Antennas on Transmission Towers -

Co-location of telco antennas on POWERGRID's extra high voltage (EHV) transmission towers is being discussed for quite some time. The idea is to promote an infrastructure-sharing arrangement with telcos which would usher in a new stream of revenues for POWERTEL, offer a cost-effective solution to telcos, and at the same time pave the way for telecom penetration cheaply and conveniently, especially in hilly terrain and rural areas. Wireless communications providers have been using electric-utility transmission towers in extra high-voltage corridors throughout the world for co-locating their equipment, power supplies, and antennas. The choice of co-location with EHV transmission lines is obviously constrained by challenging engineering problems such as effects of additional loadings on towers, effects of EMI on radio equipment, and ground potential rise (GPR). Besides, safety of personnel and equipment is an issue of great importance that needs to be addressed with utmost care and priority. POWERGRID in association with Vodafone had been toying with an idea of such a co-location as a pilot project for quite some time. An ideal site was required to be selected which would allow co-hosting of several antennas on the available structure without compromising the safety issues, at the same time ensuring line-of-site for the intended customer base and the back-haul station and also availability of power. After a protracted survey, a suitable site was located very close to Gangtok 220/132 kV substation of POWERGRID near Gangtok city.

Engineering Analysis

As a general principle when adding any additional attachment on to a tower, a structural analysis must be performed to determine wind and ice (if applicable) loading and load-carrying capacity of the structure. BIS structural standards for steel antenna towers and antenna supporting structures are stringent and diminish the spare capacity for existing structures. All analysis needs to determine and document spare loading capacity on towers, so that co-location assignments can be handled more expeditiously and confidently at lower cost.

As a part of the planning and execution process, an engineering analysis was done in connection with the co-location proposal using generally the following steps:

1. Preparation of a single line diagram with the help of structural drawings of the selected tower type and showing the height/location of each antenna proposed to be placed on this tower along with the proposed lead-in connections.

2. Structural safety due to installation of antennas on the power transmission tower by analyzing the tower with and without the proposed antennas and associated additional loading as per latest code IS-802 (1995) "Use of Structural Steel in Overhead Transmission Line."

3. Radio Frequency Electromagnetic Emissions (RFEME) analyses are required to be conducted in the event the tower is used for co-location of several antennas by different service-providers. In the present case such an analysis was not required since the requirement was from one operator only. These analyses, along with adherence to best execution practices for tower climbing, and hardware retrofitting, alignment of antennas are essential for maintaining personnel safety.

Brief Site Data

Location: Gangtok 132/66 kV S/S

Transmission line: 132 kV D/C Gangtok-Rangit / Melli EHV

Type of tower: D+6 D/C (# 175,132)

Location of tower: Just outside gantry

Wind zone: 4

No of antennas: GSM 3 nos + Microwave 1 no

Positioning of antennas: Lower edge of GSM at height 6m from ground on 3 sides. MW antenna is about 0.5 m from the upper edge of one of the GSM antennas

Weight of the antennas: 2-3 kg each

Other Physical data: Attachment pole mount for each antenna is around 10-15 kgs. 20-25 m of outdoor type cable ladder was required to route RF/ IF cable

Schematic Arrangement

It may be noted that both the immediate western and eastern directions of the subject tower have D type towers. (Refer to Figure 1)

Details of work done:

In addition to antennas as described above, the following have been placed below the tower by Vodafone team within the confinement of 4 legs:

1. 1 Shelter (U-foam make: 1.2 x 2.2 x 3m) with 48 V 200AH battery bank (Amara Raja VRLA) and associated power plant including energy meter, distribution panel, change-over switch (AC to diesel if needed later on) etc. The indoor environment inside the shelter is non-A/C. Power supply is 3 phase 400 V through underground cable drawn from the substation.

2. BTS outdoor type (600 x 600 x 2000 mm) Nokia make (Ultra BTS) IP-41 compliance. The equipment is fan-cooled and it does not require any air-conditioning. The following photographs showing GSM and backhaul antennas on the tower and shelter plus BTS equipment may be referred to:

Special Precautions

Special precautions have been taken by POWERGRID and Vodafone to ensure the following:

Sufficient clearance, as defined by IE rules, between the power conductors and the top of the antennas has been ensured to take care of personnel safety and electrical induction on the installed system.
The original tower ground and the new Vodafone ground have been interconnected so as to ensure that no voltage difference between the two ground systems exists in case of earth-faults.
The BTS cabinet is EMC compatible so no problems are anticipated on EMI considerations. The physical dimensions of the antennas and fixtures and cable-feeder system to the antennas may have insignificant wind-load effects on the tower considering the type of tower and configuration of the conductors.

Customer Base

The system has been catering to Vodafone customers at Ranipul market in the adjacent valley near the Gangtok 132/66 kV substation and also covers part of the highway between Siliguri and Gangtok.

Observation and Monitoring

Since it was the first known instance of such co-location in India, the performance of the base station needs to be observed and monitored on a continuous basis. The same is being done from the Vodafone NoC at Siliguri for the last more than two months i.e., since dedicating the link for commercial operation by Vodafone. It is particularly important to observe the performance of the link during a condition of short-circuit / earth fault on the transmission line of which the subject transmission tower is a part.

Conclusion

In the light of various tasks performed during last week of November and detailed testing carried out on 1st to 6th December 2008 and witnessed by the Vodafone and POWERGRID teams on 6th December, it may be confirmed that the proposed objective was successful. If line of sight is available, sharing transmission towers for co-location of antennas is eminently feasible taking due cognizance of additional load margin available on the targeted tower(s), EMI/EMC and safety issues, integrated and full-proof earthing system. The other challenge is arranging a power supply for the BTS equipment. It is high time that the concept may be allowed to be emulated elsewhere.

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