Attachment Radiation Hazard

This document pretains to SES-LIC-20070809-01057 for License on a Satellite Earth Station filing.

IBFS_SESLIC2007080901057_593208

Radiation Hazard Analysis Enlace Christian Church This analysis predicts the radiation levels around a proposed earth station complex, comprised of one (reflector) type antennas. This report is developed in accordance with the prediction methods contained in OET Bulletin No. 65, Evaluating Compliance with FCC Guidelines for Human Exposure to Radio Frequency Electromagnetic Fields, Edition 97-01, pp 26-30. The maximum level of non-ionizing radiation to which employees may be exposed is limited to a power density level of 5 milliwatts per square centimeter (5 mW/cm2) averaged over any 6 minute period in a controlled environment and the maximum level of non-ionizing radiation to which the general public is exposed is limited to a power density level of 1 milliwatt per square centimeter (1 mW/cm2) averaged over any 30 minute period in a uncontrolled environment. Note that the worse-case radiation hazards exist along the beam axis. Under normal circumstances, it is highly unlikely that the antenna axis will be aligned with any occupied area since that would represent a blockage to the desired signals, thus rendering the link unusable. Earth Station Technical Parameter Table Antenna Actual Diameter 1.8 meters Antenna Surface Area 2.5 sq. meters Antenna Isotropic Gain 39.6 dBi Number of Identical Adjacent Antennas 1 Nominal Antenna Efficiency (ε) 67.50% Nominal Frequency 6.2 GHz Nominal Wavelength (λ) 0.0484 meters Maximum Transmit Power / Carrier 69.4 Watts Number of Carriers 1 Total Transmit Power 69.4 Watts W/G Loss from Transmitter to Feed 0.0 dB Total Feed Input Power 69.40 Watts Near Field Limit Rnf = D²/4λ =16.74 meters Far Field Limit Rff = 0.6 D²/λ = 40.18 meters Transition Region Rnf to Rff In the following sections, the power density in the above regions, as well as other critically important areas will be calculated and evaluated. The calculations are done in the order discussed in OET Bulletin 65. 1.0 At the Antenna Surface The power density at the reflector surface can be calculated from the expression: PDrefl = 4P/A = 10.909 mW/cm² (1) Where: P = total power at feed, milliwatts A = Total area of reflector, sq. cm In the normal range of transmit powers for satellite antennas, the power densities at or around the reflector surface will exceed safe levels. This area will not be accessible to the general public. Operators and technicians will receive training specifying this area as a high exposure area. 1

Procedures will be established that will assure that all transmitters are rerouted or turned off before access by maintenance personnel to this area is possible. 2.0 On-Axis Near Field Region The geometrical limits of the radiated power in the near field approximate a cylindrical volume with a diameter equal to that of the antenna. In the near field, the power density is neither uniform nor does its value vary uniformly with distance from the antenna. For the purpose of considering radiation hazard it is assumed that the on-axis flux density is at its maximum value throughout the length of this region. The length of this region, i.e., the distance from the antenna to the end of the near field, is computed as Rnf above. The maximum power density in the near field is given by: PDnf = (16ε P)/(π D²) = 7.364 mW/cm² (2) from 0 to 16.74 meters Evaluation Uncontrolled Environment: Does Not Meet Uncontrolled Limits Controlled Environment: Does Not Meet Uncontrolled Limits 3.0 On-Axis Transition Region The transition region is located between the near and far field regions. As stated in Bulletin 65, the power density begins to vary inversely with distance in the transition region. The maximum power density in the transition region will not exceed that calculated for the near field region, and the transition region begins at that value. The maximum value for a given distance within the transition region may be computed for the point of interest according to: PDt = (PDnf)(Rnf)/R = dependent on R (3) where: PDnf = near field power density Rnf = near field distance R = distance to point of interest For: 16.74 < R < 40.2 meters We use Eq (3) to determine the safe on-axis distances required for the two occupancy conditions: Evaluation Uncontrolled Environment Safe Operating Distance,(meters), Rsafeu: 123.3 Controlled Environment Safe Operating Distance,(meters), Rsafec: 24.7 4.0 On-Axis Far-Field Region The on- axis power density in the far field region (PDff) varies inversely with the square of the distance as follows: PDff = PG/(4πR²) = dependent on R (4) where: P = total power at feed G = Numeric Antenna gain in the direction of interest relative to isotropic radiator 2

R = distance to the point of interest For: R > Rff = 40.2 meters PDff = 3.154 mW/cm² at Rff We use Eq (4) to determine the safe on-axis distances required for the two occupancy conditions: Evaluation Uncontrolled Environment Safe Operating Distance,(meters), Rsafeu : See Section 3 Controlled Environment Safe Operating Distance,(meters), Rsafec : See Section 3 5.0 Off-Axis Levels at the FarField Limit and Beyond In the far field region, the power is distributed in a pattern of maxima and minima (sidelobes) as a function of the off-axis angle between the antenna center line and the point of interest. Off-axis power density in the far field can be estimated using the antenna radiation patterns prescribed for the antenna in use. Usually this will correspond to the antenna gain pattern envelope defined by the FCC or the ITU, which takes the form of: Goff = 32 - 25log(Θ) for Θ from 1 to 48 degrees; -10 dBi from 48 to 180 degrees (Applicable for commonly used satellite transmit antennas) Considering that satellite antenna beams are aimed skyward, power density in the far field will usually not be a problem except at low look angles. In these cases, the off axis gain reduction may be used to further reduce the power density levels. For example: At one (1) degree off axis At the far-field limit, we can calculate the power density as: Goff = 32 - 25log(1) = 32 - 0 dBi = 1585 numeric PD1 deg off-axis = PDffx 1585/G = 0.542 mW/cm² (5) 6.0 Off-Axis power density in the Near Field and Transitional Regions According to Bulletin 65, off-axis calculations in the near field may be performed as follows: assuming that the point of interest is at least one antenna diameter removed from the center of the main beam, the power density at that point is at least a factor of 100 (20 dB) less than the value calculated for the equivalent on-axis power density in the main beam. Therefore, for regions at least D meters away from the center line of the dish, whether behind, below, or in front under of the antenna's main beam, the power density exposure is at least 20 dB below the main beam level as follows: PDnf(off-axis) = PDnf /100 = 0.07364 mW/cm² at D off axis (6) See Section 8 for the calculation of the distance vs. elevation angle required to achieve this rule for a given object height. 3

7.0 Region Between the Feed Horn and Sub-reflector Transmissions from the feed horn are directed toward the subreflector surface, and are confined within a conical shape defined by the feed horn. The energy between the feed horn and subreflector is conceded to be in excess of any limits for maximum permissible exposure. This area will not be accessible to the general public. Operators and technicians should receive training specifying this area as a high exposure area. Procedures must be established that will assure that all transmitters are rerouted or turned off before access by maintenance personnel to this area is possible. 8.0 Evaluation of Safe Occupancy Area in Front of Antenna The distance (S) from a vertical axis passing through the dish center to a safe off axis location in front of the antenna can be determined based on the dish diameter rule (Item 6.0). Assuming a flat terrain in front of the antenna, the relationship is: S = (D/ sin α) + (2h - D - 2)/(2 tan α) (7) Where: α = minimum elevation angle of antenna D = dish diameter in meters h = maximum height of object to be cleared, meters For distances equal or greater than determined by equation (7), the radiation hazard will be below safe levels for all but the most powerful stations (> 4 kilowatts RF at the feed). For D= 1.8 meters h= 2.0 meters Then: α S 10 10.9 meters 15 7.3meters 20 5.5 meters 25 4.5 meters 30 3.8 meters 9.0 Summary The earth station site will be protected from uncontrolled access with barriers and other means. There will also be proper emission warning signs placed and all operating personnel will be aware of the human exposure levels at and around the earth station. The applicant agrees to abide by the conditions specified in Condition 5208 provided below: Condition 5208 - The licensee shall take all necessary measures to ensure that the antenna does not create potential exposure of humans to radiofrequency radiation in excess of the FCC exposure limits defined in 47 CFR 1.1307(b) and 1.1310 wherever such exposures might occur. Measures must be taken to ensure compliance with limits for both occupational/controlled exposure and for general population/uncontrolled exposure, as defined in these rule sections. Compliance can be accomplished in most cases by appropriate restrictions such as fencing. Requirements for restrictions can be determined by predictions based on calculations, modeling or by field measurements. The 4

FCC's OET Bulletin 65 (available on-line at www.fcc.gov/oet/rfsafety) provides information on predicting exposure levels and on methods for ensuring compliance, including the use of warning and alerting signs and protective equipment for worker. 5


Document Created: 2007-09-19 12:15:16
Document Modified: 2007-09-19 12:15:16
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