Comms3 PDF - Electromagnetic Wave Propagation

Transmission line - Metallic conductor system used to Propagation Constant – used to express the attenuation transfer electrical energy from one point to another and the phase shift per unit length of a transmission line. using electrical current flow....

Transmission line - Metallic conductor system used to Propagation Constant – used to express the attenuation transfer electrical energy from one point to another and the phase shift per unit length of a transmission line. using electrical current flow. Velocity factor – the ratio of the actual velocity of Transverse Electromagnetic Wave (TEM) – propagates propagation of an electromagnetic wave through a given primarily in the non-conductor (dielectric) that separates medium to the velocity of propagation though a vacuum. the two conductors of the transmission line. Dielectric Constant – the relative permittivity of a Wave Velocity - the speed at which a wave travels material. through a medium, carrying energy from one place to Electrical Length – used extensively for transmission-line another without permanently displacing the medium calculations and antenna design. itself. It depends on the properties of the medium, such as its density, elasticity, and temperature. Five types of transmission line losses Frequency – the rate at which the periodic wave repeats. Conductor loss - inherent unavoidable loss Wavelength – the distance of one cycle occurring in Radiation loss - directly proportional to frequency space. Dielectric heating loss - difference of potential between two conductors of a metallic transmission Single-ended or Unbalanced Transmission Lines – one line wire is at ground potential, whereas the other wire is at Coupling loss - occurs whenever a connection is made signal potential. to or from a transmission line or when two sections of transmission line are connected together. Balanced Transmission Lines – with two-wire balanced lines, both conductors carry current; however, one Corona - a luminous discharge that occurs between conductor carries the signal, and the other conductor is the two conductors of a transmission line when the the return path. difference of potential between them exceeds the breakdown voltage of a dielectric insulator. Open-wire Transmission Line – two-wire parallel conductors. Consists simply of two parallel wires, closely Incident wave - voltage that propagates from the source spaced and separated by air. toward the load. Twin-lead Transmission Line - same as open-wire Reflected wave - voltage that propagates from the load transmission line except that the spacers between the toward the source two conductors are replaced with a continuous solid Resonant Transmission Line - energy is alternatively dielectric that ensures uniform spacing along the entire transferred back and forth between the magnetic and cable. electric fields of the distributed inductance and Twisted-wire pair Transmission Line – formed by capacitance of the line. twisting two insulated conductors around each other. Flat or Nonresonant Transmission Line - transmission Often stranded in units. line with no reflected power. Shielded-cable Transmission Line – is a parallel two-wire Reflection coefficient – vector quantity that represents transmission line consisting two copper conductors the ratio of reflected voltage to incident voltage or separated by a solid dielectric material reflected current to incident current. Concentric Transmission Line – often used for high data Standing waves – two travelling waves set up an transmission rates to reduce losses and isolate interference pattern transmission paths Standing-wave ratio – the ratio of the maximum voltage Characteristic Impedance - is a complex quantity that is to the minimum voltage or the maximum current to the expressed in ohms, ideally independent of line length, minimum current of a standing wave on a transmission and cannot be directly measured. line. BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi CHAPTER 14 ELECTROMAGNETIC WAVE PROPAGATION # DEFINITIONS TERMS Propagation of electromagnetic waves often called 1) radio-frequency (RF) propagation or simply radio Free-space propagation. 2) Electrical energy that has escaped into free space. Electromagnetic wave The orientation of the electric field vector in respect 3) Polarization to the surface of the Earth. 4) Polarization remains constant Linear Polarization Horizontal Polarization 5) Forms of Linear polarization and Vertical Polarization Polarization vector rotates 360◦ as the wave moves 6) one wave-length through the space and the field Circular Polarization strength is equal at all angles of polarization. 7) Field strength varies with changes in polarization. Elliptical Polarization Used to show the relative direction of 8) Rays electromagnetic wave propagation. Formed when two points of equal phase on rays 9) Wavefront propagated from the same source are joined together. A single location from which rays propagate equally 10) Point source in all directions. 11) Invisible force field produced by a magnet, such as a Magnetic Field conductor when current is flowing through. Prepared By : MA. ELAINE L. CORTEZ 86 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi Strength of a magnetic field (H) produced around a 12) conductor is expressed mathematically as: Invisible force fields produced by a difference in 13) Electric fields voltage potential between two conductors. Electric filed strength (E) is expressed mathematically 14) 2 as: Dielectric constant of the material separating the two 15) Permittivity conductors. 16) The permittivity of air or free space is approximately. 8.85 x 10 -12 F/m The rate at which energy passes through a given 17) Power density surface area in free space. Intensity of the electric and magnetic fields of an 18) Field intensity electromagnetic wave propagating in free space. Mathematically power density is expressed as: 19) P = €H W/m2 The characteristic impedance of a lossless transmission medium is equal to the square 1/2 20) Zs = (µo 0) root of the ratio of its magnetic permeability to its electric permittivity. Point source that radiates power at a constant rate 21) Isotropic radiator uniformly in all directions. Power density is inversely proportional to the square of 22) Inverse Square Law the distance from the source. Prepared By : MA. ELAINE L. CORTEZ 87 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi 23) Propagation medium. Isotropic medium Waves propagate through free space, they spread 24) Attenuation out, resulting in a reduction in power density. 25) Reduction of Power. Absorption Loss Reduction in power density with distance is equivalent 26) Wave attenuation to a power loss. 27) Spherical spreading of the wave. Space attenuation 28) One with uniform properties throughout. Homogeneous medium Absorption coefficient varies considerably with 29) location, thus creating a difficult problem for radio Inhomogeneous medium systems engineers. Refraction, Reflection, 30) Optical properties of Radio Waves. Diffraction and Interference 31) Bending of the radio wave path. Refraction Square root of the dielectric constant and is 32) Refractive index; n = (k) expressed in: (k) Equivalent dielectric constant relative to free 33) space (vacuum). K = (1- 81N/f2)1/2 Boundary between two media with different densities. 34) Plane Prepared By : MA. ELAINE L. CORTEZ 88 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi Imaginary line drawn perpendicular to the interface 35) Normal at the point of incidence. Angle formed between the incident wave and the 36) Angle of Incidence normal. Angle formed between the refracted wave and the 37) Angle of Refraction normal. Ratio of velocity of propagation of a light ray in free 38) space to the velocity of propagation of a light ray in Refractive Index a given material. Perpendicular to the direction of propagation 39) Density gradient (parallel to the waveform) 40) To cast or turn back. Reflect Ratio of the reflected to the incident voltage 42) Reflection Coefficient intensities. Portion of the total incident power that is not Power transmission 43) reflected. coefficient 44) Fraction of power that penetrates medium 2. Absorption coefficient Incident wave front strikes an irregular surface, it is 45) Diffuse reflection randomly scattered in many directions. Specular (mirrorlike) 46) Reflection from a perfectly smooth surface. reflection 47) Semirough surfaces Surfaces that fall between smooth and irregular. Prepared By : MA. ELAINE L. CORTEZ 89 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi Semirough surface will reflect as if it were a smooth surface whenever the cosine of the angle of Rayleigh criterion 48) incidence is greater than λ/8d, where d is the depth of the surface irregularity and λ is the wavelength of Cos θi > λ/8d the incident wave. Modulation or redistribution of energy within a 49) wavefront when it passes near the edge of an Diffraction opaque object. Diffraction occurs around the edge of the obstacle, 51) Shadow zone which allows secondary waves to “sneak” around the corner of the obstacle. States that the total voltage 54) intensity at a given point in space is the sum of the Linear Superposition individual wave vectors. Electromagnetic waves travelling within Earth’s 52) Terrestial waves atmosphere. Communications between two or more points on Terrestial radio 53) Earth. communications 54) Used for high-frequency applications. Sky waves Earth –guided electromagnetic wave that travels 55) Surface wave over the surface of earth. Relative Surface Conductivity Seawater Good Flat, loamy soil Fair Relative Conductivity of Earth Surfaces Large bodies of 56) Fair freshwater Rocky terrain Poor Desert Poor Jungle Unusable Prepared By : MA. ELAINE L. CORTEZ 90 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi 1. Ground waves require a relatively transmission power. 2. Ground waves are limited to very low, low, and medium frequencies. 57) Disadvantages of surface waves. 3. Requiring large antennas. 4. Ground losses vary considerably with surface material and composition. 1. Given enough transmit power, ground waves can be used to communicate between any two locations in the 58) Advantages of ground wave propagation. world. 2. Ground waves are relatively unaffected by changing atmospheric conditions. Travel essentially in a straight line between the 59) Direct waves transmit and receive antennas. Line-of-Sight (LOS) 60) Space wave propagation with direct waves. transmission The curvature of Earth presents a horizon to space 61) Radio Horizon wave propagation. Occurs when the density of the lower atmosphere is 62) such that electromagnetic waves are trapped Duct propagation between it and Earth’s surface. Lowest layer of the ionosphere and is located 63) D Layer approximately between 30 miles and 60 miles (50 km to 100 km) above Earth’s surface. Prepared By : MA. ELAINE L. CORTEZ 91 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi Located approximately between 60 miles and 85 64) E Layer miles (100 km to 140 km) above Earth’s surface. 65) The upper portion of the E layer. Sporadic E layer Made up of two layers, F 1 and F 2 layers. 66) F Layer Highest frequency that can be propagated directly 67) upward and still be returned to Earth by the Critical frequency ionosphere. Maximum vertical angle at which it can be 68) propagated and still be refracted back by the Critical Angle ionosphere. A measurement technique used to determine the 69) Ionospheric Sounding critical frequency. Height above the Earth’s surface from which a 70) Virtual Height refracted wave appears to have been reflected. Highest frequency that can be used for sky wave Maximum Usable 71) propagation between two specific points on Earth’s Frequency (MUF) surface. MUF = critical 72) Secant law. frequency/cosθi Operating at a frequency of 85% of the MUF provides Optimum Working 73) more reliable communications. Frequency (OWF) Minimum distance from a transmit antenna that a sky 74) Skip distance wave at a given frequency will be returned to Earth. Prepared By : MA. ELAINE L. CORTEZ 92 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi The area between where the surface waves are 75) completely dissipated and the point where the first Quiet, or skip, zone sky wave returns to Earth. Formed by the ionosphere is raised, allowing sky 76) waves to travel higher before being returned to Ceiling Earth. 77) Define as the loss incurred by an electromagnetic Free-space path loss waves as it propagates in a straight line through a vacuum with no absorption or reflection of energy from nearby objects. 78) Occurs simply because of the inverse square law. Spreading loss 79) Variation in signal loss. Fading 80) To accommodate temporary fading, an additional Fade margin loss is added to the normal path loss Fm = 30 logD + 10log (6ABf) – 10log (1-R) – 70 Prepared By : MA. ELAINE L. CORTEZ 93 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi CHAPTER 12 METALLIC CABLE TRANSMISSION MEDIA # DEFINITIONS TERMS Provides a conduit in which electromagnetic signals Guided Transmission 1) are contained. Media Unguided Transmission 2) Emitted then radiated through air or a vacuum. Media Used to propagate electromagnetic signals between Cable Transmission 3) two locations in a communications system. Medium Most common means of interconnecting devices in Cable Transmission 4) local area networks. Systems Metallic conductor system used to transfer electrical 5) energy from one point to another using electrical Transmission line current flow. longitudinal and 6) Two basic kinds of waves. transverse 7) The rate at which the periodic wave repeats. Frequency Currents that flow in opposite directions in a 8) Metallic circuit currents balanced wire pair. 9) Currents that flow in the same direction. Longitudinal currents 10) Cancellation of common mode signals. Common mode rejection One wire is at the ground potential, whereas the Single-ended or 11) other is at signal potential. unbalanced A circuit device used to connect a balanced 12) transmission line to an unbalanced load. balun Prepared By : MA. ELAINE L. CORTEZ 78 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi Most common metallic cables used to interconnect Parallel-conductor 14) data communications systems and computer transmission lines and networks. coaxial transmission lines. Formed by twisting two insulated conductors around 15) Twisted-pair each other. Unshielded twisted pair 16) Types of twisted pair. and Shielded twisted pair Coupling that takes place when a transmitted signal 17) is coupled into the received signal at the same end Near-end crosstalk of the cable. Pair 1: blue/white stripe and blue Pair 2: orange/white Standard color code specified by the EIA for CAT-5 stripe and orange 18) cable. Pair 3: green/white strip[e and green Pair 4: brown/white stripe and brown 19) Woven into a mesh. Braid Name given to the area between the ceiling and the root in a single-story building or between the ceiling 20) Plenum and the floor of the next higher level in a multi-story building. Used for high data transmission rates to reduce losses 21) Coaxial and isolate transmission path. Refers to the woven stranded mesh that surrounds 22) Shielding some types of coaxial cables. One layer of foil insulation and one layer of braided 23) shielding. Dual shielded Prepared By : MA. ELAINE L. CORTEZ 79 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi Rigid air-filled; solid 24) Types of coaxial cables. flexible 26) Uniformly distributed throughout the length of the line. Distributed parameters 27) Transmission characteristics of a transmission line. Secondary constants 28) Impedance seen looking into an infinitely long line. Surge impedance Expressed the attenuation and the phase shift per 29) Propagation constants unit length of a transmission line. Ratio of the actual velocity of propagation of an 30) electromagnetic wave through a given medium to Velocity factor. the velocity of propagation through a vacuum. 31) Relative permittivity of a material. Dielectric constant Conductor loss, radiation loss, dielectric heating 32) Several ways in which signal power is lost. loss, coupling loss and corona Voltage that propagates from the source toward the 33) Incident voltage load. Voltage that propagates that propagates from the 34) Reflected voltage load toward the source. 35) Transmission line with no reflected power. Flat or non resonant line Vector quantity that represents the ratio of reflected 36) voltage to incident voltage or reflected current to Reflection coefficient incident current. 37) Incident power is absorbed by the load. Matched line Prepared By : MA. ELAINE L. CORTEZ 80 BOOK REVIEW IN COMMUNICATIONS Electronic Communications System By Wayne Tomasi Unmatched or 38) Incident power returned (reflected) to the source. mismatched line 39) Two travelling waves set up an interference pattern. Standing wave Ratio of the maximum voltage to the minimum 40) voltage or the maximum current to the minimum Standing-wave ratio current of a standing wave on a transmission line. Used to matched transmission lines to purely resistive Quarter-wavelength 41) loads whose resistance is not equal to the transformers characteristic impedance of the line. A technique that can be used to locate an Time domain 42) impairment in a metallic cable reflectometry (TDR) 43) Return signal. Echo Simply a flat conductor separated from a ground 44) Microstrip plane by an insulating di-electric material. Simply a flat conductor sandwiched between two 45) Stripline ground planes. Prepared By : MA. ELAINE L. CORTEZ 81