standards and guidelines for communication sites - Radio And ...
standards and guidelines for communication sites - Radio And ... standards and guidelines for communication sites - Radio And ...
TOWER DESIGN AND CONSTRUCTION CHAPTER 2: SITE DESIGN AND DEVELOPMENT COAXIAL GROUNDING KITS SHELTER IN-LINE PROTECTOR 1 2 COAXIAL GROUNDING KITS SHELTER IN-LINE PROTECTOR 3 4 FIGURE 2-1 TRANSMISSION LINE ENTRY POINTS (IN ORDER OF PREFERENCE) NOTE: The point of entry for RF transmission lines is especially important at facilities that are normally occupied, such as dispatch centers, and at facilities that are located on above-ground platforms (usually for the purpose of flood control). See Figure 3-6 on page 3-9 for an example of a shelter located on an above-ground platform. 2.12.2.2 DISTANCE BETWEEN TOWER AND BUILDING COAXIAL GROUNDING KITS SHELTER COAXIAL GROUNDING KITS IN-LINE PROTECTOR SHELTER IN-LINE PROTECTOR Increasing the distance between the tower and building may reduce the susceptibility of a communications site to damage from lightning. Increasing the distance between the tower and building has the following benefits: • It decreases the magnetic field associated with lightning that is coupled into the building. The amount of magnetic field coupled into the building decreases by the square of the distance. For example, the magnetic field coupled into a building would decrease by a factor of nine (9) if the distance between the tower and building is increased by a factor of three (3). • It reduces the amount of energy that reaches the building via the RF transmission lines. This is because of the increase in inductance of the longer transmission lines. • It reduces the amount of lightning energy that is propagated through the earth from the tower grounding (earthing) electrode system to the building grounding electrode system. • Nine metres (30 feet) is considered a good compromise between protection level benefits and the length added to RF transmission line. 2-20 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES TOWER DESIGN AND CONSTRUCTION 2.12.2.3 METALLIC TELECOMMUNICATIONS/DATA LINES Whenever practical, metallic telecommunications/data lines should be eliminated from the facility. Metallic telecommunications/data lines provide a conductive path into the facility for lightning energy. Elimination of metallic telecommunications/data lines through the use of fiber optic cable (or other isolation device) provides isolation from lightning-induced ground potential rise (GPR) and lightning energy. 2.12.2.4 ISOLATION OF BUILDING AND TOWER FROM ICE BRIDGE/CABLE BRIDGE NOTE: Applies to tower and/or building-supported ice bridges and cable bridges. In order to reduce the amount of lightning energy diverted toward the equipment building/shelter, and to provide seismic isolation between the building and tower, it is recommended that the cable bridge/ice bridge be secured to the tower using a non-conductive slip-joint type device. When a slip-joint type device is used, grounding of the cable bridge/ice bridge shall be completed as described in “Cable Bridge/Ice Bridge Grounding (Earthing)” on page 4-71. 2.12.2.5 USE OF AIR TERMINALS ON TOWERS The use of air terminals (lightning rods) on a tower may be appropriate in some circumstances to protect antennas from a direct lightning strike. If the tower is over 45.7 m (150 ft.) tall, side-mounted antennas are vulnerable to direct lightning strikes. Side-mounted antennas installed at a height greater than 45.7 m (150 ft.) may be protected through the use of horizontal lightning rods. The horizontal lightning rods are attached to the tower, just above and below the antenna. PROTECTED AREA SIDE- MOUNTED ANTENNA PROTECTED AREA NON-PROTECTED AREA HORIZONTAL LIGHTNING RODS NON-PROTECTED AREA FIGURE 2-2 USING HORIZONTAL LIGHTNING RODS TO PROTECT SIDE-MOUNTED ANTENNA 68P81089E50-B 9/1/05 2-21
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STANDARDS AND GUIDELINES FOR COMMUNICATION SITES TOWER DESIGN AND CONSTRUCTION<br />
2.12.2.3 METALLIC TELECOMMUNICATIONS/DATA LINES<br />
Whenever practical, metallic tele<strong>communication</strong>s/data lines should be eliminated from the facility.<br />
Metallic tele<strong>communication</strong>s/data lines provide a conductive path into the facility <strong>for</strong> lightning energy.<br />
Elimination of metallic tele<strong>communication</strong>s/data lines through the use of fiber optic cable (or other<br />
isolation device) provides isolation from lightning-induced ground potential rise (GPR) <strong>and</strong> lightning<br />
energy.<br />
2.12.2.4 ISOLATION OF BUILDING AND TOWER FROM ICE BRIDGE/CABLE BRIDGE<br />
NOTE: Applies to tower <strong>and</strong>/or building-supported ice bridges <strong>and</strong> cable bridges.<br />
In order to reduce the amount of lightning energy diverted toward the equipment building/shelter, <strong>and</strong> to<br />
provide seismic isolation between the building <strong>and</strong> tower, it is recommended that the cable bridge/ice<br />
bridge be secured to the tower using a non-conductive slip-joint type device. When a slip-joint type<br />
device is used, grounding of the cable bridge/ice bridge shall be completed as described in “Cable<br />
Bridge/Ice Bridge Grounding (Earthing)” on page 4-71.<br />
2.12.2.5 USE OF AIR TERMINALS ON TOWERS<br />
The use of air terminals (lightning rods) on a tower may be appropriate in some circumstances to protect<br />
antennas from a direct lightning strike. If the tower is over 45.7 m (150 ft.) tall, side-mounted antennas<br />
are vulnerable to direct lightning strikes. Side-mounted antennas installed at a height greater than<br />
45.7 m (150 ft.) may be protected through the use of horizontal lightning rods. The horizontal lightning<br />
rods are attached to the tower, just above <strong>and</strong> below the antenna.<br />
PROTECTED<br />
AREA<br />
SIDE-<br />
MOUNTED<br />
ANTENNA<br />
PROTECTED<br />
AREA<br />
NON-PROTECTED AREA<br />
HORIZONTAL<br />
LIGHTNING RODS<br />
NON-PROTECTED AREA<br />
FIGURE 2-2 USING HORIZONTAL LIGHTNING RODS TO PROTECT SIDE-MOUNTED ANTENNA<br />
68P81089E50-B 9/1/05 2-21
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