Antennas and Propagation - Lecture 15

Antennas and PropagationLecture 15OverviewElectric and Magnetic Field CouplingEM RadiationsPeriod, Frequency, and WavelengthPhase Lag and Phase LeadAntennas Why???Antenna AnalogyWhy Separate TX and RX Antennas Transmission are RequiredTransmission Line as an AntennaWorking of an AntennaFar And Near FieldsAntennas CharacteristicsPolarizationAntenna and WavelengthAntenna GainAntenna Length2Electric and Magnetic Field Coupling3Electric FieldElectric and Magnetic Fields CouplingElectric and Magnetic Field Coupling4two laws (from Maxwell Equation)1. A Moving Electric Field Creates a Magnetic (H) field 2. A Moving Magnetic Field Creates an Electric (E) field Magnetic Field Lines are closed loops surrounding the currents that produce themElectromagnetic (EM) radiationElectromagnetic (EM) radiation is caused by charged particles that are accelerated. Charged particles have an electric field. Moving charged particles create a magnetic field, which in turn creates electromagnetic radiation sometimes called an electromagnetic wave or electromagnetic field. Therefore, changing currents are required to create electromagnetic radiation. Electromagnetic radiation has both a magnetic and electric field.5Period, Frequency, and WavelengthT = period, time for one cyclef = frequency (cycles/s = Hz) = 1/Tλ = wavelength (m)c = speed of light in vacuum = 3E8 m/sc= λ*fWhat is T, f, and λ?Ans: 2 s, 0.5 Hz, 6E8 m 6Phase (time delay)Phase: relative timing of two signalsCould measure absolute time like secondsMore common to use a radians or degreesSignal 1 = sin(θ)Signal 2 = sin(θ-pi/4)7Phase Lag8Phase Lead910Why Antenna ???11A general model of all communication systems.Analogy12You need to understand the basics of communications. You have one side sending a message and one receiving it. Our ears work like antennas. We can capture vibrations between 20 Hz and 20,000 Hz. So when we talk about antennas on electronics, they do the same job but with higher frequencies. Our voice gets lost because of distance, obstructions like walls or just because the decibels produced by humans have a limit of roughly 70 decibels at around 3 feet. So in order to expand that distance we use radios and radios use antennas instead of ears! 13HISTORYThe first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving. He published his work in Annalen der Physik und Chemie (vol. 36, 1889).14INTRODUCTIONAn antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter applies an oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). 15INTRODUCTION Transmitting Antenna: Any structure designed to efficiently radiate electromagnetic radiation in a preferred direction is called a transmitting antenna.In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified. An antenna can be used for both transmitting and receiving. Receiving Antenna: Any structure designed to efficiently receive electromagnetic radiation is called a receiving antenna16Requirement of separate TX and RX antennas?Because, they have different requirements:TX antennas need to deliver strongest possible signal into target area compared to other antennas.Efficiency and gain are most important factors.RX antennas need to have best Signal to Noise Ratio (SNR) – gain and efficiency are not necessary.17BASIC STRUCTUREIt is a metallic conductor system capable of radiating and receiving em waves.Typically an antenna consists of an arrangement of metallic conductors (“elements"), electrically connected (often through a transmission line) to the receiver or transmitter. An oscillating current of electrons forced through the antenna by a transmitter will create an oscillating magnetic field around the antenna elements, while the charge of the electrons also creates an oscillating electric field along the elements. 18These time-varying fields radiate away from the antenna into space as a moving electromagnetic field wave.Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back and forth, creating oscillating currents in the antenna.Antenna reciprocity : can be used as transmitter and receiver.In two way communication same antenna can be used as transmitter and receiver.BASIC STRUCTURE.19Antennas may also contain reflective or directive elements or surfaces not connected to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern. Antennas can be designed to transmit or receive radio waves in all directions equally (omnidirectional antennas), or transmit them in a beam in a particular direction, and receive from that one direction only ( directional or high gain antennas).BASIC STRUCTURE20WHY ANTENNAS ?Need of antenna arisen when two person wanted to communicate between them when separated by some distance and wired communication is not possible.Antennas are required by any radio receiver or transmitter to couple its electrical connection to the electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss. 21 Radio transmitters and receivers are used to convey signals (information) in systems including broadcast (audio) radio, television, mobile telephones , point-to-point communications links (telephone, data networks), satellite links.Radio waves are also used directly for measurements in technologies including Radar, GPS, and radio astronomy.In each and every case, the transmitters and receivers involved require antennas, although these are sometimes hidden (such as the antenna inside an AM radio or inside a laptop computer equipped with wi-fi).WHY ANTENNAS ?22WHERE USED?Antennas are used in systems such as radio and television broadcasting, point to point radio communication, wireless LAN, radar and space explorationAntennas are most utilized in air or outer spaceBut can also be operated under water or even through soil and rock at certain frequencies for short distances232425Converting a Transmission Line into an AntennaBending at right angles produces an efficient radiatorOptimum radiation occurs when the length is 1/2 of a wavelengthMagnetic fields now support each otherAntennas: How Stuff Works264Antennas: How Stuff Works27Electromagnetic RadiationAny antenna can be successfully measured on either a near-field or far-field range, with appropriate implementation. There are significant cost, size, and complexity details which will lead to a recommendation of one type over the other. In general, far-field ranges are a better choice for lower frequency antennas and where simple pattern cut measurements are required, and near-field ranges are a better choice for higher frequency antennas and where complete pattern and polarization measurements are required.Antennas with a periodic signal create electromagnetic radiationTwo types of electromagnetic radiationNear fieldFar field28Near Field (Inductive Coupling)Area from the antenna to the point where the electromagnetic field forms. Field starts at the antenna as purely magneticInductive (like a transformer) or capacitive couplingMagnetic field decreases by a factor of 1/(r3) in free space, where r is distance between the tag and reader antennaEnough power for cryptographic functions if tag close to reader29Far Field (Radiative Coupling)Area some distance from the transmitting antenna at which the electromagnetic wave has fully formed and separated from the antenna. The electric and magnetic fields propagate as an electromagnetic (EM) wave.In the far field, inductive coupling is not possibleEM field decreases by a factor of 1/r, where r is distance between the tag and reader antenna30Approximating Boundary Between Near and Far FieldCase 1: If antenna size is comparable to the wavelength (like UHF),r = 2f(d2)/cd = maximum antenna dimensionf = frequencyc = speed of lightCase 2: If antenna size much smaller than wavelength (like HF),r = c/(2*pi*f)31Near-field/Far-field BoundariesBandDistance (meters)Distance (feet)LF3821146HF3.511UHF0.160.53233Antennas34An antenna is a device that provides a transition between guided electromagnetic waves in wires and electromagnetic waves in free space.Antenna Physical Characteristics 35The antenna’s size and shape largely determines the frequencies it can handle and how it radiates electromagnetic waves.36The polarization of an antenna refers to the orientation of the electric field it produces.Polarization is important because the receiving antenna should have the same polarization as the transmitting antenna to maximize received power.Antenna Polarization 37Antenna PolarizationHorizontal PolarizationVertical PolarizationCircular PolarizationElectric and magnetic field rotate at the frequency of the transmitterUsed when the orientation of the receiving antenna is unknownWill work for both vertical and horizontal antennasRight Hand Circular Polarization (RHCP)Left Hand Circular Polarization (LHCP)Both antennas must be the same orientation (RHCP or LHCP)38The dimensions of an antenna are usually expressed in terms of wavelength ().Low frequencies imply long wavelengths, hence low frequency antennas are very large.High frequencies imply short wavelengths, hence high frequency antennas are usually small.Wavelength and Antennas 39Basic Antenna40Basic AntennaLet’s start by looking at the radiation pattern of an isotropic point source.Power from an isotropic point source is equally distributed in all directionsIt is completely unfocused.Isotropic Point SourceAntenna only existstheoretically41Antenna gain (G) Because an antenna is a passive device, the power radiated can not be greater than the input power.Antenna gain is expressed as a ratio of the effective radiated output power (Pout) to the input power (Pin)The gain of an antenna is a measure of power transmitted relative to that transmitted by an isotropic source.Antenna gain relative to an isotropic source is expressed in decibels as dBi.42Effective Radiated Power The effective radiated power (ERP or EIRP) is the gain of an antenna (with respect to an isotropic radiator) multiplied by its input power.For example, a highly directional antenna with a gain of 7 has an input power of 1-kW. Its ERP is therefore 7 kW.43SummaryElectric and Magnetic Field CouplingEM RadiationsPeriod, Frequency, and WavelengthPhase Lag and Phase LeadAntennas Why???Antenna AnalogyWhy Separate TX and RX Antennas Transmission are RequiredTransmission Line as an AntennaWorking of an AntennaFar And Near FieldsAntennas CharacteristicsPolarizationAntenna and WavelengthAntenna GainAntenna Length44