Tutorial: Part I, Part II, Part III, Part IV,Part V, Part VI and Part VII

Capsule Summary: Chapter 3
Electrical currents and charges radiate, but the net effects usually cancel; an antenna is a special structure arranged to avoid such cancellation and create electromagnetic waves from electrical currents and voltages. These waves are usually periodic in nature and characterized in terms of sines and cosines, or complex exponential functions. They are converted into voltages and currents in electrical circuits. The size of the voltages and related power varies over a large range, so power and gain are usually measured logarithmically, using dBm and dB.

Wave must be modulated in order to transmit information. When a signal is modulated, the width of its frequency spectrum increases. Modulations used for RFID readers are constrained by the need to provide power to passive tags and are thus profligate users of spectrum relative to the amount of information transmitted.

The currents induced in tag antennas, like other uncompensated currents, radiate, leading to backscattered waves. The load connected to the antenna can be varied to change the amount of induced current and thus the backscattered wave, enabling a tag to communicate with a reader even though t has no transmitter. This reflected signal adds to other, large reflections from the system and ambient, so there is no simple relationship between the tag state and reader signal; thus, tag modulations are all variants of frequency-shift keying.

The amount of power needed to turn on a tag IC is the main limit on the range of passive tags. Directional antennas increase the power that reaches tags in the main beam of the antenna, but regulations limit the transmitter power and antenna gain that can be used, so the range in air is typically only a few meters. Radiation from antennas is polarized, and if the polarization from the reader antenna does not agree with the polarization the tag can receive, the power received is reduced, and the tag may not be read.

In realistic environments, propagation is greatly complicated by reflections from surfaces as well as diffraction around obstacles, leading to local fading and requiring that tags and reader be moved in some fashion to ensure that all the tags have a chance to be read.

Further Reading
Signal and Signal Processing
"Digital Modulation and Coding", S. Wilson, Prentice-Hall 1996. For the serious student; the fundamentals of signal modulation and detection, developed with considerably more rigor than we have used here.

Backscatter Links
"Communication by Means of Reflected Power", Harry Stockman, Proc I.R.E., October, 1948, p. 1196

Antennas
Antenna Theory (3rd Edition), C. Balanis, Wiley 2005
Antenna Theory and Design (2nd Edition), W. Stutzman and G. Thiele, Wiley 1997
Antennas (3rd Edition), J. Kraus and R. Marhefka, McGraw-Hill 2001
"RF Engineering for Wireless Networks", D. Dobkin, Elsevier 2004, Chapter 5; see also p. 350 for references covering the microwave properties of common construction materials.

Reflection from Dielectric Surfaces
"Physics of Waves", W. Elmore and M. Heald, Dover 1985, Chapter 8
"Classical Electricity and Magnetism (2nd Edition)", W. Panofsky and M. Philips, Addison Wesley 1962, Chapter 11

About the Author
Daniel Dobkin is an RFID consultant, writer and teacher. He holds six patents as inventor or co-inventor. He is the author of such books as: Principles of Chemical Vapor Deposition and RF Engineering for Wireless Networks, and The RF in RFID. Additionally, he is a published author of 25 technical publications. He has taught RFID courses internationally in Singapore for the SMa/RFID Focus; and domestically at SDForum, Mitre Corporate University, and San Jose State University. Daniel is a Stanford University PhD in Applied Physics. He has MS and BS degrees from CalTech.

Printed with permission from Newnes, a Division of Elsevier. Copyright 2007. "The RF in RFID: Passive UHF RFID in Practice by Daniel M. Dobkin. ISBN-10: 0750682094For more information about this title and other similar books, please visit www.elsevierdirect.com.