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ENGINEERING CONSIDERATIONS APPENDIX A: ELECTROMAGNETIC ENERGY INFORMATION A.3.3 COLLOCATED BROADCAST TRANSMITTERS Areas with broadcast transmitters can have fields created by grating lobes from the antenna or fields developed directly by the main radiating beam. On broadcast-only sites these are the only field that must be considered in EME analysis. On collocated sites, the EME fields are a combination of the fields generated by two-way transmitters and broadcast stations. If the exposure from each contributor is considered independently and then added, the total MPE situation can be evaluated. The fields from the broadcast transmitter act like a blanket covering the area. If the fields from a preexisting broadcast station create a level of 15% Occupational/Controlled MPE there is only 85% of the MPE budget remaining. This requires the levels from the two-way transmitters to be lower than what otherwise would be required to maintain compliance. In some conditions extra cooperation between the broadcasters and two-way licensees may be necessary to ensure site compliance. In the areas that receive grating lobes from the broadcast transmitters, careful measurements must be done before compliance can be analyzed. Consideration must be given to anyone working on antenna systems. If a person must climb into the fields of the broadcast antenna, coordination ahead of time must be done to reduce the transmitter power. Special consideration and care shall be utilized when a person is required to climb through a field known to exceed 100% Occupational/Controlled MPE. On some sites the broadcast towers are mounted adjacent to the two-way tower. In this situation the fields from the broadcast transmitter will be very intense on the two-way tower. Maintenance activities must be coordinated when the broadcast station is collocated. The FCC requires broadcasters to cooperate during maintenance situations; however, they may elect special times to conduct maintenance. A.3.4 LOCATION OF DIRECTIONAL ANTENNAS Directional antennas in the horizontal plane present a focused pattern for maximum coverage into a specific area. Even in the near field the levels in the beam of the antenna can be significantly higher than behind or on either side. Consideration must be given to the area and location the antenna is directed. Directional high-powered transmitting antennas should be located where the energy in excess of the Occupation/Control MPE is directed away from any area frequented by workers. Additionally directional antennas shall not be installed where they can produce fields higher than the General Population/Uncontrolled MPE in uncontrolled areas. A-16 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES ENGINEERING CONSIDERATIONS A.3.5 ANTENNA SELECTION Antenna selection is important because it is directly linked to EME levels. The requirement for more antennas within a given horizontal space has created new designs of antennas. Within one radome several antennas can now be stacked on top of each other. The standard configurations are double (two), triple (three) and Quad (four) co-linear arrays. Aperture length directly affects the power density created. In the near field, a 4.6 m (15 ft.) antenna driven with 500 Watts will have one-third the power density of an antenna 1.52 m (5 ft.) long. Remember that near the antenna, the power density is related to the surface area of a cylinder placed over the antenna. A cylinder having one-third the height will have one-third the surface area and, hence will result in three times the power density. This is complicated even more when the 1.52 m (5 ft.) antenna is placed with other antennas in a common radome. This allows the power density, created by each antenna, to combine and increase the potential exposure of an individual. The technique of using triple and quad antennas is becoming increasingly popular as the space on hilltops and towers becomes scarcer. Paging transmitters, sectored antenna systems, and digital networks represent only a few of the services requiring individual antennas. There is a finite antenna density that can be accomplished within a given area. Creative methods of combining or increasing the antenna structures must be developed. Consideration should be given to connecting lower power transmitters to the bottom portion of triple and quad radome antennas. A.3.6 MOUNTING DENSITY OF ANTENNAS While the RF fields from one antenna may be below the MPE allowed, the combination of fields from several antennas can produce levels exceeding the Occupation/Control MPE. This can be easily seen in Figure A-9 and Figure A-10 which show the fields produced by one antenna and the fields produced by five antennas mounted at roof level with all transmitters keyed simultaneously. 11.D-8 14.D-9 17.D-10 21.D-11 12.C-8 15.C-9 C 18.C-10 22 22.C-11 24.C-112 13.B-8 16.B-9 19.B-10 23.B-11 255.B-12 FIGURE A-9 COMPOSITE RF FIELDS WITH ONE ANTENNA TRANSMITTING 68P81089E50-B 9/1/05 A-17 27.D-13 28.C-13 29.B-13
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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 369 and 370: 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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