Satellite communications earth stations are perhaps the most visible RF sources found in the environment. Large parabolic dish antennas, normally painted white to minimize heating from the sun and possible surface deformation, can create extremely high effective radiated power (ERP) levels. It is not unusual for the ERP of a high power earth station to approach the terawatt (TW) level.
RF field surveys conducted at numerous earth station facilities by Richard Tell Associates have typically demonstrated very low levels of electromagnetic energy in publicly accessible areas. As with any antenna, despite relatively high input power to the antenna, near-field power densities are directly related to the physical aperture area of the dish and since most of these antennas are large, the power densities found, even directly within the aperture can be surprisingly modest. These observations do not hold, however, for locations directly in front of the feed horn of the dish.
Measurement projects have been conducted at major satellite teleports around the country to support claims of compliance with applicable RF exposure limits. Theoretical studies have also been performed, such as for the FAA's Wide Area Augmentation System, in which near-field analysis methods have been employed. The large dish to the right is located in Brewster, Washington.
In a typical sat-com earth station facility, numerous dish antennas may be employed for communications with various satellites, feeding video, data and command and control signals to the satellites orbiting the earth. In other cases, portable sat-com equipment may be situated temporarily close to public access points and require evaluation. In the photo below, a two portable systems are being tested at an educational facility in Texas.
In fact, at some facilities, the antennas may be so densely mounted that the ground is almost always shaded such as the above right photo at an earth station in Culver City, California. To properly evaluate the intensity of side-lobe radiation, surveys must be made on the site itself and at nearby locations, such as multi-story buildings that may be located beneath the main beam of the antennas.
Field surveys at satellite earth stations, except for the aperture of the dish itself, typically require more sensitive equipment than normally used for RF hazard studies. This is because of the high directivity of the antennas with very low side-lobe fields. A useful approach to these measurements has been found to consist of a standard gain horn antenna connected to a broadband, high sensitivity power detector, inline bandpass filter and portable power meter.
This combination provides for extremely high sensitivity, permitting measurements in the picowatts per square centimeter range. The power density may be determined simply by an accurate measurement of the power captured by the antenna divided by the effective aperture area of the antenna. This type of measurement system has been used to advantage in numerous studies and illustrates the utility of assembling the right equipment for the job.
Theoretical studies of RF fields produced by circular dish antennas are often used to evaluate potential exposure levels, particularly in the near field. The electromagnetic field, plane wave equivalent power density in front of a dish can be estimated from the input power to the dish, its aperture dimension and distance from the aperture plane. The figure below illustrates the how the power density, designated as S, can be estimated in the near-field, transition zone and far-field regions. Note that the classical engineering definition of the start of the far field at 2 D²/λ is entirely too conservative; in reality, the power density is found to begin decreasing as inverse square law after only 0.6 D²/λ. It is also noted that in the true near-field region, the peak power density can be simply estimated from power and aperture cross-sectional area alone, somewhat analogous to estimating the field adjacent to vertical collinear antennas used in the wireless industry where gain really plays no role.
At distances beyond 0.25 D²/λ, the peak plane-wave equivalent power density begins to vary with distance.
