Quiz 18: Terrestrial Communication Systems

Trade & Technology

Fibre Optic link: It is used for communication purpose as it helps in transmitting information over a distance. It is mainly used where wired line connectivity and higher data carrying capacity is required. Microwave Link: It is also used for communication purpose but it transmits information in the form of radio waves. It is wireless network and requires point to point connectivity. Following is the comparison between microwave and fibre optic links: img

Repeater is a combinational circuit of transmitter and receiver that performs the signal amplification followed by signal retransmission. Generally, the microwave systems require the relays at intervals of 40 km. Due to this advantage these systems are installed in long-haul difficult terrain, like mountainous areas very easily. These terrestrial microwave systems use either single link (one hop) or multi links (more hops) for signal transmission. Use Figure 18.1 in the textbook to observe the single link and multi-link systems. The multi-link uses repeaters as shown in 18.1(b) in the textbook. Repeaters are needed for terrestrial system in order to extend the signal transmission beyond the line of sight propagation range of a single link. Usage of repeaters increases the damage of circuit components that disable the link. Hence, beyond the line of sight range the repeater stations are not located from one after other. All these practical limitations avoid certain sites for repeaters installation. Whereas the inspected sites such as high points of the terrain allows repeater installation. And also eliminates limitations of repeaters in these areas. The terrestrial systems prefers low power transmitters, either parabolic or hog horn antennas with high gain. These antennas use narrow beam for power transmission, which rises the effective power and decrease the interference generated in the systems that are used for signal propagation. so repeaters are used in high terrain areas for multi-link propagation systems. Therefore, due to consideration of all factors repeaters are needed in terrestrial microwave systems.

Line of Sight propagation is achieved when there can be direct radiation from the transmitting (source) antenna to the receiving (destination) antenna. However, when radio wave propagates from source to destination then it undergoes reflection, refraction, diffraction and absorption from the various obstacles/Objects present nearby. As compare to visible light, Radio waves have the longest wavelength in the electromagnetic spectrum, due to which they suffer more signal loss from the medium which they pass. Whereas visible light is the light which is visible to our eyes and it can be used by the transmitter and receiver efficiently, when they are in the line of sight. In vacuum both radio wave and Visible light travel with the speed of light ( img ). Now in normal conditions, when Radio wave and visible light travel then the radio wave travel about one-third farther than visible light. The reason of this can be as radio waves have longer wavelengths, so they get reflected by the ionosphere, due to which there are continuous bounces between ground and the ionosphere, that is, the radio wave essentially skip around the earth. As a result of which they propagate for great distance. Whereas visible light has shorter wavelength, so they aren't reflected efficiently by the ionosphere, as a result of which there energy dissipates over distance.

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