Antennas and Propagation

Lecture 15

Overview Electric and Magnetic Field Coupling EM Radiations Period, Frequency, and Wavelength Phase Lag and Phase Lead Antennas Why??? Antenna Analogy Why Separate TX and RX Antennas Transmission are Required Transmission Line as an Antenna Working of an Antenna Far And Near Fields Antennas Characteristics

Polarization Antenna and Wavelength Antenna Gain Antenna Length 2

Electric and Magnetic Field Coupling

3Electric Field

Electric and Magnetic Fields Coupling

Electric and Magnetic Field Coupling

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two 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 them

Electromagnetic (EM) radiation

Electromagnetic (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.

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Period, Frequency, and Wavelength

T = period, time for one cycle f = frequency (cycles/s = Hz) =

1/T λ = wavelength (m) c = speed of light in vacuum =

3E8 m/s c= λ*f What is T, f, and λ?

Ans: 2 s, 0.5 Hz, 6E8 m

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Phase (time delay) Phase: relative timing of two

signals Could measure absolute time

like seconds More common to use a radians

or degrees Signal 1 = sin(θ) Signal 2 = sin(θ-pi/4)

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Phase Lag

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Phase Lead

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Why Antenna ???

11A general model of all communication systems.

Analogy

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You 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!

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HISTORY The 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).

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INTRODUCTION An 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).

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INTRODUCTION … 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 antenna

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Requirement 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.

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BASIC STRUCTURE It 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.

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These 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….

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Antennas 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 STRUCTURE…

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WHY 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.

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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 ?

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WHERE USED? Antennas are used in systems such as radio and

television broadcasting, point to point radio communication, wireless LAN, radar and space exploration

Antennas are most utilized in air or outer space But can also be operated under water or even through

soil and rock at certain frequencies for short distances

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Converting a Transmission Line into an Antenna

Bending at right angles produces an efficient radiator

Optimum radiation occurs when the length is 1/2 of a wavelength

Magnetic fields now support each other

Antennas: How Stuff Works

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Antennas: How Stuff Works

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Electromagnetic 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 radiation

Near field Far field 28

Near Field (Inductive Coupling)

Area from the antenna to the point where the electromagnetic field forms. Field starts at the antenna as purely magnetic

Inductive (like a transformer) or capacitive coupling Magnetic field decreases by a factor of 1/(r3) in free space, where r

is distance between the tag and reader antenna Enough power for cryptographic functions if tag close to reader

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Far 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 possible EM field decreases by a factor of 1/r, where r is distance between the tag

and reader antenna

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Approximating Boundary Between Near and Far Field

Case 1: If antenna size is comparable to the wavelength (like UHF),

r = 2f(d2)/c d = maximum antenna dimension f = frequency c = speed of light

Case 2: If antenna size much smaller than wavelength (like HF),

r = c/(2*pi*f)

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Near-field/Far-field Boundaries

Band Distance (meters) Distance (feet)

LF 382 1146

HF 3.5 11

UHF 0.16 0.5

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Antennas

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An antenna is a device that provides a transition between guided electromagnetic waves in wires and electromagnetic waves in free space.

Antenna Physical Characteristics

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The antenna’s size and shape largely determines the frequencies it can handle and how it radiates electromagnetic waves.

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The 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

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Antenna Polarization Horizontal Polarization Vertical Polarization Circular Polarization

Electric and magnetic field rotate at the frequency of the transmitter

Used when the orientation of the receiving antenna is unknown

Will work for both vertical and horizontal antennas

Right Hand Circular Polarization (RHCP) Left Hand Circular Polarization (LHCP)

Both antennas must be the same orientation (RHCP or LHCP)

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The 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

c

c ff

or

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Basic Antenna

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Basic Antenna Let’s start by looking at the radiation pattern of

an isotropic point source. Power from an isotropic point source is

equally distributed in all directions It is completely unfocused.

Isotropic Point Source

Antenna only existstheoretically

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Antenna 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.

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Effective 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.

ERP input power antenna gain

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Summary Electric and Magnetic Field Coupling EM Radiations Period, Frequency, and Wavelength Phase Lag and Phase Lead Antennas Why??? Antenna Analogy Why Separate TX and RX Antennas Transmission are Required Transmission Line as an Antenna Working of an Antenna Far And Near Fields Antennas Characteristics

Polarization Antenna and Wavelength Antenna Gain Antenna Length 44