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