TABLE OF CONTENTS In Chapter 3, we learned that an antenna is a device to produce a distribution of currents and charges that do not cancel when observed from far away. We also introduced several properties of antennas that bear on their utility in an RFID system:
Gain and radiation pattern: the extent to which the power radiated from an antenna is concentrated in some directions in preference to others;
Effective aperture: the equivalent area from which a receiving antenna collects energy;
Polarization: the orientation of the electric field radiated by the antenna.
There are three other key parameters that we got to ignore when thinking about link budgets, but which become very important when we need to hook a reader to an antenna:
Impedance: how much voltage is required to cause a given current to flow in the antenna?
Bandwidth: over what frequency range does the impedance remain reasonably constant?
Size and cost: what we give away to get small and cheap.
In this chapter, we'll look at how we get the gain and polarization we want, and how antenna impedance and bandwidth must be traded against antenna size. Armed with these fundamentals, we can examine the requirements for different reader applications and see how they map to preferred antenna types. We will touch upon how antennas affect implementation: how to ensure that the beam covers the tags, what polarizations and orientations to use, and something about the specialized connectors...
