lesson title: electromagnetics and antenna overview dale r. thompson computer science and computer...

Lesson Title: Electromagnetics and Antenna

Overview

Dale R. ThompsonComputer Science and Computer Engineering Dept.

University of Arkansas

http://rfidsecurity.uark.edu 1

This material is based upon work supported by the National Science Foundation under Grant No. DUE-0736741.

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).

Copyright © 2008 by Dale R. Thompson {d.r.thompson@ieee.org}

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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Electromagnetic Radiation

• Antennas with a periodic signal create electromagnetic radiation

• Two types of electromagnetic radiation– Near field– Far field

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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/(r^3) 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(d^2)/cd = maximum antenna dimensionf = frequencyc = 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 BoundariesBand Distance (meters) Distance (feet)

LF 382 1146

HF 3.5 11

UHF 0.16 0.5

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Periodic Signal Voltage

• v(t) = vocos(ωt)

• ω = 2*pi*f

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Power in Direct Current

• P = VI• V = IR• P = V^2/R

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Power of Periodic Signal

• Pavg = Vo2/(2R)

• Vo = peak voltage

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Power of Periodic Signal

• Root-mean-square (RMS) voltage• Vrms = Vo/sqrt(2)

• Pavg = Vrms2/R

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Decibels (dB)

• Useful to describe signals with power spectrum (Power vs frequency)

• Signal power ranges from 10-15 to 102 watts• Logarithmic notation: 10log(x) = x• GdB = 10log10(Pout/Pin)

• GdB = 20log10(Vout/Vin)

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Absolute Power

• dBm is absolute power with reference to a milliwatt

• dBm = 10log10(P/(1 mW))

• dBW = 10log10(P/(1 W))

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Isotropic Antenna

• Assume antenna radiates same power density in all directions

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Antenna Gain

• Focus energy is a particular direction• Power gain above isotropic antenna or a

dipole antenna

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Half-Wave Dipole

• 2.2 dB gain above an isotropic antenna (2.2 dBi)

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Dipole Pattern

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Effective Isotropic Radiated Power (EIRP)

• Power required if using an isotropic antenna to get the same power as the power from the main beam of a directional antenna

• Includes transmitter power and gain of antenna

• EIRP = PTX (dBm) + GTX (dBi)

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Effective Radiated Power (ERP)

• Power required if using a half-wave dipole antenna to get the same power as the power from the main beam of a directional antenna

• Includes transmitter power and gain of antenna

• ERP = PTX (dBm) + GTX (dBd)

• dBi = dBd + 2.2

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Linear Polarization

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Mismatched Polarization

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Circular Polarization• Electric field rotates as a function

of time around direction of propagation

• Orientation of electric field varies with time

• Right-hand polarization (RHP)• Left-hand polarization (LHP)• Common for reader antenna to

use circular polarization and the tag to use linear so that the system is less sensitive to tag orientation!

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Bistatic configuration

• One reader antenna is used for transmitting and a different antenna is used for receiving

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Monstatic configuration

• The same reader antenna is used for both transmitting and receiving

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Reader Antennas

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Tag Antennas

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Contact InformationDale R. Thompson, Ph.D., P.E.Associate ProfessorComputer Science and Computer Engineering Dept.JBHT – CSCE 5041 University of ArkansasFayetteville, Arkansas 72701-1201

Phone: +1 (479) 575-5090FAX: +1 (479) 575-5339E-mail: d.r.thompson@ieee.orgWWW: http://comp.uark.edu/~drt/

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Copyright Notice, Acknowledgment, and Liability Release

• Copyright Notice– This material is Copyright © 2008 by Dale R. Thompson. It may be freely redistributed in its entirety

provided that this copyright notice is not removed. It may not be sold for profit or incorporated in commercial documents without the written permission of the copyright holder.

• Acknowledgment– These materials were developed through a grant from the National Science Foundation at the

University of Arkansas. Any opinions, findings, and recommendations or conclusions expressed in these materials are those of the author(s) and do not necessarily reflect those of the National Science Foundation or the University of Arkansas.

• Liability Release– The curriculum activities and lessons have been designed to be safe and engaging learning

experiences and have been field-tested with university students. However, due to the numerous variables that exist, the author(s) does not assume any liability for the use of this product. These curriculum activities and lessons are provided as is without any express or implied warranty. The user is responsible and liable for following all stated and generally accepted safety guidelines and practices.

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