There are few antennas that cause more concern than satellite transmission antennas. The large antennas are frightening to the public, and the high gain of large satellite antennas often causes concern with professionals.
Antennas that simply receive energy from satellites and cannot transmit are referred to as “receive only” (RO) antennas and can never be an RF safety concern. Satellite antennas used to transmit energy to the satellites, or “uplink” the signal, are potential RF safety hazards. Some very high-power, large aperture satellite antennas and small satellite antennas can have very high field levels in the main beam, but most larger earth stations have relatively low RF field levels in the main beam. Satellite antennas do not have significant energy outside the cylindrical beam of energy formed by the antenna.
False Sense of Security
Unfortunately, it is not uncommon to have self-proclaimed—but uneducated—experts make measurements around satellite-uplink antennas. When they find insignificant RF field levels on the ground, they declare the site “compliant,” and people are led to believe that there is nothing to worry about. You might also see reports that focus on some very low-level RF fields found on the ground. Invariably, these are not true RF field levels but measurement artifacts caused by zero drift in the survey system. See Measurement Artifacts. The main point, which is almost always missed, is that the real risk is to the maintenance staff working on the antennas.
RF Safety Hazards with Satellite-Uplink Antennas
For the majority of satellite antennas, the only RF safety hazards are for maintenance personnel. There are two scenarios in which maintenance personnel could be exposed to excessive levels of RF energy near a satellite-uplink antenna even though the RF field level in the main beam may not be a concern.
The RF energy coming out of the feed horn and, in Cassegrain design antennas, between the feed horn and the subreflector, is extremely concentrated because the energy has not been distributed over the large area of the antenna reflector. Should a person go into this area while the system is transmitting, he or she would be exposed to RF field levels in orders of magnitude above recognized RF safety standards.
Waveguide transmission line leaks can occur at any time. Workers must be concerned with leaks in the waveguide if their faces get very close to the site of the leak. The greatest concern over waveguide leaks is the potential for eye damage. The eyes are especially vulnerable to the high levels of RF energy that can be present very close to a leak site. Flexible waveguide, which is used with most satellite antennas, is especially prone to developing leaks. A leak of as little as 15 Watts can result in an RF field level that is 10 times the FCC’s MPE limit for Occupational/Controlled exposure. Medical journals have reported that exposure to RF field levels of this magnitude for as little as 10 minutes can result in the formation of cataracts in the eye.
Analyzing Satellite-Uplink Antennas
Reflector antennas such as satellite-uplink antennas can be analyzed with confidence. The resultant calculations are inherently conservative and result in higher values than actual measurements since the calculations are based on the maximum possible energy level rather than the average level.
With these antennas, all the energy is confined to a cylindrical beam. Energy starts at a focal point in a manner similar to a bulb in a flashlight and reflects off the antenna reflector. The intensity of the energy in this beam depends primarily on two factors—the amount of energy fed into the antenna and the size of the antenna. Although larger diameter antennas have a higher gain—a measure of how efficiently the energy in the beam can be focused in the distance—from an RF safety perspective, the smaller the antenna, the greater the concern. This is because gain does not become a factor for several hundred feet from the antenna. For satellite antennas, gain begins to come into play hundreds of feet above the ground since the antennas are all pointed upward toward satellites in orbit above Earth’s equator. When you are close enough to a satellite antenna to get into the beam, the energy level or energy density depends on the size of the antenna. The smaller the diameter of the antenna is, the smaller the beam is. The smaller the beam is, the greater the energy density becomes since the energy is distributed over a smaller area. A simple analogy is to adjust a spotlight from a broad beam to a much smaller, brighter spot of light. It is important to understand that the only RF safety concerns are for individuals who get into the cylindrical beam of energy. While technically there is a very small amount of energy outside the cylindrical beam, it can be safely ignored because even the energy level very close to the beam is an extremely small fraction of the level inside of it.
RF Safety Solutions often analyzes satellite-uplink facilities using FCC-recommended and approved methodology and provides a comprehensive, easy-to-read report without any need to visit the site. This approach requires the client to take pictures and gather some information about the antennas. It yields essentially the same results as an on-site survey (without a check for leaks) at a significantly lower cost. Since leaks can occur at any time, a one-time survey would simply provide information on the conditions at a particular point in time. Leaks can then be addressed in other ways.
Contact RF Safety Solutions to discuss your facility.