Antennas and PropagationReview/RecapLecture 17

Overview*Antenna FunctionsIsotropic AntennaRadiation PatternParabolic Reflective AntennaAntenna GainSignal Loss in Satellite CommunicationNoise TypesRefractionFadingDiffraction and ScatteringFast and Slow FadingFlat and Selective FadingDiversity Techniques

Review Question: Antenna FunctionalityQ:- What two functions are performed by an antenna?

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Antenna Definition An antenna is defined as usually a metallic device (as a rod or wire) for radiating or receiving radio waves.

The IEEE Standard Definitions of Antenna defines the antenna or aerial as a means for radiating or receiving radio waves. In other words the antenna is the transitional structure between free-space and a guiding device, as shown in Figure*

Why Antennas of Different ShapesIn addition to receiving or transmitting energy, an antenna in an advanced wireless system is usually required to optimize or accentuate the radiation energy in some directions and suppress it in others.Thus the antenna must also serve as a directional device in addition to a probing device.It must then take various forms to meet the particular need at hand, and it may be a piece of conducting wire, an aperture, a patch, an assembly of elements (array), a reflector, a lens, and so forth. For wireless communication systems, the antenna is one of the most critical components. A good design of the antenna can relax system requirements and improve overall system performance.The antenna serves to a communication system the same purpose that eyes and eyeglasses serve to a human*

Basic Antenna FunctionsAs Antenna resides between cable/waveguide and the medium air, the main function of antenna is to match impedance of the medium with the cable/waveguide impedance. Hence antenna is impedance transforming device. The second and most important function of antenna is to radiate the energy in the desired direction and suppress in the unwanted direction. This basically is the radiation pattern of the antenna. This radiation pattern is different for different types of antennas. *

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The Role of AntennasAntennas serve four primary functionsSpatial filterdirectionally-dependent sensitivityPolarization filterpolarization-dependent sensitivityImpedance transformertransition between free space and transmission linePropagation mode adapterfrom free-space fields to guided waves(e.g., transmission line, waveguide)*

Spatial filterAntennas have the property of being more sensitive in one direction than in another which provides the ability to spatially filter signals from its environment.Directive antenna.Radiation pattern of directive antenna. *

Polarization filterAntennas have the property of being more sensitive to one polarization than another which provides the ability to filter signals based on its polarization.In this example, h is the antennas effective height whose units are expressed in meters.*

Impedance transformerIntrinsic impedance of free-space, E/H

Characteristic impedance of transmission line, V/IA typical value for Z0 is 50 .

Clearly there is an impedance mismatch that must be addressed by the antenna.*

Propagation Mode AdapterIn free space the waves spherically expand following Huygens principle: each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves.

Within the sensor, the waves are guided within a transmission line or waveguide that restricts propagation to one axis.

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Propagation Mode AdapterDuring both transmission and receive operations the antenna must provide the transition between these two propagation modes.*

*Antenna purposeTransformation of a guided EM wave in transmission line (waveguide) into a freely propagating EM wave in space (or vice versa) with specified directional characteristicsTransformation from time-function in one-dimensional space into time-function in three dimensional space The specific form of the radiated wave is defined by the antenna structure and the environment Space waveGuided wave

*Antenna functionsTransmission linePower transport medium - must avoid power reflections, otherwise use matching devices RadiatorMust radiate efficiently must be of a size comparable with the half-wavelength ResonatorUnavoidable - for broadband applications resonances must be attenuated

Ans:- Two functions of an antenna are: For transmission of a signal, radio frequency 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 radio-frequency electrical energy and fed into the receiver.*Q:- What two functions are performed by an antenna?Review Question: Antenna Functionality

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Isotropic AntennaQ:- What is an isotropic antenna?*

Isotropic AntennaIsotropic antenna or isotropic radiator is a hypothetical (not physically realizable) concept, used as a useful reference to describe real antennas. Isotropic antenna radiates equally in all directions. Its radiation pattern is represented by a sphere whose center coincides with the location of the isotropic radiator.*

Reference Antenna for Gain Gain is Measured Specific to a Reference Antenna isotropic antenna often used - gain over isotropicIsotropic antenna radiates power ideally in all directionsGain measured in dBiTest antennas field strength relative to reference isotropic antenna at same power, distance, and angle-Isotropic antenna cannot be practically realizede.g. A lamp is similar to an isotropic antenna

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Isotropic*

*An Isotropic Source: GainEvery real antenna radiates more energy in some directions than in others (i.e. has directional properties)Idealized example of directional antenna: the radiated energy is concentrated in the yellow region (cone). Directive antenna gain: the power flux density is increased by (roughly) the inverse ratio of the yellow area and the total surface of the isotropic sphereGain in the field intensity may also be considered - it is equal to the square root of the power gain.

*Antenna Gain Measurement Antenna Gain = (P/Po) S=S0

Isotropic AntennaQ:- What is an isotropic antenna?

Ans:- An isotropic antenna is a point in space that radiates power in all directions equally.*

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Review: Radiation PatternQ:- What information is available from a radiation pattern?*

Radiation PatternIn the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source.Particularly in the fields of fiber optics, lasers, and integrated optics, the term radiation pattern may also be used as a synonym for the near-field pattern or Fresnel pattern. This refers to the positional dependence of the electromagnetic field in the near-field, or Fresnel region of the source. The near-field pattern is most commonly defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it.The far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, and the radiation pattern deduced from it by computation. The far-field radiation pattern can also be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can also compute the near field.*

Antenna Radiation PatternRadiation patternGraphical representation of radiation properties of an antennaDepicted as two-dimensional cross sectionThe radiation pattern of an antenna is a plot of the far-field radiation from the antenna. More specifically, it is a plot of the power radiated from an antenna per unit solid angle, or its radiation intensity U [watts per unit solid angle]. This is arrived at by simply multiplying the power density at a given distance by the square of the distance r, where the power density S [watts per square metre] is given by the magnitude of the time-averaged Poynting vector:U=rS*

*Radiation pattern The radiation pattern of antenna is a representation (pictorial or mathematical) of the distribution of the power out-flowing (radiated) from the antenna (in the case of transmitting antenna), or inflowing (received) to the antenna (in the case of receiving antenna) as a function of direction angles from the antennaAntenna radiation pattern (antenna pattern): is defined for large distances from the antenna, where the spatial (angular) distribution of the radiated power does not depend on the distance from the radiation sourceis independent on the power flow direction: it is the same when the antenna is used to transmit and when it is used to receive radio waves is usually different for different frequencies and different polarizations of radio wave radiated/ received

*Power Pattern Vs. Field pattern The power pattern is the measured (calculated) and plotted received power: |P(, )| at a constant (large) distance from the antenna The amplitude field pattern is the measured (calculated) and plotted electric (magnetic) field intensity, |E(, )| or |H(, )| at a constant (large) distance from the antenna The power pattern and the field patterns are inter-related: P(, ) = (1/)*|E(, )|2 = *|H(, )|2P = powerE = electrical field component vectorH = magnetic field component vector = 377 ohm (free-space, plane wave impedance)

*Normalized patternUsually, the pattern describes the normalized field (power) values with respect to the maximum value.Note: The power pattern and the amplitude field pattern are the same when computed and when plotted in dB.

*3-D patternAntenna radiation pattern is 3-dimensionalThe 3-D plot of antenna pattern assumes both angles and varying, which is difficult to produce and to interpret3-D pattern

*2-D patternTwo 2-D patternsUsually the antenna pattern is presented as a 2-D plot, with only one of the direction angles, or variesIt is an intersection of the 3-D one with a given plane usually it is a = const plane or a = const plane that contains the patterns maximum

*Example: a short dipole on z-axis

*Principal PatternsPrincipal patterns are the 2-D patterns of linearly polarized antennas, measured in 2 planesthe E-plane: a plane parallel to the E vector and containing the direction of maximum radiation, and the H-plane: a plane parallel to the H vector, orthogonal to the E-plane, and containing the direction of maximum radiation

*Example

*Antenna Mask (Example 1)Typical relative directivity- mask of receiving antenna (Yagi ant., TV dcm waves)

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*Antenna Mask (Example 2)Reference pattern for co-polar and cross-polar components for satellite transmitting antennas in Regions 1 and 3 (Broadcasting ~12 GHz) 0dB-3dBPhi

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Review: Radiation PatternQ:- What information is available from a radiation pattern?

Radiation Patterns in Polar and Cartesian Coordinates Showing Various Types of Lobes

Ans:- A radiation pattern is a graphical representation of the radiation properties of an antenna as a function of space coordinates.*

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Parabolic Reflective AntennaQ:- What is the advantage of a parabolic reflective antenna?

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Two Main Purposes of AntennaImpedance matching: matches impedance of transmission line to the intrinsic impedance of free space to prevent wanted reflection back to source.Antenna must be designed to direct the radiation in the desired direction.

So a parabolic antennais a high gain reflector antenna. It is used for television, radio and data communications. It may also be used for radar on the UHF and SHF sections of the electromagnetic spectrum.

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Reflector AntennaReflector antenna such as parabolic antenna are composed of primary radiator and a reflective mirror.*

Parabolic Reflector AntennaAny electromagnetic wave incident upon the paraboloid surface will be directed to the focal point.Primary antenna is used at the focal point of the parabolic reflector antenna instead of isotropic antenna. The isotropic antenna would radiate and receive radiation from all directions resulting in spillover.Primary antenna should be designed to illuminate just the reflector uniformly.*

Loss*

CharacteristicsAperture:r= radius of the diameterLarger dish has more gain than smallerClear line of sight is important

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CalculationsPhysical area:D= DiameterEffective area: = illumination efficiency Wavelength:

Gain:

3db beamwidth:*

Half Power Beamwidth

The half power graph showing the angle between the half power point on either side of maximum*

Radiation Pattern for Parabolic Antenna*

Advantage of a Parabolic AntennaThe advantage of a parabolic antenna is that it can be used as primary mirror for all the frequencies in the project, provided the surface is within the tolerance limit; only the feed antenna and the receiver need to be changed when the observing frequency is changed.An advantage of such a design is the small irradiation loss, which allows for an optimum antenna gain.It is an advantage of such an arrangement that the exciter system and/or the exciter 3 are/isprotectively located within the parabola or the parabolic reflector.Parabolic antenna is the most efficient type of a directional antenna - large front/back ratio, sharp radiation angle and small side lobes. It fits well for noisy locations where other antennas factually do not work.The antenna's Gain is adequate to the area of the reflector. The reflector can be central-focused(the focus is in the center of the dish) or offset (the focus is off the axis of the dish).

In general, they serve for connection of end users (so-called last mile) to a wireless network. However, in areas with lower intensity of Wi-Fi networks, they can be successfully used also for back-bone links. In fact, this frequency is used for connections up to maximum 10 km

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Parabolic Reflective AntennaQ:- What is the advantage of a parabolic reflective antenna?

A parabolic antenna creates, in theory, a parallel beam without dispersion. In practice, there will be some beam spread. Nevertheless, it produces a highly focused, directional beam.*

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Antenna GainQ:- What factors determine antenna gain?*

Antenna GainAntenna Gain (Directivity)Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna [usual reference is an isotropic antenna (dBi) but sometimes a simple antenna is a more practical reference; good sales trick to use an isotropic reference when a dipole is inferred resulting in a 1.64 power gain]Antenna gain doesnt increase power; only concentrates effective radiation patternEffective area (related to antenna aperture) Expressed in terms of effective areaRelated to physical size and shape of antenna related to the operational wavelength of the antennaChange in coverage by focusing the area of RF propagation*

Passive GainFocusing isotropic energy in a specific patternCreated by the design of the antennaUses the magnify glass concept*

Passive GainAntennas use passive gainTotal amount of energy emitted by antenna doesnt increase only the distribution of energy around the antennaAntenna is designed to focus more energy in a specific directionPassive gain is always a function of the antenna (i.e. independent of the components leading up to the antenna*

Passive GainAdvantageDoes not require external powerDisadvantageAs the gain increases, its coverage becomes more focusedHighest-gain antennas cant be used for mobile users because of their tight beam *

Active GainProviding an external power sourceAmplifierHigh gain transmitters

Active gain involves an amplifier

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Antenna GainRelationship between antenna gain and effective area

G = antenna gainAe = effective areaf = carrier frequencyc = speed of light ( 3 x 108 m/s) = carrier wavelength*

Antenna gainAntenna gain is increased by focusing the antennaThe antenna does not create energy, so a higher gain in one direction must mean a lower gain in another.Note: antenna gain is based on the maximum gain, not the average over a region. This maximum may only be achieved only if the antenna is carefully aimed. This antenna is narrower and results in 3dB higher gain than the dipole, hence, 3dBD or 5.14dBiThis antenna is narrower and results in 9dB higher gain than the dipole, hence, 9dBD or 11.14dBi*

Antenna gainInstead of the energy going in all horizontal directions, a reflector can be placed so it only goes in one direction => another 3dB of gain, 3dBD or 5.14dBiFurther focusing on a sector results in more gain.A uniform 3 sector antenna system would give 4.77 dB more.A 10 degree range 15dB more.The actual gain is a bit higher since the peak is higher than the average over the range.Mobile phone base stations claim a gain of 18dBi with three sector antenna system. 4.77dB from 3 sectors 13.33 dBi An 11dBi antenna has a very narrow range. *

Antenna GainThe power gain G, or simply the gain, of an antenna is the ratio of its radiation intensity to that of an isotropic antenna radiating the same total power as accepted by the real antenna. When antenna manufacturers specify simply the gain of an antenna they are usually referring to the maximum value of G.

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Antenna gain and effective areas*

Type of antennaEffective areaPower gainIsotropic2/41Infinitesimal dipole or loop1.52/41.5Half-wave dipole1.642/41.64Horn, mouth area A0.81A10A/ 2Parabolic, face area A0.56A7A/ 2turnstile1.152/41.15

Antenna GainQ:- What factors determine antenna gain?

Ans:- Effective area and wavelength*

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Satellite CommunicationQ:- What is the primary cause of signal loss in satellite communications?*

Basics: How do Satellites WorkTwo Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.The two stations can use a satellite as a relay station for their communicationOne Earth Station sends a transmission to the satellite. This is called a Uplink.The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.*

Basics: Advantages of SatellitesThe advantages of satellite communication over terrestrial communication are:The coverage area of a satellite greatly exceeds that of a terrestrial system.Transmission cost of a satellite is independent of the distance from the center of the coverage area.Satellite to Satellite communication is very precise.Higher Bandwidths are available for use.*

Basics: Disadvantages of SatellitesThe disadvantages of satellite communication:Launching satellites into orbit is costly.Satellite bandwidth is gradually becoming used up.There is a larger propagation delay in satellite communication than in terrestrial communication. *

Basics: Factors in Satellite CommunicationElevation Angle: The angle of the horizontal of the earth surface to the center line of the satellite transmission beam.This effects the satellites coverage area. Ideally, you want a elevation angle of 0 degrees, so the transmission beam reaches the horizon visible to the satellite in all directions.However, because of environmental factors like objects blocking the transmission, atmospheric attenuation, and the earth electrical background noise, there is a minimum elevation angle of earth stations.*

Basics: Factors in satellite communication .Coverage Angle: A measure of the portion of the earth surface visible to a satellite taking the minimum elevation angle into account.R/(R+h) = sin(/2 - - )/sin( + /2) = cos( + )/cos()R = 6370 km (earths radius)h = satellite orbit height = coverage angle = minimum elevation angle*

Basics: Factors in satellite communication.Other impairments to satellite communication:The distance between an earth station and a satellite (free space loss).Satellite Footprint: The satellite transmissions strength is strongest in the center of the transmission, and decreases farther from the center as free space loss increases.Atmospheric Attenuation caused by air and water can impair the transmission. It is particularly bad during rain and fog. *

Atmospheric LossesDifferent types of atmospheric losses can disturb radio wave transmission in satellite systems:Atmospheric absorptionAtmospheric attenuationTraveling ionospheric disturbances*

Atmospheric AbsorptionEnergy absorption by atmospheric gases, which varies with the frequency of the radio waves.Two absorption peaks are observed (for 90 elevation angle):22.3 GHz from resonance absorption in water vapour (H2O)60 GHz from resonance absorption in oxygen (O2)For other elevation angles:[AA] = [AA]90 cosec Source: Satellite Communications, Dennis Roddy, McGraw-Hill*

Atmospheric AttenuationRain is the main cause of atmospheric attenuation (hail, ice and snow have little effect on attenuation because of their low water content).Total attenuation from rain can be determined by:A = L [dB]where [dB/km] is called the specific attenuation, and can be calculated from specific attenuation coefficients in tabular form that can be found in a number of publicationswhere L [km] is the effective path length of the signal through the rain; note that this differs from the geometric path length due to fluctuations in the rain density. *

Traveling Ionospheric DisturbancesTraveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke radio signal fluctuations which can only be determined on a statistical basis.The disturbances of major concern are:Scintillation;Polarisation rotation.Scintillations are variations in the amplitude, phase, polarisation, or angle of arrival of radio waves, caused by irregularities in the ionosphere which change over time. The main effect of scintillations is fading of the signal.*

Satellite CommunicationQ:- What is the primary cause of signal loss in satellite communications?

Ans:- Free space loss.*

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ImpairmentsQ:- Name and briefly define four types of noise.*

Transmission ImpairmentsSignal received may differ from signal transmitted causing:Analog - degradation of signal qualityDigital - bit errorsMost significant impairments areAttenuation and attenuation distortionDelay distortionNoise

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NoiseSignal strength falls off with distance over any transmission mediumVaries with frequency

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Categories of Noise

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Categories of NoiseCrosstalk:a signal from one line is picked up by anothercan occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signalsImpulse Noise:caused by external electromagnetic interferencesnoncontinuous, consisting of irregular pulses or spikesshort duration and high amplitudeminor annoyance for analog signals but a major source of error in digital data

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NoiseThermal noise due to thermal agitation of electrons.Present in all electronic devices and transmission media.As a function of temperature.Uniformly distributed across the frequency spectrum, hence often referred as white noise.Cannot be eliminated places an upper bound on the communication system performance.Can cause erroneous to the transmitted digital data bits.*

Noise: Noise on Digital DataError in bits*

Thermal NoiseThe noise power density (amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor) is:

N0 = noise power density in watts per 1 Hz of bandwidthk = Boltzmann's constant = 1.3803 10-23 J/KT = temperature, in kelvins (absolute temperature) 0oC = 273 Kelvin*

Thermal NoiseNoise is assumed to be independent of frequencyThermal noise present in a bandwidth of B Hertz (in watts):

or, in decibel-watts (dBW),*

Noise TerminologyIntermodulation noise occurs if signals with different frequencies share the same mediumInterference caused by a signal produced at a frequency that is the sum or difference of original frequenciesCrosstalk unwanted coupling between signal pathsImpulse noise irregular pulses or noise spikesShort duration and of relatively high amplitudeCaused by external electromagnetic disturbances, or faults and flaws in the communications system*

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ImpairmentsQ:- Name and briefly define four types of noise.

Ans:- Thermal noise is due to thermal agitation of electrons. Intermodulation noise produces signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies. Crosstalk is the unwanted coupling between signal paths. Impulse noise is noncontinuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude.*

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Refraction?Q:- What is refraction?

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Law of refraction A refracted ray lies in the plane of incidence and has an angle 2 of refraction that is related to the angle of incidence 1 by: the symbols n1 and n2 are dimensionless constant, called the index of refraction *

Refraction*Refraction occurs when an RF signal changes speed and is bent while moving between media of different densities.

Refraction*

Refraction?Q:- What is refraction?

Ans:- Refraction is the bending of a radio beam caused by changes in the speed of propagation at a point of change in the medium*

FadingQ:- What is fading?*

Fading in a Mobile EnvironmentThe term fading refers to the time variation of received signal power caused by changes in the transmission medium or paths.Atmospheric condition, such as rainfallThe relative location of various obstacles changes over time

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Types of FadingFast fadingSlow fadingFlat fadingSelective fadingRayleigh fadingRician fading*

Fading in the Mobile EnvironmentFading: time variation of received signal power due to changes in the transmission medium or path(s)Kinds of fading:Fast fadingSlow fadingFlat fading independent from frequencySelective fading frequency-dependentRayleigh fading no dominant path Rician fading Line Of Sight (LOS) is dominating + presence of indirect multipath signals*

FadingQ:- What is fading?

Ans:- The term fading refers to the time variation of received signal power caused by changes in the transmission medium or path(s).*

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Q:- What is the difference between diffraction and scattering?*

Diffraction*Diffraction is a change in the direction and/or intensity of a wave as it passes by the edge of an obstacle.Diffraction occurs because the RF signal slows down as it encounters the obstacle and causes the wave front to change directionsDiffraction is often caused by buildings, small hills, and other larger objects in the path of the propagating RF signal.

DiffractionDiffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave*

Scattering*Scattering happens when an RF signal strikes an uneven surface causing the signal to be scattered. The resulting signals are less significant than the original signal.Scattering = Multiple Reflections

ScatteringScattering occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less.Irregular objects such as walls with rough surfaces,furniture and vehicles and foliage and the like scatter rays in all the direction in the form of spherical waves.*

Multipath Propagation*

Diffraction and ScatteringQ:- What is the difference between diffraction and scattering?

Ans:- Diffraction occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave. The edge in effect become a source and waves radiate in different directions from the edge, allowing a beam to bend around an obstacle. If the size of an obstacle is on the order of the wavelength of the signal or less, scattering occurs. An incoming signal is scattered into several weaker outgoing signals in unpredictable directions*

Summary*Antenna FunctionsIsotropic AntennaRadiation PatternParabolic Reflective AntennaAntenna GainSignal Loss in Satellite CommunicationNoise TypesRefractionFadingDiffraction and Scattering

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Complimentary Session for Antennas and Propagation (Lecture 17)*

Antenna Gain (Q)

*WhereSol

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QQ:- For each of the antenna types listed in Table above , what is the effective area and gain at a wavelength of 30 mm? Repeat for a wavelength of 3 mm. Assume that the actual area for the horn and parabolic antennas is m2 .*

Antenna Gain*Where

Ans*Q:- For each of the antenna types listed in Table above , what is the effective area and gain at a wavelength of 30 mm? Repeat for a wavelength of 3 mm. Assume that the actual area for the horn and parabolic antennas is m2 .

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Q*Solution

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Q*Question

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Thermal Noise*WhereQuestion:-

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The Expression Eb /N0in decibel notation,*Question:-

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Q:- It is often more convenient to express distance in km rather than m and frequency in MHz rather than Hz. Rewrite Equation using these dimensions.Solution:- The equations from Text Book are

Solution:-*

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QQ:- Suppose a transmitter produces 50 W of power.Express the transmit power in units of dBm and dBW.If the transmitter's power is applied to a unity gain antenna with a 900-MHz carrier frequency, what is the received power in dBm at a free space distance of 100 m?Repeat (b) for a distance of 10 km.Repeat (c) but assume a receiver antenna gain of 2.*

Q/A.

b)

Therefore, received power in dBm = 47 71.52 = 24.52 dBm*Q:- Suppose a transmitter produces 50 W of power.Express the transmit power in units of dBm and dBW.If the transmitter's power is applied to a unity gain antenna with a 900-MHz carrier frequency, what is the received power in dBm at a free space distance of 100 m?Repeat (b) for a distance of 10 km.Repeat (c) but assume a receiver antenna gain of 2.

Q/Ac)

d) *Q:- Suppose a transmitter produces 50 W of power.Express the transmit power in units of dBm and dBW.If the transmitter's power is applied to a unity gain antenna with a 900-MHz carrier frequency, what is the received power in dBm at a free space distance of 100 m?Repeat (b) for a distance of 10 km.Repeat (c) but assume a receiver antenna gain of 2.

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Free Space Loss*

Free Space Loss*

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******With any communications system, the signal that is received may differ from the signal that is transmitted due to various transmission impairments. For analog signals, these impairments introduce various random modifications that degrade the signal quality. For digital signals, bit errors may be introduced, such that a binary 1 is transformed into a binary 0 or vice versa. In this section, we examine the various impairments and how they may affect the information-carrying capacity of a communication link; Chapter 5 looks at measures that can be taken to compensate for these impairments.The most significant impairments areAttenuation and attenuation distortionDelay distortionNoise

Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011*NoiseFor any data transmission event, the received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception. The latter, undesired signals are referred to as noise. Noise is the major limiting factor in communications system performance.Noise may be divided into four categories:Thermal noiseIntermodulation noiseCrosstalkImpulse noiseData and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011Thermal noise is due to thermal agitation of electrons. It is present in all electronic devices and transmission media and is a function of temperature. Thermal noise is uniformly distributed across the bandwidths typically used in communications systems and hence is often referred to as white noise. Thermal noise cannot be eliminated and therefore places an upper bound on communications system performance. Because of the weakness of the signal received by satellite earth stations, thermal noise is particularly significant for satellite communication.The amount of thermal noise to be found in a bandwidth of 1 Hz in any device or conductor isN0 = kT (W/Hz)whereN0 = noise power density in watts per 1 Hz of bandwidthk = Boltzmann's constant = 1.38 1023 J/KT = temperature, in kelvins (absolute temperature), where the symbol K is used to represent 1 kelvinExample 3.3 Room temperature is usually specified as T = 17C, or 290 K. At this temperature, the thermal noise power density isN0 = (1.38 10-23) 290 = 4 1021 W/Hz = 204 dBW/Hzwhere dBW is the decibel-watt, defined in Appendix 3A.The noise is assumed to be independent of frequency. Thus the thermal noise in watts present in a bandwidth of B hertz can be expressed asN = kTBor, in decibel-watts,N= 10 log k + 10 log T + 10 log B= 228.6 dBW + 10 log T + 10 log BExample 3.4 Given a receiver with an effective noise temperature of 294 K and a 10-MHz bandwidth, the thermal noise level at the receiver's output isN=228.6 dBW + 10 log(294) + 10 log 107=228.6 + 24.7 + 70=133.9 dBWWhen signals at different frequencies share the same transmission medium, the result may be intermodulation noise. The effect of intermodulation noise is to produce signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies. For example, if two signals, one at 4000 Hz and one at 8000 Hz, share the same transmission facility, they might produce energy at 12,000 Hz. This noise could interfere with an intended signal at 12,000 Hz.Intermodulation noise is produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium. Ideally, these components behave as linear systems; that is, the output is equal to the input times a constant. However, in any real system, the output is a more complex function of the input. Excessive nonlinearity can be caused by component malfunction or overload from excessive signal strength. It is under these circumstances that the sum and difference frequency terms occur.*Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011*Crosstalk has been experienced by anyone who, while using the telephone, has been able to hear another conversation; it is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying multiple signals. Crosstalk can also occur when microwave antennas pick up unwanted signals; although highly directional antennas are used, microwave energy does spread during propagation. Typically, crosstalk is of the same order of magnitude as, or less than, thermal noise.All of the types of noise discussed so far have reasonably predictable and relatively constant magnitudes. Thus it is possible to engineer a transmission system to cope with them. Impulse noise, however, is noncontinuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude. It is generated from a variety of causes, including external electromagnetic disturbances, such as lightning, and faults and flaws in the communications system.Impulse noise is generally only a minor annoyance for analog data. For example, voice transmission may be corrupted by short clicks and crackles with no loss of intelligibility. However, impulse noise is the primary source of error in digital data communication. For example, a sharp spike of energy of 0.01 s duration would not destroy any voice data but would wash out about 560 bits of digital data being transmitted at 56 kbpsData and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011ACS/antennas and propagationby Ya Bao http://eent3.sbu.ac.uk/staff/baoyb/acs*Thermal noise due to agitation of electronsPresent in all electronic devices and transmission mediaCannot be eliminatedFunction of temperatureParticularly significant for satellite communication

ACS/antennas and propagationby Ya Bao http://eent3.sbu.ac.uk/staff/baoyb/acs*ACS/antennas and propagationby Ya Bao http://eent3.sbu.ac.uk/staff/baoyb/acs**One of the two antennas is moving relative to the others. The relative location of various obstacles changes over time.*Rapid variation in signal strength occurs over distances of about one-half a wavelength. 900Mhz mobile cellular application a wavelength is 0.33m. Spatial variation of amplitude.Flat fading, all frequency components of the received signal fluctuate in the same proportions simultaneously. Affects unequally the different spectral components. Occurs a portion of the whole bandwidth, selective.