Answer to Question #773 Submitted to "Ask the Experts"

Q

If a person is in the direct line at the same level (height) as the main beam of a telecommunications antenna, how far away should they be? I've read in various publications 500 feet. Is that true?

A

When considering exposure to the radiowaves from telecommunications antennas, it is important to distinguish between "radiated power" and "effective radiated power" (ERP). The radiated power is the actual power delivered to the antenna—ERP is an artificial construct used to compare the properties of a directional antenna with one that propagates energy uniformly in all directions. Specifically, ERP is a measure of how well a directional antenna concentrates the radiated energy in a specific direction. (An analogy can be drawn in a comparison between an ordinary light bulb and a spotlight. At the same distance, the light in the beam of a 100 watt [W]spotlight appears much brighter than the light from an ordinary 100 W lamp because the spotlight concentrates the light into a smaller solid angle. Conversely, the light outside the beam of a spotlight appears less bright than the light from an ordinary lamp even though the power of each is the same.)

The base-station antennas used for cellular service are mostly directional antennas (called "sector antennas" that look like a rectangular panel—approximately 4 ft tall by 6 to 10 inches wide and deep). The ERP of such antennas is defined as the product of the power into the antenna and the gain of the antenna. Regardless of the gain, the actual power radiated from the antenna is always slightly less than the power delivered to the antenna. The gain of typical antennas used for cellular service, such as the DAPA Model 2980.002 mentioned in the expansion that you sent us of your question, is about 10 to 20 depending on the antenna. This means that when compared with a hypothetical antenna that radiates uniformly in all directions (called isotropic), the intensity of the signal from a cellular antenna, in the direction of maximum intensity, is about 20 to 30 times greater at the same distance as that from the hypothetical isotropic antenna. But, increasing the intensity of the signal in one direction means decreasing it in others compared with the isotropic antenna. (Sort of like squeezing a balloon.)

The sector antennas used for cellular service propagate the energy in a fan-shaped beam, typically 120 degrees wide in the horizontal plane and 6 to 10 degrees wide in the vertical plane. Thus, most of the energy is directed well above structures near the antenna and the intensity of the signals is quite low in these locations. Moreover, since each sector covers 120 degrees, only one sector has to be considered when considering exposure of people near the site (since one cannot be in more than one sector at one time). Also, while there may be three or four antennas covering a sector, two are usually receive-only antennas. For typical systems, the total number of channels per sector is about 21—which means that the intensity of the signal is proportional to 21 times power for each channel.

As indicated above, ERP is equal to the product of the antenna input power and the antenna gain. If the antenna gain is 10, the antenna input power necessary to produce a 100 W ERP is 100/10 = 10 W. Thus the total power radiated in any of the three sectors would about 210 W—not 5,700 W or 17,100 W. In fact, the maximum power of the output amplifier that drives the antennas is about 220-240 W for the combined signals for each sector.

The 100 W ERP is the maximum that would be used. As the cells are split into smaller cells by adding sites between existing sites, the ERP of all of the adjacent cells is reduced. (The cells are split—not the channels. The capacity of the channels can be increased using newer digital technology but that does not change the antenna power.) We have carried out measurements on numerous installations and find that the actual ERP at many sites is between 2.5 and 16 W. 

With regard to the levels at 500 feet directly in the main beam of the antenna, even if all 21 channels operate at 100 ERP each, the power density would be less than 1 millionth of a watt per square centimeter (µW/cm2). At 200 feet from the base of the tower they would be far lower than 1 µW/cm2.

There is a consensus of the world's scientific community as to what constitutes safe levels of exposure. This consensus is reflected in contemporary safety standards for exposure of the public, including those used in the United States and throughout the western world. The conclusion is that 24 hour/day exposure at levels below approximately 500 µW/cm2 is safe. These standards, which incorporate large factors of safety, have evolved over the past five decades and are based on an enormous amount of research. It is important to note that in spite of the tremendous amount of research carried out in this field during the past few decades, none of the results warranted a significant change to the recommendations and guidelines over the same period.

Exposure to RF energy in the vicinity of the site in question (even if operated with 21–100 W ERP channels) is far below any contemporary exposure limits and is considered safe by the US public health agencies such as the Food and Drug Administration, the Environmental Protection Agency, and international organizations such as the World Health Organization.

R.C. Petersen
Manager, Wireless & Optical
Technologies Safety Department