sure that all contractors and employees that access the site have such training, otherwise the Commission will deem the site “uncontrolled” and the public RF exposure standard will apply.

If you have had a power density survey performed at your site, review the report and make sure that nothing has changed at the site with regard to the stations that operate from the site. Check and make sure that any RF warning signs that were installed are still visible. And, check to make sure that fencing and locks on gates are intact.

Finally, if you have any doubts about whether or not you comply with the RF exposure standards, contact an engineer. We can perform calculations for your station as well as conduct power density surveys of broadcast sites. We have RF meters that can determine overall RF levels in specific areas as well as individual station contributions in multi-station environments. In addition, we provide RF safety training classes for those that need to access your site.