In this chapter, you learned about the basic theory of electromagnetic radiation. Intuitively understanding how electromagnetic radiation behaves, both in the near field and the far field, particularly around materials commonly found in your environment, is critical to quickly and easily understanding the performance experienced by your RFID installation even before the readers are installed.
In the first section, I showed you the wonders and basic characteristics of electromagnetic radiation. You learned that electromagnetic radiation comes in two primary types of concern for RFID systems: near-field dynamic magnetic fields and electromagnetic waves. The magnetic fields are energy storage fields, whereas the electromagnetic waves are energy propagation fields.
You discovered that magnetic fields are created by fixed magnets as well as electric current traveling through a wire. The right-hand rule may be used to determine the direction of the magnetic field generated by electric currents. You learned that loops of wire increase the strength of the magnetic field within the loops, or coils. Small-diameter coils have higher magnetic field strengths within them than do larger-diameter coils when they are both tuned to the same frequency.
You also learned that dynamic, or time-varying, electric currents create dynamic, or time-varying, magnetic fields. And you learned that dynamic, time-varying magnetic fields create dynamic, time-varying electric fields by exerting force upon charges located within the magnetic field. By using time-varying currents, coiled loop reader antennas are able to induce currents within coiled loop tag antennas. When the tag is sufficiently close to the reader antenna, the induced current is sufficient to power the operation of the tag.
I discussed that electromagnetic waves result from time-varying electric currents and have numerous properties including frequency, amplitude, phase, and polarization. You also learned that they are linear, allowing for both constructive and destructive interference at a point in space without destroying the intersecting waves.
You learned that electromagnetic waves have both an electric and a magnetic component that are perpendicular to one another. Tags may couple to either or both of these components in varying degrees to harvest their operating power.
You learned that all materials attenuate the amplitude of electromagnetic waves as they propagate through them. You also learned that electromagnetic waves are reflected to greater and lesser extents at every surface boundary.
After showing you the basic characteristics of electromagnetic radiation, I discussed the Complete Laws of Electrodynamics as described by Maxwell's equations. You learned that Maxwell's equations describe the interrelationship between dynamic electric fields and dynamic magnetic fields. These fields are not interrelated in the static case.
Electric fields tend to emanate from charge sources, similar to shrapnel from hand grenades. You also saw that magnetic fields form closed loops, similar to the lines of creamer formed by a tempest in a teacup. You learned that conducting surfaces attract electric fields and that conducting surfaces inhibit the flow of magnetic fields near them.
Finally, you learned that although Maxwell's equations describe the behavior of electromagnetic waves regardless of their frequency, multiple frequencies are allowed by regulations and used by the various RFID systems because they exhibit significantly different functional characteristics across the gigahertz of frequency diversity within the radio frequency spectrum.