standards and guidelines for communication sites - Radio And ...
standards and guidelines for communication sites - Radio And ... standards and guidelines for communication sites - Radio And ...
ENVIRONMENTAL EVALUATION APPENDIX A: ELECTROMAGNETIC ENERGY INFORMATION A.2.4.3 RED ZONE The red zone is any area where the spatial-averaged levels fall above 100% of Occupational/Controlled MPE. When locations are found to require red zoning, special procedures, engineering, or restricted access must be implemented to ensure compliance. Some procedures that can be implemented are: • Restrict Access • Lock-out/Tag-out of transmitters during maintenance of antenna system • Control of antenna types used for site design • Re-engineer site to reduce EME fields • Measure and consider uptime. (“Uptime” is the percentage that a transmitter will likely be keyed. See “Uptime,” on page A-18 for detailed information.) A.2.5 CHARACTERIZATION ZONING The level of RF energy to which one is exposed is called Exposure. The quantity of exposure depends on the duration and strength of the field. In most cases, the characteristics of a site will determine the EME exposure potential. Understanding these characteristics will aid in predicting and preventing levels that exceed the FCC Guidelines and allow the site manager to establish the proper procedures for personnel who frequent these areas. A.2.5.1 BUILDINGS Building sites are normally in dense, metropolitan cities. The buildings used are normally the highest structures in the city and offer the unique opportunity of height without the need for a long feedline. The facility which houses the radio transmitters is normally close to the antennas which reduces the loss between the antenna and transmitter, allowing maximum power to the antenna. While this maximum power provides extended range, it increases the EME levels around the antennas. The main determinants of EME are frequency, power into the antenna, and aperture height. The greater the power, the higher the EME field. The shorter the aperture, the higher the EME field for a given power. On buildings, the antennas are generally mounted on the roof. This mounting arrangement is normally laid out on a single plane and distributed in a grid arrangement, within the confines of the roof. The mounting is normally on a pipe structure and the separation can be as close as 914 mm (3 ft.) in some cases. This arrangement provides for maximum mounting density, but it may leave little space for the workers performing maintenance. Any worker attempting to change an antenna, repair a cable, or perform general maintenance may be exposed to high levels of RF energy from other antennas surrounding the work area. Proper engineering design should be used to prevent this situation. By reducing all the fields on a building the potential for high exposure is eliminated and provides the best compliance resolution. A-12 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES ENVIRONMENTAL EVALUATION A.2.5.2 TOWERS Towers are antenna supporting structures that can be found in various locations ranging from central metropolitan, to isolated rural locations. Normally, the towers are designed to elevate the antennas in accordance with the intended coverage area. This can vary from 30.5 m (100 ft.) for cellular to 610 m (2000 ft.) for two-way communications. Regardless of the height of the supporting structure, the characteristics are the same. The application of the antennas that are being supported determine these characteristics. Cellular towers usually have directional antennas mounted on a single face to define a sector. There may be several faces and several directional antennas per face. A two-way tower can have several antennas mounted in a star configuration to maximize the density of antennas at a position on the tower. Additionally there can be several star mounts on a single tower. With respect to EME, the cellular configuration presents less exposure to people working on the tower than the two-way tower configuration because the RF radiation of the directional antenna is aimed away from the tower. There is a significant power difference between the front and the back side of the antenna. This difference is called front-to-back ratio. While the front-to-back ratio can be as great as 25 dB in the far field it is less well developed in the near field. There is still reduction of the exposure of the worker in the near field behind, as compared to the front of the antenna, but the amount may be considerably less than the advertised far field front-to-back ratio. The situation on two-way towers is significantly different. As workers climb up the tower they may encounter several antenna mounts at various locations on the tower. These mounting areas can contain various types of transmitters ranging from paging transmitters with hundreds of watts of power to large antennas for transmitters in the 35 MHz frequency range. While the antennas and the resulting mounting arrangement can be considerably different, in some conditions the EME levels may approach or exceed the FCC guidelines. In the case of the paging transmitter, the antenna will normally be an omni configuration with an aperture length of 1.2 m to 4.6 m (4 to 15 ft.). The antenna will be mounted from 1.2 m to 1.8 m (4 to 6 ft.) from the tower. Fields directly adjacent to the aperture will present the highest levels. Because of this, workers shall use caution while working or stopping directly in front of these antennas unless the transmitters are deactivated. If the antenna is grouped with other antennas at the same level more than one transmitter may need to be deactivated. Another important characteristic of paging is the duty cycle of the transmitter. The importance of duty cycle will be discussed in detail later. 3G G3 6E6 4GG6 5F6 FIVE TRANSMITTING ANTENNAS 9D7 3G3 3G FOUR TRANSMITTING ANTENNAS THREE TRANSMITTING ANTENNAS TWO TRANSMITTING ANTENNAS FIGURE A-7 EME ZONE MAP OF A TOWER MOUNTED STAR CLUSTER MOUNT OR CANDELABRA (RESOLUTION 1 SQ. FT.) 68P81089E50-B 9/1/05 A-13 6E6 E6 4G6 4G 5F6 5F 9D7 9D 3G3 6E6 4GG6 5F66 9D77 3G3 6E6 4G6 5F F6 9D7
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ENVIRONMENTAL EVALUATION APPENDIX A: ELECTROMAGNETIC ENERGY INFORMATION<br />
A.2.4.3 RED ZONE<br />
The red zone is any area where the spatial-averaged levels fall above 100% of Occupational/Controlled<br />
MPE. When locations are found to require red zoning, special procedures, engineering, or restricted<br />
access must be implemented to ensure compliance. Some procedures that can be implemented are:<br />
• Restrict Access<br />
• Lock-out/Tag-out of transmitters during maintenance of antenna system<br />
• Control of antenna types used <strong>for</strong> site design<br />
• Re-engineer site to reduce EME fields<br />
• Measure <strong>and</strong> consider uptime. (“Uptime” is the percentage that a transmitter will likely be keyed.<br />
See “Uptime,” on page A-18 <strong>for</strong> detailed in<strong>for</strong>mation.)<br />
A.2.5 CHARACTERIZATION ZONING<br />
The level of RF energy to which one is exposed is called Exposure. The quantity of exposure depends<br />
on the duration <strong>and</strong> strength of the field. In most cases, the characteristics of a site will determine the<br />
EME exposure potential. Underst<strong>and</strong>ing these characteristics will aid in predicting <strong>and</strong> preventing<br />
levels that exceed the FCC Guidelines <strong>and</strong> allow the site manager to establish the proper procedures <strong>for</strong><br />
personnel who frequent these areas.<br />
A.2.5.1 BUILDINGS<br />
Building <strong>sites</strong> are normally in dense, metropolitan cities. The buildings used are normally the highest<br />
structures in the city <strong>and</strong> offer the unique opportunity of height without the need <strong>for</strong> a long feedline. The<br />
facility which houses the radio transmitters is normally close to the antennas which reduces the loss<br />
between the antenna <strong>and</strong> transmitter, allowing maximum power to the antenna. While this maximum<br />
power provides extended range, it increases the EME levels around the antennas. The main<br />
determinants of EME are frequency, power into the antenna, <strong>and</strong> aperture height. The greater the power,<br />
the higher the EME field. The shorter the aperture, the higher the EME field <strong>for</strong> a given power.<br />
On buildings, the antennas are generally mounted on the roof. This mounting arrangement is normally<br />
laid out on a single plane <strong>and</strong> distributed in a grid arrangement, within the confines of the roof. The<br />
mounting is normally on a pipe structure <strong>and</strong> the separation can be as close as 914 mm (3 ft.) in some<br />
cases. This arrangement provides <strong>for</strong> maximum mounting density, but it may leave little space <strong>for</strong> the<br />
workers per<strong>for</strong>ming maintenance. Any worker attempting to change an antenna, repair a cable, or<br />
per<strong>for</strong>m general maintenance may be exposed to high levels of RF energy from other antennas<br />
surrounding the work area. Proper engineering design should be used to prevent this situation. By<br />
reducing all the fields on a building the potential <strong>for</strong> high exposure is eliminated <strong>and</strong> provides the best<br />
compliance resolution.<br />
A-12 68P81089E50-B 9/1/05
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