# Lesson Title: Electromagnetics and Antenna Overview Dale R. Thompson Computer Science and Computer Engineering Dept. University of Arkansas

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Lesson Title: Electromagnetics and Antenna Overview Dale R. Thompson Computer 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. http://rfidsecurity.uark.edu 2

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 http://rfidsecurity.uark.edu 3

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) http://rfidsecurity.uark.edu 4

Phase Lag http://rfidsecurity.uark.edu 5

Electromagnetic Radiation Antennas with a periodic signal create electromagnetic radiation Two types of electromagnetic radiation – Near field – Far field http://rfidsecurity.uark.edu 7

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 http://rfidsecurity.uark.edu 8

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 http://rfidsecurity.uark.edu 9

Approximating Boundary Between Near and Far Field Case 1: If antenna size is comparable to the wavelength (like UHF), r = 2f(d^2)/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) http://rfidsecurity.uark.edu 10

Near-field/Far-field Boundaries BandDistance (meters)Distance (feet) LF3821146 HF3.511 UHF0.160.5 http://rfidsecurity.uark.edu 11

Periodic Signal Voltage v(t) = v o cos(ωt) ω = 2*pi*f http://rfidsecurity.uark.edu 12

Power in Direct Current P = VI V = IR P = V^2/R http://rfidsecurity.uark.edu 13

Power of Periodic Signal P avg = V o 2 /(2R) V o = peak voltage http://rfidsecurity.uark.edu 14

Power of Periodic Signal Root-mean-square (RMS) voltage V rms = V o /sqrt(2) P avg = V rms 2 /R http://rfidsecurity.uark.edu 15

Decibels (dB) Useful to describe signals with power spectrum (Power vs frequency) Signal power ranges from 10 -15 to 10 2 watts Logarithmic notation: 10 log(x) = x G dB = 10log 10 (P out /P in ) G dB = 20log 10 (V out /V in ) http://rfidsecurity.uark.edu 16

Absolute Power dBm is absolute power with reference to a milliwatt dBm = 10log 10 (P/(1 mW)) dBW = 10log 10 (P/(1 W)) http://rfidsecurity.uark.edu 17

Isotropic Antenna Assume antenna radiates same power density in all directions http://rfidsecurity.uark.edu 18

Antenna Gain Focus energy is a particular direction Power gain above isotropic antenna or a dipole antenna http://rfidsecurity.uark.edu 19

Half-Wave Dipole 2.2 dB gain above an isotropic antenna (2.2 dBi) http://rfidsecurity.uark.edu 20

Dipole Pattern http://rfidsecurity.uark.edu 21

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 = P TX (dBm) + G TX (dBi) http://rfidsecurity.uark.edu 22

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 = P TX (dBm) + G TX (dBd) dBi = dBd + 2.2 http://rfidsecurity.uark.edu 23

Linear Polarization http://rfidsecurity.uark.edu 24

Mismatched Polarization http://rfidsecurity.uark.edu 25

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! http://rfidsecurity.uark.edu 26

Bistatic configuration One reader antenna is used for transmitting and a different antenna is used for receiving http://rfidsecurity.uark.edu 27

Monstatic configuration The same reader antenna is used for both transmitting and receiving http://rfidsecurity.uark.edu 28

Tag Antennas http://rfidsecurity.uark.edu 30

Contact Information Dale R. Thompson, Ph.D., P.E. Associate Professor Computer Science and Computer Engineering Dept. JBHT – CSCE 504 1 University of Arkansas Fayetteville, Arkansas 72701-1201 Phone: +1 (479) 575-5090 FAX: +1 (479) 575-5339 E-mail: d.r.thompson@ieee.org WWW: http://comp.uark.edu/~drt/ http://rfidsecurity.uark.edu 31

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. http://rfidsecurity.uark.edu 32

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