Answer to Question #12533 Submitted to "Ask the Experts"

What is a safe distance for the new two-way home satellite dishes? People now access two-way Internet from their satellite provider, but I don't see any safety information for them. We have one aimed directly through our upstairs bedroom from next door.

Home-based satellite internet systems were first introduced in 2003 to provide for internet connectivity, principally in rural areas deficient in other forms of internet access. These systems generally comply with Federal Communications Commission (FCC) exposure limits for the general public with wide margins of safety.

The home-based equipment consists of a relatively small dish-type antenna, similar to that used for satellite television (TV) reception, directed toward a satellite in geosynchronous orbit above the equator. A feed horn antenna is used to illuminate the dish reflector with microwave energy from a small transmitter that is closely mounted to the feed horn. Depending on the particular internet service, the transmit frequency may range somewhere between 14.5 gigahertz (GHz) and 30 GHz. The transmitters operate at remarkably low power levels, typically in the range of 1 to 3 watts (W) maximum power. In operation, however, depending on the particular satellite service and local weather conditions, the duty cycle of the transmitter may range from less than 1% to approximately 30%. (Duty cycle refers to the percentage of time that the transmitter full power is present and is important for determining the average power density produced by the system.)

The intensity of the transmitted signal from a home-based satellite system is normally described in terms of its power density in the unit watts per square meter (W m^-2) or milliwatts per square centimeter (mW cm^-2), this being the amount of power propagated away from the dish antenna per unit area of space. Because the dish antenna is very directional, most of the transmitted energy of the microwave signal is generally contained within a cylindrical beam approximately the same size as the dish. Much lesser levels of microwave energy fall outside the beam.

Because of the low power, only relatively low power densities are created by the system. For evaluating the potential for hazardous exposure to the transmitted signal, the transmitted power density is compared to limits that have been established for human exposure. In the frequency range used by home-based satellite internet systems, the exposure limit set by the FCC for the general public is a power density of 10 W m^-2 or 1 mW cm^-2 as averaged over a 30-minute period or exposure. This exposure limit is believed to be a factor of 50 times lower than the level at which hazardous exposure would exist.  

Based on the power level and antenna dimensions of these systems, theoretical analyses have shown that the greatest power density that can occur in front of the dish antenna is generally less than 1 mW cm^-2, at the closest point to the dish and, hence, is compliant with the FCC's more stringent limit for public exposure. As distance from the dish increases and for areas that are not aligned perfectly with the pointing direction of the dish, the power density will become substantially less. The region of most intense microwave energy is the small region located between the feed horn and the dish surface. To eliminate the possibility that someone might place their hand in front of the feed horn where the microwave power density could potentially be much higher than in front of the dish antenna, home-based systems incorporate an automatic shut-down feature. The systems are designed to turn off the transmitted signal if the received signal coming from the satellite is blocked. Normally, this process happens in much less than one second. Thus, exposure to higher intensities of the transmitted signal are eliminated. In some cases, companies affix cautionary labels to the dish alerting individuals to not block the region between the feed horn and the dish surface.

Companies providing home-based satellite internet access must file technical information with the FCC in which they provide supportive data to show compliance with the FCC exposure limits prior to receiving a license to install these systems.

Finally, because of the narrow beam of the transmitted signal, for proper operation of the system the antenna must be oriented so that its signal will not be blocked by intervening structures. The question states that an adjacent neighbor's satellite dish is directed through an upstairs window. If the dish is, in actuality, directed through the home, it is unlikely that it would operate. Accurate visual determination of the beam direction of upward directed dish antennas while looking toward the front of the dish from a distance is generally difficult and it is likely that the most intense point of the beam of transmitted energy, while aligned closely with the home, is not exposing the home. Nonetheless, the intensity of the signal, regardless of its actual orientation would be expected to be substantially less than the FCC limit for safe exposure.  

Gary H. Zeman, ScD, CHP