Changing currents are required


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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.

  • 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.



T = period, time for one cycle

  • 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


Phase: relative timing of two signals

  • 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)







Antennas with a periodic signal create electromagnetic radiation



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

  • 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



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.

  • 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



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

  • 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)




v(t) = vocos(ωt)

  • v(t) = vocos(ωt)

  • ω = 2*pi*f



P = VI

  • P = VI

  • V = IR

  • P = V^2/R



Pavg = Vo2/(2R)

  • Pavg = Vo2/(2R)

  • Vo = peak voltage



Root-mean-square (RMS) voltage

  • Root-mean-square (RMS) voltage

  • Vrms = Vo/sqrt(2)

  • Pavg = Vrms2/R



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

  • 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)



dBm is absolute power with reference to a milliwatt

  • dBm is absolute power with reference to a milliwatt

  • dBm = 10log10(P/(1 mW))

  • dBW = 10log10(P/(1 W))



Assume antenna radiates same power density in all directions

  • Assume antenna radiates same power density in all directions



Focus energy is a particular direction

  • Focus energy is a particular direction

  • Power gain above isotropic antenna or a dipole antenna



2.2 dB gain above an isotropic antenna (2.2 dBi)

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





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

  • 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)



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

  • 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







Electric field rotates as a function of time around direction of propagation

  • 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!



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

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



The same reader antenna is used for both transmitting and receiving

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







Dale R. Thompson, Ph.D., P.E.

  • 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/



Copyright Notice

  • 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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