Telecommunications Equipment .1 Background

Cellular telephone antenna sites have become increasingly difficult to obtain. This has made use of overhead transmission line structures preferred sites for use as antenna support structures. The structures are already built and the public more readily accepts alterations to existing structures rather than the installing new structures. Joint use also increases the value of the existing infrastructure and provides an income flow to the utility owner. This section evaluates the impact of installing antennas and associated equipment on transmission line structures.

Figure 7-1: Direct mount of telecommunications antenna to a double circuit 230 kV lattice tower between shield wire and top conductor arm. Note location of communication control building and

coax cable installed on cable ladder.

A minimum of three antennas located 120° apart are required for each carrier. There may be as many as four antennas located in each sector. Amplifiers may be required for each antenna. One or two coax cables are attached to each antenna and run down the structure to a control cabinet or enclosure which can be located on the ground (Figure 7-1 close to the tower or inside the main legs) or on a special platform. Service voltage electricity supply is necessary (usually supplied from an external source).

Figure 7-2: Worker on pipe extension to a double circuit 115 kV lattice steel tower with antenna mount platform

7.1.2 Considerations for Adding Telecommunications Equipment

i. The optimal elevation of the antennas is determined by the cellular engineer to obtain maximum signal coverage. Antennas can be mounted below or above phase conductors or on the top of a tower above the earth wire (if any). If the elevation is below the top of the structure, the antennas are mounted directly on the structure. If the elevation is above the top of the structure, an extension is designed to support the antennas at the required elevation (Figure 7-2);

ii. It is very important to perform a complete structural analysis of the tower, reviewing the added loads caused by wind and ice loads on the cell antennas;

iii. The review of electrical clearances (relative to safety distances) between the live parts (conductors, fittings, etc) and towers to permit the safe installation and maintenance of antennas without requiring live line procedures to be used;

iv. Where enclosures with electronic equipments, air conditioned equipments, etc should to be located? On the ground? On a platform installed on the tower?;

v. Installation of work platforms or ladders to facilitate the maintenance activities by telecommunications technicians;

vi. The source electricity source supply for cellular equipment and how to protect the lower voltage system against overvoltages and surges due to short circuits or lightning strikes;

vii. Supply electricity equipment can be fed from the low voltage network which is the most common case or from the medium voltage network;

viii. Environmental issues required by regional jurisdictions due to antenna is a source of EMF of high frequency and depending on a regulation their installation may require a special permission which is usually proceded by the environmental impact assessment;

ix. A formal contract or agreement between the asset owner and the telecommunications company is required. This agreement should take in considerations different issues such as:

a. Detailed responsibilities of each party;

b. Detailed design of cellular system and structural modifications to tower structures;

c. Safety procedures for employees of both companies including radiation exposure;

d. Detailed procedures for operating both companies systems taking each other’s equipment into consideration;

x. The safety of radio base stations (RBS) is an important issue. Lightning and earth potential rise must be taken into consideration during the engineering process. International Telecommunication Union (ITU) developed two recommendations in this field:

K56 (07/2003): Protection of radio base stations against lightning discharges;

K57 (09/2003): Protection measures for radio base stations sited on power line towers;

K56 provides a quantitative procedure in order to protect radio base stations for wireless access network against lightning discharges;

K57 specifies measures to be taken with respect to safety and risk of damage to equipment through the simultaneous effects of lightning stroke earth potential rise, when power line towers are used for locating radio base stations;

xi. Operators of overhead lines don’t usually own the land areas under transmission lines. These are the property of land owners. For a certain compensation the access to overhead lines is guaranteed, but this doesn’t usually include the right of leasing a tower to a third party for a business not connected with transmission of electrical power. Thus an agreement with a land owner can be necessary, in particular when a container is situated on a floor;

xii. Reengineering of the ground system of the existing tower or separate grounding system for antennas installation;

xiii. Special measures (if necessary) to protect the installation against vandalism.

7.1.3 Technical and Practical Limitations

i. Lack of lower voltage electricity supply for cellular equipment;

ii. Legal issues ( Licensing or environmental concerns);

iii. Location and height of a tower ( May not provide coverage for cell system);

iv. The presence of nesting endangered birds on the tower could dictate when the installation or the maintenance of antennas can take place.

7.1.4 Case Studies

Case Study 7-1 (USA) – Adding Cell Site Systems to Transmission Structures

Since the introduction of personal cell phones in the USA in the late 90’s, the use of existing transmission structures has been utilized for location of antennae. Typically, they have been located above the conductors due to the need for height. One arrangement is shown in Figure 1 (typical steel pole and tower). Another arrangement to achieve additional height is to use an auxiliary “pole extension”, which extends above the structure (Figure 2). In some cases, there is significant structural reinforcing to incorporate both types (as shown in Figure 2 for the tower). Evaluation of the structures is required, as are detailed engineering design plus specialized crews for construction

Figure 1

Figure 2

Case Study 7-2: (Poland) – Antennas Installation on a 400 kV Tower

A telecommunication operator was interested in installation of antennas on a 400 kV tower. The operator of a line accepted this location on a condition to locate antennas below the attachment points of phase conductors and in a distance of at least 3.5 meter from live parts. The control container was located on a height of 17 meter. The tower construction was checked for additional loads which resulted in local reinforcement of the tower body. Special platform to carry the container was designed as well as four triangular shaped smaller platforms to enable installation and maintenance of antennas.

The tower is typically equipped with climbing steps on one leg, but special ladder was provided for antennas operator staff. In the bottom part this ladder is protected (a key locked cover) from climbing by unwanted persons. Electricity supply is provided by the low voltage cable. The installation has its own grounding system.

7.2 Fibre Optics