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Cover Page Preface Acknowledgement Contents Chapter 1 Chapter 2 Chapter 3
Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Conclusion

Chapter 2
Microwave Transmission & Towers

2.1 Microwave Transmission

MTT’s Microwave Transmission Network can be categorized into the following categories and during my training period I was able to familiarize with the planning, installation and maintenance of those systems.

Those Categories are:
1. Trunk Transmission

a)       SDH

b)       PDH

c)       MINI – LINK

2. Access Networks

Under the Access Networks, services are provided mainly using two types of systems namely

            1. Point to Multi Point Systems

                        Air Span System

                        Bosch System

            2. Point to point Systems 

            MDS Radios (Microwave Data Systems)

                        MDS 450D Radio and the MDS 400S Radio (LEDR)

            PECOM Radios

And accessories for these systems like

Switches, Cross Connectors, Multiplexes and other related devices.

2.2 A Brief History of Transmission Systems

In the early 1970s, digital transmission systems began to appear, utilizing a method known as Pulse Code Modulation (PCM), first proposed by STC in 1937. PCM allowed analog waveforms, such as the human voice, to be represented in binary form, and using this method it was possible to represent a standard 4 kHz analog telephone signal as a 64 kbit/s digital bit stream. Engineers saw the potential to produce more cost effective transmission systems by combining several PCM channels and transmitting them down the same copper twisted pair as had previously been occupied by a single analog signal.

In Europe, and subsequently in many other parts of the world, a standard TDM scheme was adopted whereby thirty 64 kbit/s channels were combined, together with two additional channels carrying control information, to produce a channel with a bit rate of 2.048 Mbit/s.

As demand for voice telephony increased, and levels of traffic in the network grew ever higher, it became clear that the standard 2 Mbit/s signal was not sufficient to cope with the traffic loads occurring in the trunk network. In order to avoid having to use excessively large numbers of 2 Mbit/s links, it was decided to create a further level of multiplexing. The standard adopted in Europe involved the combination of four 2 Mbit/s channels to produce a single 8 Mbit/s channel. This level of multiplexing differed slightly from the previous in that the incoming signals were combined one bit at a time instead of one byte at a time i.e. bit interleaving was used as opposed to byte interleaving. As the need arose, further levels of multiplexing were added to the standard at 34 Mbit/s, 140 Mbit/s, and 565 Mbit/s to produce a full hierarchy of bit rates.

2.3 Towers

Transmission Towers plays a major roll in the MTTs’ communication network. Self Supporting Towers (or the Free Stand Towers) and the Guy Mast Towers (or the Guy Wired Towers) are used in all the tower sites that belongs to MTT. MTT’s main towers and their heights are summarized below,

  1. Mulleriyawa                   Self Supported – Medium Duty – 90m
  2. Mahagalkanka               Guy Mast – 90m
  3. Hunnasgiriya                 Guy Mast – 90m  

Self Supported – 60m  

  1. Nayabedda                    Guy Mast – 66m 
  2. Gongala                        Guy Mast – 120m

Self Supported – Medium Duty – 60m

  1. Avissawella                   Guy Mast – 60m
  2. Kaluthara                      Guy Mast – 60m
  3. Galle                             Guy Mast – 60m 
  4. Mathara                        Self Supported – Medium Duty – 60m
  5. Hambanthota                 Guy Mast – 60m
  6. Aberfoyle                       Guy Mast – 60m
  7. Kochchikade                 Guy Mast – 60m
  8. Bingiriya                        Guy Mast – 60m
  9. Puttalum                       Guy Mast – 90m
  10. Dambulla                       Guy Mast – 75m
  11. Anuradhapura                Guy Mast – 90m
  12. Polonnaruwa                  Guy Mast – 60m
  13. Minuwangala                 Guy Mast – 45m
  14. Laggala                         Guy Mast – 60m
  15. Kurunagala                    A small tower is used, with the Celltel Tower
  16. Hatton                           Guy Mast – 60m
  17. Set Building                   Two small towers of each 6m in height

Even though different kind of towers are available in different heights, it is important to select the right tower according to the requirement. During the period of training I had the chance to climb one or two towers and came across many things about the tower loading, tower earthling and other related things. And I came across several installations of different kinds of antennas on the tower and had a good knowledge about different types of antenna brackets, mounting techniques and method of installing them on the tower

Selecting the Right Tower

 

 
 

 

 

 

 

 

 

   

Fig. 2.1 Towers

Cost, Technology of transmission and the area of coverage are the main factors, which influence the selection of tower.  
The mostly used towers are the

·         Self Supported towers and

·         Guy Mast Towers 

Self-Supporting Towers

A self-supporting tower is a free-standing space frame. It requires less land than a guyed tower and can accommodate more and larger antennas. Because of its bolted construction, it can easily be modified to increase antenna capacity. Because of these reasons a self-supporting tower is usually the best choice when microwave communication is needed in addition to cellular. Self - supporting towers can be provided as three-sided or four-sided structures.

A triangular tower is usually preferred over the square tower because it has fewer parts and the fourth side of a square tower does not add as much capacity to the tower as it does cost. A three-sided tower is lighter and more economical to erect and requires a less expensive foundation.

Three Self Supporting Tower Models are Available:

Light Duty – Heights to 90 feet

Medium Duty – Heights to 150 feet

Heavy Duty – Heights to 490 feet

Guyed Towers

Guyed towers are the most economical support systems available. Guyed towers will usually provide lower steel, foundation, and erection costs. They also have a larger range of heights. The guyed tower is composed of a central small-faced tower-mast, which is tethered and supported by guy wires. These guy cables are the main means of support for a guyed tower. The cables extend from the tower at designated pull-offs or torque stabilizers and are anchored into the ground at various distances away from the tower. The guy wires serve to stabilize the tower from twist and sway. To accomplish this, the guy wires are maintained at calculated tensions in relation to the length and diameter of the guy wire. Guyed towers can accommodate a variety of applications by simply increasing the face width and member size. A 36-inch (915 mm) face width tower can easily accommodate most cellular applications with cellular platforms, antennas, and transmission lines. A 46-inch (1170 mm) face width can carry a large range of microwave communications from high-performance antennas to horn antennas with access platforms. Towers of this sort can carry a combination of cellular and microwave communications. There are several factors to consider when determining what type of tower is best for the required application  Fig. 2.2 Guyed TowerLand Requirements..We have to have towers in rural areas and as well as in suburbs, therefore the amount of land used for the tower and the other buildings is a considerable thing. Guyed towers require more land than self-supporting towers. For example, a 250 ft (76 m) guyed tower may require more than four acres, while a 250 ft (76 m) self-supporting tower requires less than one acre. Cost Comparison...

Guyed towers typically cost less than self-supporting towers due to reduced steel usage. Most self-supporting foundations are more costly than guyed tower foundations. Erection time for guyed towers is generally less than for self-supporting towers.

Maintenance...

To keep the tower and the buildings in the good condition through out the lifetime, maintenance is needed. In general, self-supporting towers require slightly less maintenance. It is recommended that guyed towers undergo an annual inspection including a determination of guy wire tensions. Proper tensioning of guy wires ensures minimum deflection of antennas under extreme wind conditions.

Tower Specifications... Tower design must meet:

 Customer Specifications

  • Configuration
  • Height
  • Design Code or Standard (EIA/TIA-222-F)

Loading Requirements

  • Wind Speed and Ice Loading
  • Antennas
  • Ladders
  • Platforms

Operational Requirements

  • Limitations on Movement (Twist and Sway) to insure satisfactory performance of Microwave Antennas

Required Tower Design Information...

In order to tailor a tower product to the specific need, and to provide a tower structure that is the most efficient and economical solution to the particular requirement, certain information must be identified.
Required Information:

  • Tower type (guyed or self-supporting).
  • Tower height.
  • Any future increase in tower height.
  • Basic wind speed for design.
  • Exposure coefficient, Kz, and gust response factor, Gh, if other than recommended by  EIA/TIA-222-F.
  • Basic wind speed for rigidity.
  • Antenna types (manufacturers and models).
  • Antenna sizes.
  • Antenna angular deflection limits (twist and sway).
  • Initial and future antenna elevations, azimuths and operating frequencies.
  • Horizontal separation and sectorization angle (normally 120°) of cellular antennas.
  • Ice shields for initial and future antennas.
  • Initial and future waveguide and coaxial cable information.
  • Tower lighting and marking requirements.
  • Grounding (earthing) requirements.
  • Type of climbing facility and safety climb device required.
  • Tower base topography and details.
  • Soil reports for foundation designs.
  • Special documentation requirements.

 All of the above are the factors which have to  be taken into the design of the specific tower requirements.

Chapter 3 >>