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ENGINEERING CONSIDERATIONS APPENDIX A: ELECTROMAGNETIC ENERGY INFORMATION Star cluster mounts (see Figure A-7) or candelabras present a significant issue in the management of EME on towers. If there are five to eight antennas mounted in a circle and these antennas are located 1.5 m (5 ft.) from the tower, there is the potential for an EME level in the center that exceeds the limits. Because the center is the tower, workers must ensure they understand the fields while entering this area. Figure A-7 shows the computed effects of several transmitters using the EME modeling program described above. Each square pixel represents 929 cm 2 (1 sq. ft.) of resolution. This simulates the effects of five PD-10017 antennas with 100 W into the antenna at 900 MHz. A worker entering this area may be exposed to EME levels above the applicable MPE and shall take appropriate steps, such as moving quickly through the area, to assure compliance with recognized exposure guidelines. What makes this situation difficult to manage is the fact that the field and the resultant high EME levels from all the antenna fields overlap and add. While this situation can exist, the fields are reduced by the cable loss associated with the height of the candelabra, and are therefore more manageable. Most candelabras are mounted on top of a tower. Because of the cable loss associated with towers, the power into the antenna is significantly lower than buildings and hilltop sites. This loss between the transmitter and antenna reduces the power and ultimately the fields produced. Higher frequencies have higher line loss, which significantly reduces the power at the antenna. This fact is very important and proves to significantly reduce the fields produced on tall towers. Candelabra and star mounts present unique compliance and maintenance situations due to the additive nature of EME exposure at these locations. Insertion losses of transmission lines reduce the power into the antennas and reduce the likelihood of strong fields on clusters located at high levels on tall towers. For equal transmitter power, the higher the frequency, the higher the insertion loss; thus EME levels are lower on tall towers. A.3 ENGINEERING CONSIDERATIONS A.3.1 ANTENNA ELEVATION One common technique for reducing the RF levels expected on large roof tops is to elevate the antennas above the roof. Elevating the antennas raises the EME fields above the roof and reduces the power density to which an individual at roof level will be exposed. The results of elevating antennas are illustrated in Figure A-8. These data are based on the EME fields produced by an 850 MHz SMR antenna. Ten 150 Watt transmitters through a combiner drive the antenna. The resultant 550 Watts of power is fed into a 3.96 m (13 ft.) omni antenna. This type of antenna configuration is not unusual on rooftops. A-14 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES ENGINEERING CONSIDERATIONS Percentage of MPE 450 % 400 % 350 % 300 % 250 % 200 % 150 % 100 % 50 % 0 % FIGURE A-8 EXPOSURE VS. ANTENNA HEIGHT ABOVE ROOF The resultant exposure possible can be above the MPE when the antenna is mounted at the roof level. From the chart, fields in excess of 200% of the Occupational/Controlled MPE are encountered within 305 mm (1 ft.) of the antenna. While this seems extremely close, a technician walking down the center of an antenna grid with 1.2 m (4 ft.) centers will be 610 mm (2 ft.) from any antenna at any time. At a distance of 610 mm (2 ft.) from this antenna mounted at roof level, it is possible for the exposure to be over 100% of the same MPE. If this situation is compounded with several antennas having the same power density, the levels in this walking area could be above the MPE. For this reason every effort should be given to reducing the fields at the roof level. The most effective technique for reducing the fields on a building, while maintaining constant radiated power, is raising the antenna. Raising the antenna 1.2 m (4 ft.) above the roof reduces the EME field strength at the roof level to about 50% of the MPE at 305 mm (1 ft.) from where the antenna was. If the antenna is raised 1.8 m (6 ft.) above the roof the fields are reduced more than 90%. The most effective technique for reducing EME levels is elevating antennas on buildings and extending antennas away from the tower. A.3.2 EXTENDING ANTENNAS AWAY FROM TOWERS Roof Level 1 ft above roof 2 ft above roof 3 ft above roof 4 ft above roof 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Distance from Antenna Tower contractors climbing the tower must pass through fields created by active antennas on the tower. Antennas mounted on short sidearms or mounted directly to the tower produce high levels of exposure to tower climbers. It is a good engineering practice to mount omnidirectional antennas a minimum of 1.52 m (5 ft.) from the tower. 68P81089E50-B 9/1/05 A-15
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© 2005 MOTOROLA, INC. ALL RIGHTS R
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TABLE OF CONTENTS Chapter 1. Introd
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STANDARDS AND GUIDELINES FOR COMMUN
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CHAPTER 1 INTRODUCTION 1 This manua
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CHAPTER 2 SITE DESIGN AND DEVELOPME
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CHAPTER 3 COMMUNICATION SITE BUILDI
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CHAPTER 4 EXTERNAL GROUNDING (EARTH
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CHAPTER 5 INTERNAL GROUNDING (EARTH
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CHAPTER 6 POWER SOURCES 6 6.1 LOCKO
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CHAPTER 7 SURGE PROTECTIVE DEVICES
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CHAPTER 8 MINIMIZING SITE INTERFERE
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Page 367 and 368: STANDARDS AND GUIDELINES FOR COMMUN
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Page 405 and 406: APPENDIX A ELECTROMAGNETIC ENERGY I
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APPENDIX D GROUNDING (EARTHING) ELE
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APPENDIX E GENERAL CONVERSIONS AND
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APPENDIX F R56 COMPLIANCE CHECKLIST
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I NDEX Index Index AC power cabling
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