Antennas are an essential part of modern communication systems, used to transmit and receive radio frequency signals. These antennas require a support structure, such as towers, to ensure they are at the correct height for optimal transmission and reception. However, the installation of these structures presents unique electrical protection considerations, which must be taken into account to ensure safety.
In this article, we will discuss the different types of antenna support structures, their installation, and the electrical protection considerations that must be considered.
Antenna Locations
Antennas can be mounted on either wood or metal supports. If mounted on a building, the antenna support structure (tower) must not damage the structural integrity of the building or void the warranty of the roof. Occasionally, the antenna might be mounted on the side wall of a building or on a parapet wall using special braces or supports.
Types of Support Structures
The typical antenna support structures include guyed metallic, self-supporting metallic, and wooden structures (poles). These towers can be installed at grade level (surface), on building roofs, or side walls.
Guyed Metallic Towers
Guyed metallic towers are structures with upright support members (legs) mounted on a foundation or pier that require multiple anchors and down guys. These structures provide an economical solution for supporting tall antennas.
Self-Supporting Metallic Towers
Self-supporting metallic towers are free-standing structures with upright support members (legs) mounted on a foundation or pier that need no other supporting elements. These structures provide excellent stability and can support taller antennas.
Wooden Structures (Poles)
Wooden structures, also known as poles, are either free-standing or guyed structures either mounted on a foundation or partially buried. These structures are often used in areas with environmental regulations that prohibit the use of metallic towers.
Electrical Protection Considerations
Antennas and their supporting structures are at risk of being struck by lightning, which can cause significant damage. To mitigate this risk, proper electrical protection measures must be implemented. These measures include grounding the tower and antenna, installing lightning rods or air terminals, and surge protection devices.
The presence of an antenna and its supporting structure on the roof of a building is not expected to significantly increase the probability of lightning striking the building. However, if lightning does strike, the antenna and its supporting tower may be the focal point of the strike. Therefore, it is essential to ensure that the tower and antenna are properly grounded to protect against lightning strikes.
The Importance of Proper Grounding for Towers and Antennas
Towers and antennas are crucial components of any communication system. They enable the transmission and reception of signals over long distances. However, they are also highly susceptible to electrical surges, such as lightning strikes. Therefore, it is important to ensure that towers and antennas are properly grounded to prevent damage to the equipment and ensure the safety of personnel.
Grounding Requirements for Antenna Support Structures (Towers)
Antenna support structures, or towers, can be made of wood or metal and installed at grade level, on building roofs, or side walls. There are three types of typical antenna support structures: guyed metallic, self-supporting metallic, and wooden structures (poles). Regardless of the type of tower, proper grounding is essential to protect the equipment and personnel.
Grounding Electrode System
All towers and satellite dish mountings should have a grounding electrode system consisting of a ground ring and ground rods. A #2 AWG solid, bare copper (SBC) conductor should be buried around the tower at a minimum depth of 760 mm (30 in) below grade, and at least 610 mm (24 in) from the base of the tower or mounting. The ends of the #2 AWG conductor should be connected using exothermic welding or a listed connector to form a continuous ring.
Ground Rods
Driven ground rods minimum 2.5 m (8 ft) in length and 16 mm (5/8 in) diameter should be placed at the base of each tower leg, a minimum of 460 mm (18 in) below grade. The connection of the ground rods to the ground ring should be accomplished using exothermic welding or listed connectors. Ground rods should be copper, copper-clad steel, stainless steel, or zinc-clad steel.
Bonding Conductor
The base of each tower leg should be bonded to the ground ring with a #2 AWG conductor, which should be as short and straight as possible. The metallic frame supporting a satellite dish should also be bonded to the ground ring with a #2 AWG conductor.
Monopole or Guyed Towers
For monopole or guyed towers, there should be at least two ground rods, driven a minimum of 25 mm (1 in) below grade, equally spaced, and connected to the ground ring using exothermic welding or listed connectors. Grounds of monopole tower mounted antennas should be connected to the ground ring with a #2 AWG SBC conductor using exothermic welding or listed connectors.
Equipment Building Ground Ring
The tower ground ring should be bonded to the equipment building ground ring with at least two #2 AWG SBC conductors buried at a minimum depth of 760 mm (30 in) between the tower and building ground rings.
Metal Fence
If there is a metal fence within 1.8 m (6 ft) of the tower, the tower ground ring should be bonded to the fence with a #2 AWG SBC conductor.
Proper grounding is crucial to ensure the safety of personnel and prevent damage to equipment from electrical surges. The requirements for grounding antenna support structures, or towers, include a grounding electrode system, ground rods, bonding conductors, and connection to the equipment building ground ring or metal fence. Adhering to these guidelines will ensure that towers and antennas function efficiently and safely.
Understanding the Importance of Down Conductors and Radial Grounding Conductors in Antenna Sites
Antenna sites are critical communication infrastructures that require proper grounding and lightning protection to ensure their safe and reliable operation. Proper grounding prevents damage to equipment and reduces the risk of injury or death due to electrical hazards. Among the essential components of grounding systems are down conductors and radial grounding conductors, which play a crucial role in dissipating lightning strikes and other electrical surges.
Down Conductors
Down conductors are conductive pathways that connect the roof ground ring of a tower or antenna mast to a buried ground ring or grounding rods. They serve as the primary means of dissipating lightning strikes and other electrical surges from the structure to the ground. The J-STD-607-A standard recommends that all roof-mounted towers or antenna masts should have at least two down conductors, each from opposite sides of the roof ground ring down the building wall.
In addition, the standard mandates the use of additional down conductors for every 30 meters (100 feet) of building length. This is in addition to the down conductors used in the lightning protection system, which is necessary to provide a separate path for lightning strikes that may occur near the structure.
Roof Conductors
Roof conductors are the metallic pathways that run along the roof of the building and connect to the down conductors. They help ensure the safe dissipation of lightning strikes and other electrical surges by providing a continuous path for current flow from the roof ground ring to the down conductors. The J-STD-607-A standard recommends supporting roof conductors every 1 meter (3 feet) using either no-nail paste down cable fasteners or pan-type base ridge cable supporters.
Radial Grounding Conductors
Radial grounding conductors are necessary when ground rods cannot be driven or drilled, making it impossible to create a buried ground ring. In such cases, a network of buried grounding conductors should be provided radiating from each base of the antenna and building corner and connecting to the ground rings.
The J-STD-607-A standard mandates the use of a #2 AWG SBC conductor connected to the ground ring and buried at least 760 millimeters (30 inches) below grade level. If adequate contact cannot be reached with the soil at 7.6 meters (25 feet), each radial wire should be extended as far as necessary to obtain the desired resistance. The conductor length should not be less than 7.6 meters (25 feet).
Low resistance in radial grounding arrangements is desirable but not essential. Low resistance in the dissipating path of strike currents into the earth is of secondary importance compared to the major objective of controlling voltage gradients and voltage differences between structures close to the radio equipment.
Understanding Waveguide and Coaxial Cable Grounding Requirements
In the world of telecommunications and broadcasting, waveguide and coaxial cables play a vital role in transmitting signals from one point to another. However, these cables are also susceptible to lightning strikes, which can cause damage to the equipment and endanger the lives of people in the vicinity. To prevent such mishaps, it is essential to understand the grounding requirements for waveguide and coaxial cables.
Bonding to the Tower
One of the primary requirements for grounding waveguide and coaxial cables is bonding them to the tower. The shields of these cables should be bonded to the tower at the top and bottom of the structure. If the tower is more than 60 meters (200 feet) tall, the bonding should also be done at the midpoint or every 15 meters (50 feet). This helps to prevent any potential difference that may exist between the cable and the tower.
Grounding to the Building
When the waveguide or coaxial cable enters the building, it should be bonded to the building’s external grounding electrode system using a #2 AWG conductor. Once inside the building, the cable’s shield should be bonded to the building’s interior grounding electrode system as close as possible to the entrance. This helps to prevent any potential difference that may exist between the cable and the building.
Metallic Entrance Plate
If there is a metallic waveguide or coaxial cable entrance plate, it should be bonded to both the exterior and interior grounding system with a #2 AWG conductor. The waveguide or coaxial cable shield should be bonded to the metallic entrance plate on both the outside and inside of the building with a #2 AWG conductor. This helps to provide an effective grounding path and reduces the potential for damage due to lightning strikes.
Lightning Surge Arrester
The coaxial cable should be protected by a lightning surge arrester, which is bonded to the exterior grounding electrode system with the proper size grounding conductor specified by the manufacturer. This helps to provide additional protection to the equipment by diverting the lightning surge to the ground.
Conduit for Lighting Power Conductors
If the tower is lighted, the conduit for the lighting power conductors should be bonded and grounded as described for waveguide and coaxial cable shields. This helps to ensure that any potential difference between the lighting power conductors and the tower is eliminated, thereby providing additional protection to the equipment.