Lesson Title:
Electromagnetics and Antenna
Overview
Copyright © 2008 by Dale R. Thompson {d.r.thompson@ieee.org}
Dale R. Thompson
Computer Science and Computer Engineering Dept.
University of Arkansas
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).
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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)/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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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 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/
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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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