Radiation safety for small Ka-band satellite interactive terminals

The coming years will see the widespread deployment of small, low-cost, interactive satellite terminals. Unlike the popular Ku-band direct broadcast receiveonly terminals, the Ka-band user terminals will transmit to the satellite. Due to the small size of these terminals, on the order of 1 m in diameter, they may be installed on the premises of homes and businesses where people may be present in the vicinity. To keep subscriber costs low, system designers wish to achieve blanket licensing on their most popular terminals, avoiding the cost of licensing each terminal installation. To accomplish this the terminals must be demonstrated to be safe, to include radiated power density levels, given that certain installation guidelines are observed. However, power densities generated even by these small terminals may exceed levels that have been established as safe for human exposure. Exact power densities are a function of the given terminal design, but are largely driven by EIRP and antenna size limitations imposed on the terminal. So the system designer will want to have an idea of the range of power density levels that may be created near the installation by a given terminal specification that includes EIRP and a maximum aperture size constraint imposed by marketability requirements. The system designer will also want to be aware of installation constraints created by a given choice of terminal characteristics. Electromagnetic field restrictions have been derived from research into the effects of radiation on biological matter, which shows that increasing the body temperature by 1◦C results in tissue damage. The specific power absorption required to produce such a temperature rise is estimated to be 4 W/kg for whole-body absorption. Applying safety factors, it has been determined that the resulting limits on power density should be set at 50 W/m2 (5 mW/cm2) for occupational exposure and 10 W/m2 (1 mW/cm2) for general public exposure. These levels are mentioned in both FCC and international documentation. While these levels apply to whole-body exposure, international guidelines imply that localized power densities may be up to 20 times higher averaged over 1 cm2. The averaging time specified in FCC guidelines [1] is 6 min for occupational exposure and 30 min for general exposure. The international guidelines [2] define the averaging time as a function of frequency as 68/f1.05, or about 2 min at 30 GHz. There are four regions (Fig. 1) which are of interest in a power density level analysis: the interior of the antenna, the main reflector spillover, the near field, and the subreflector spillover (if applicable). With the exception of the subreflector spillover region, the power densities can be estimated as a function of transmitter and antenna size and sidelobe level. So the levels can be estimated from the specified requirements regardless of the specific antenna design geometry. The analysis that follows will do that for the first three regions mentioned. In the subreflector spillover region, however, the levels are highly dependent on the size of the sub and whether a cassegrain or gregorian configuration is used. Power densities for a few representative cases will be estimated in the subreflector spillover region.