In the past 10 years, there has been tremendous growth in the use of roof-mounted transmission devices. This topic is little understood within the fire suppression community. Until recently, some fire departments were completely unaware of this “new-age” communication tool, yet they have been...
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In the past 10 years, there has been tremendous growth in the use of roof-mounted transmission devices. This topic is little understood within the fire suppression community. Until recently, some fire departments were completely unaware of this “new-age” communication tool, yet they have been scattered throughout their cities for years.
It is important for all firefighters and chief officers to be aware of what these tools are and how they might affect their safety during any type of rooftop operation. The devices in question are microwave transmission dishes, “whip antennas” (vertical rods) and broadcasting towers. The presence of these transmitters can alter the ways fire departments function on a roof. For instance, some departments automatically send a team to the roof on any working high-rise fire. The firefighters’ pre-designated assignment is to stand by and await further instruction from the incident commander in the event any type of action needs to be taken at roof level. This assignment should now be approached with considerable caution.
Transmission devices include:
- Microwave dish antennas. Most fire departments are already aware of cell phone antennas and their related risks, which are commonly found on low-rise buildings. However, high-rise office buildings have begun adding rooftop microwave transmitters to transmit data (for banking, trading or other business purposes). When these business entities send data out, it is usually done through fiber optics, microwave links or both.
Most microwave dishes are relatively safe and ironically, the bigger and more intimidating the dishes, the less power they emit and the safer they are. Large dishes (see photo 1) typically emit only a few watts of power, so you can stand right in front of them and not put yourself in harm’s way. The smaller dishes (see photo 2) can produce a more concentrated beam of microwave radiation and, hence, can lead to a higher level of exposure to personnel operating near them. Beyond several feet from the antenna, the radiation exposure dissipates quickly and is usually within applicable human exposure standards. These dishes are pointed at other (reception) dishes (see photo 3) on other roofs and are, in a sense, “speaking to each other,” albeit one way.
These devices are not always on the main roof level. They may also be found on roof setbacks on taller buildings with decreasing-sized floor plates on upper floors. Reception dishes are completely harmless, but normally can’t be discerned by simply looking at them. “High-intensity” microwave transmitters are sometimes mounted up high on elevator penthouses (see photo 4) or by other means, away from potential human exposure. These can emit up to 1,000 watts of effective radiated power (ERP). Although Federal Communications Commission (FCC) regulations require roof signage warning of potential danger areas (see signs on page 76), the author has personally seen many roofs with little to no signage to that affect. The FCC rules apply only to its licensees and do not apply to building owners and managers. FCC limits for human exposure are based on a safety factor of 10, so even if a rooftop transmitting site exceeds FCC limits, while this could represent a regulatory breech, it still may be safe for firefighters to operate (depending upon signal strength) and how long one is exposed.
- “Whip antennas.” The more deceiving and more intense exposure sources are represented by “whip antennas,” or vertical transmitting rods. There are two types – bare element and fiberglass rods. Bare element rods are typically used in two-way radio applications. These are rods you do not want to touch, as your skin can be burned by making direct contact. The fiberglass rods are the safer of the two, due to their insulating properties. These rods can be mounted virtually anywhere on a roof (see photo 5) and unlike the microwave dishes, they emit radio frequency (RF) energy uniformly 360 degrees (known as omni-directional).
What separates the microwave dishes from the “whip antennas” is the difference in wavelength. Microwave dishes and panel antennas operate at higher frequencies with shorter wavelengths, while “whip antennas” operate at lower frequencies with longer wavelengths. The more intense RF fields are usually from antennas that transmit signals for pagers. Cell phone and two-way radio transmitters are typically lower power. Pager, cellular telephone base station and two-way radio antennas often emit RF radiation in short “bursts.” The strength of the signal surges when these devices are utilized. For instance, RF exposure can only occur when a radio is keyed. Otherwise, the transmitter is inactive. A pager antenna can emit up to 500 watts of effective radiated power. The ERP with microwave dishes is less concentrated than with rod antennas, thus are safer despite the higher power range. Some buildings run these rods along the perimeter of the roofline, which can prove to be an issue if a window-washer rescue may need to be performed using rappelling techniques.
- Broadcasting towers. The larger, permanently mounted antennas are known as broadcasting towers (or masts) and can be found on some of the tallest buildings in North America. They are the most potentially hazardous. These transmit television and FM radio signals to surrounding communities for many, many miles (see photo 6). The higher the power level, the greater the potential for exceeding established safe limits for exposure. They range from one to five megawatts of effective radiated power. These are the strongest sources of RF energy found on rooftops and particular caution should be used if required to access regions near the actual antennas.