Towers for telecommunication

Communication Towers are structures used in establishing line of sight for Microwave or UHF links . When a microwave or any wireless link is established between two points physical structures of dimensions of the same order or larger as that of the wavelength of the signal acts as an obstruction. To minimize the effects of this obstruction two methods can be adopted

a) Use frequency with large wavelengths so that the obstructions are of insignificant dimension and so the signals can reach from the transmitters to the receiver. This method is adopted in the case of Vhf and HF ranges that is where the frequency is less than about 300 MHZ . Here however again the range is affected by the height of the antenna and varies as the square root of the height of the antenna. Therefore to increase the range of communication the height of antenna is to be increased and for this purpose tower structures are useful to lift the antenna at suitable height above the ground.

b) To locate the transmitting and the receiving antenna at a height above the structures coming between the receiving and the transmitting antenna that is to establish what is known as the line of sight. Selection of the height of the tower requires survey of topographical maps and also the identification of physical structures not indicated in the topographical maps. Topographical maps are available from Survey organizations .

a) Height designed to achieve required range as in the situation (a) above or to ensure line of sight as in case (b). The clearance in the case of case (b) is not restricted to the physical height of the obstruction and to the physical height the following factors have to be added

Clearance have to be given for the curvature of the Earth which is quite significant when we are considering distances of some 40 to 50 Kms.

Clearance for the physical space occupied by the electromagnetic energy which is concentrated mostly within what is known as the 1st Fresnel zone. . This clearance increases with reduction of frequency and so for lower frequencies more space will have to kept between the physical obstruction and the line of sight.

Click to see diagram depicting the conditions for realizing line of sight.

b) For high frequencies above 400 MHz the line of sight communication is used. In this case the antennas used are parabolic reflectors which can be gridpaks or simple plain reflectors. For frequencies unto about 3000 Mhz grid pak antennae can be used. These reflectors have to be aligned perfectly and the required accuracy is governed by the beam width ofd the antenna which typically is within 3 degrees and at high frequencies will be less than 1 degree. The higher the gain of the antenna the narrower is the beam width and vice versa. Higher antenna diameter allows higher gain and so communication over larger distances. The communication tower requires to ensure that due to wind pressure its sway and twist do not cause loss of line of sight due ot the beam transmitted being missed by the receiver when the beam goes by the side of the antenna due to twist and sway. The design of the tower structure therefore requires that it is sufficiently stable and has low twist and sway. Structural analysis of tower design through computers is done to determine the twist and sway of a tower. While doing such calculation the effective wind load of the antenna is to be considered.

Tower heights typically vary between 25 meters from ground level to about 100 meters. The weight of tower which depends on many factors typically vary between 5 mtonnes to 110 mtonnes. Tower costs can be typically US$ 1200 per m tonne

COMMUNICATION TOWERS