Chapter#6 1 King Saud University College of Applied studies and Community Service 1301CT By: Nour Alhariqi

 The communication channel is the medium of transmission that provides the connection between the transmitter and the receiver.  It can be a physical or non-physical link  A communications channel moves electromagnetic energy from one or more source to one or more receiver. 2

 The channels can be classified as :  Analog Channels: These channels can carry analog signals. Examples: telephone system, commercial radio system  Digital Channels: These channels can carry digital signals. Example: computer communications 3

 Channels also can be classified as either bounded or unbounded.  In bounded (guided) channels, signals are confined to the medium and do not leave it. Examples: twisted pair, coaxial cable, and optical fiber  In unbounded (unguided) channels, the signals originated by the source travel freely into the medium and spread throughout the medium.  Unguided media employ an antenna for transmitting through air or water 4

 The three commonly used guided media are twisted pair, coaxial cable, and optical fiber  A twisted pair consists of two insulated copper wires twisted together in a helical form 5

 Coaxial cable consists of two conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor. 6

 Fiber optic cables: These cables carry the transmitted information in the form of a fluctuating beam of light in a glass fiber. 7

 An antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy.  For transmission of a signal, electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water).  For reception of a signal, electromagnetic energy impinging on the antenna is converted into electrical energy and fed into the receiver. 8

 Isotropic antenna radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center.  Omni-directional antenna radiates power in a circle.  Dish and Yagi are two common types of directional Antennas. 9

 A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight (LOS).  Ground waves: The signal follows the curvature of the earth’s surface  Sky waves: The signal bounces back and forth between the earth’s surface and the earth’s ionosphere (for the higher HF frequencies).  Because it depends on the Earth's ionosphere, it changes with the weather and time of day. 10

 Line of sight propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station.  It is limited by the curvature of the Earth for ground-based stations (100 km, from horizon to horizon).  To facilitate beyond-the-horizon propagation, satellite or terrestrial repeaters are used 11

 We will talk about two ranges of frequencies: microwave range and radio range 12

 Microwave signals are higher frequency signals.  Due to a higher frequency of operation, microwave systems carry large quantities of information.  It is highly directional so it follow line-of-sight (LOS) propagation.  The required antenna is smaller due to shorter wavelength (due to higher frequencies). Take note that the size of the antenna required to transmit a signal is proportional to the wavelength (λ) of the signal. 13

 Microwave is quite suitable for point-to-point transmission and it is also used for satellite communications.  Radio frequency is lower suitable for omnidirectional applications.  It follow ground or sky wave propagation 14

 In unguided channels, signals are not only transmitted directly from source to destination but also a lot of paths from source to destination by reflection, diffraction, …etc.  So the receiver receive multiple copies (components) of transmitted signal.  Line of sight (LOS) is the fastest component reaching to destination. 15

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