Transverse electromagnetic (TEM) waves and also described how metallic wires can be used as a transmission medium to transfer TEM waves from one point to another. However, very often in electronic communication systems, it is impractical or impossible to interconnect two pieces of equipment is separated by large a metallic wire or cable. This is especially true when the equipment is separated by large spans of water , rugged mountains , or harsh desert terrain or when communicating with satellite transponders orbiting 22,300 miles above earth. also when the transmitters and receivers are mobile, as with two-way radio communications and cellular telephone, providing connections with metallic facilities is impossible. Therefore , earth s atmosphere is often used as a transmission medium.
Free-space propagation of electromagnetic waves is often called radio-frequency propagation or simply radio propagation. Although free space implies a vacuum propagation through earth s atmosphere is often referred to as free -space propagation and can often be treated as just that, the primary difference being that earth s atmosphere introduces losses and impairments to the signal that are not encountered in a vacuum. TEM waves will propagate through any dielectric material including air.TEM waves , however do not propagate well through lossy conductors, such as seawater, because the electric fields cause currents to flow in the material that rapidly dissipate the waves energy.
Radio waves are electromagnetic waves and like light propagate through free space in a straight line with a velocity of approximately 300,000,000 meters per second . Other forms of electromagnetic waves include infrared, ultraviolet, x rays and gamma rays. To propagate radio waves through earth s atmosphere , it is necessary that the energy be radiated from the source, then the energy must be captured at the receive end. Radiating and capturing energy are antenna functions and the properties of electromagnetic waves were explained The purpose of its behavior and optical properties of radio waves propagating through earth s atmosphere.
An electromagnetic waves is electrical energy that has escaped into free space. Electromagnetic waves travel in a straight line at approximately the speed of light and are made up of magnetic and electric fields that are at right angles to each other and at right angles to the direction of propagation. The essential properties of radio waves are frequency intensity dirction of travel, and place of polarization.
Radio waves are a form of electromagnetic radiation similar to light and heat. They deffer from these radiations in the manner in which they are generated and detected and in their frequency range . A radio waves consists of traveling electric and magnetic fields, with the energy evenly divided between the two types of fields.
The polarization of a plan electromagnetic waves is simply the orientation of the electric field vector in respect to the surface of the earth. If the polarization remains consists , it is described as linear polarization. Horizontal polarization and vertical polarization. If the electric field is propagating parallel to the earth s surface, the wave is said to be horizontally polarized. If the electric field id propagating perpendicular to the earth s surface, the wave is said to be vertically polarized. If the polarization vector rotates 360 degree as the waves moves one wave length through space and the field strength is equal to all angles of polarization, the waves is described as having circular polarization. When the field strength varies with changes in polarization, this is described as elliptical polarization .
A rotating waves can turn in either direction. If the vector rotates in a clockwise direction, it is right handed, and if the vector rotates in a counterclockwise direction , it is considered left handed. Electromagnetic waves are invisible ; therefore , they must be analyzed by indirect methods using methods. They concepts of rays and wavefronts are aids to the effects of electromagnetic wave propagation through free space . A ray is a drawn along the direction of electromagnetic wave propagation; however , it does not necessarily represent the propagation or a single electromagnetic wave.
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