Propagation and Interference of Waves
Electromagnetic waves are more than just a convenient model for electromagnetic energy propagation. They are an accurate model of how the energy propagation field behaves. As with all waves, electromagnetic waves interact with one another whenever they intersect at a point in space. Depending on the phase, amplitude, and polarization, intersecting waves may either constructively interfere or destructively interfere. This is one of the basic properties of linear waves. The observed wave at a point of intersection is the addition of all of the waves at that point. Constructive interference increases the amplitude of the detectable wave at that point. Destructive interference decreases the amplitude of the detectable wave.
Wireless communication within an indoor environment is often plagued by prolonged deep fades that degrade and possibly prohibit communication between the transmitter and the receiver. The electromagnetic waves radiated by the transmitter undergo multiple reflections and diffractions through an often highly cluttered environment. Every object in the environment both absorbs and reflects electromagnetic waves. Conductors, such as metals, reflect the electromagnetic waves at UHF frequencies with little loss to the wave's energy. Other, non-conducting materials, such as cardboard, also reflect electromagnetic energy incident upon it, although typically at much reduced energy.
Fundamental physics, through the Uniform Theory of Diffraction, tells us that at every boundary between two materials, electromagnetic waves incident upon that boundary will be both transmitted from one material to the other and reflected back into the material in which they are traveling. Conducting materials, such as metals, act similar to perfect reflectors for UHF radiation. Materials such as glass, concrete, and cardboard are effectively RF transparent for waves that are incident upon them with an angle of incidence of 90 degrees, but they become less transparent as the angle of incidence becomes more oblique.
Some materials, such as water, act as both good reflectors of electromagnetic waves and good attenuators, or absorbers, of electromagnetic energy. The partial reflection of a wave results in the energy of the wave being separated to traverse multiple paths. The result is that a partial reflection attenuates the partially transmitted wave by the amount of energy reflected at the boundary.
By passing through several materials and being reflected by several more, an electromagnetic wave traverses a path through the environment. In addition to attenuating the wave as it travels through the environment, the environment may impact the polarization of the wave. Two long parallel metal strips separated by a few inches, for example, will filter the UHF waves that are incident upon them by allowing waves that are polarized parallel to the metal strips to pass through the space between the strips while waves polarized perpendicular to the metal strips will be reflected.
When two waves that have traversed different-length paths intersect at a point, they will be out of phase with one another. The phase difference is due to differences in the time required to traverse the different paths. Most phase differences cause destructive interference and may cause the observed wave at a point to appear to have a different frequency than what was originally transmitted.
Electromagnetic waves are linear, meaning that the wave experienced at a point in space and time is the sum of the waves that intersect at that point. Because of reflections, attenuation, and different path lengths caused by objects in the environment, the waves that arrive at a point in space may have an amplitude that sums to zero or nearly zero. Passive RFID tags are not able to harvest sufficient operating power from low-amplitude, hence, low-power, locations. When these near-zero amplitude locations are surrounded by much-higher amplitude locations where passive RFID tags are able to operate, the low-amplitude location is called a null. The position of nulls may be changed or the nulls may be eliminated by changing the position of the objects in the environment or changing the frequency being radiated by the antenna. When the environment is static, standing waves may result. This phenomenon can be described as a standing wave or a null . The most common occurrence is when the two waves intersect each other exactly half a wavelength out of phase and completely cancel the signals. This creates the null spot where a tag would not be read.
Real World Scenario-Environmental Coincidences
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In Chapter 3, "Site Analysis," you'll learn how to perform an analysis of the interrogation zone before actually setting up a reader. This is referred to as creating a path loss contour map (PLCM) that shows how RFID will react when put in a very specific location. Without a foundation in physics, following the PLCM procedure will leave you scratching your head if you come across a problem.
One of the ODIN technologies engineers was on-site at a client's facility performing a series of PLCMs when he found that one of the interrogation zones had a significant null (RF dead spot), and tags would not be read if a reader was set up at that location. He had to find out why. After looking around the area and moving some RF-transparent items (cardboard boxes), he uncovered ladder racks for data cables that measured 12˝ × 12˝ for each step. Because he was an expert in RF physics, he knew that 12˝ is about 33 cm, which is almost exactly the wavelength of an RF wave at 915 MHz. The racks were acting like a sponge, sucking in the RF energy. Moving them out of the area resulted in a perfect interrogation zone location. Lots of headaches and frustration were saved by knowing the physics of RFID.
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James Clerk Maxwell was the first to correctly assemble the complete laws of electrodynamics in his classic treatise in 1873. Modern electromagnetism theory is based on the four fundamental equations known as Maxwell's equations.
Before Maxwell, the laws of electrodynamics, including Gauss's Law, Ampere's Law of Magnetostatics, and Faraday's Law, were laws of electrostatics, and did not predict waves. These laws correctly described what is known as the near field (that is, the electrostatic field of an electric charge and the magnetostatic field of a current loop). These laws described the observable impact of electric charges and magnetic fields close to the source but failed to describe the distant impact of these forces.
In the static case, when all electric charges are permanently fixed or if they all move at a steady state, the electric field and the magnetic field are not interconnected. This allows us to study electricity and magnetism as two distinct and separate phenomena.
Up until Maxwell challenged conventional wisdom the separation of electricity and magnetism was the accepted state of the world. He corrected Ampere's Law of Magnetostatics to become Ampere's Law as corrected by Maxwell, so that consistency with the Law of Conservation of Charge now occurred. Maxwell added a term indicating that vortices of magnetic fields can be displacement current density (time-varying electric flux density) as well as conduction current density. The resulting corrected equations define the complete laws of electrodynamics and predict electromagnetic waves. Heinrich Rudolf Hertz confirmed experimentally that these waves exist.
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The Complete Laws of Electrodynamics
The Complete Laws of Electrodynamics define the relationship between the electric field quantities and the magnetic field quantities. These quantities are the electric field vector (E), the electric flux vector (D) (where E = εD and ε is the electrical permittivity of the material), the magnetic field vector (H), and the magnetic flux vector (B) (where H = μB and μ is the magnetic permeability of the material). The Complete Laws of Electrodynamics are as follows:
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Faraday's Law Faraday's Law states that any magnetic field which is changing in the time dimension creates an equal change in the electromotive force. To be more specific the circulation of the electric field vector (E) around a closed contour is equal to minus the time rate of change of magnetic flux through a surface bounded by that contour. This only holds true if the positive direction of the surface is related to the positive direction of the contour by the right-hand rule.
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Ampere's Law as Modified by Maxwell The circulation of the magnetic field vector (H) around a closed contour is equal to the sum of the conduction current and the displacement current passing through a surface bounded by that contour. Again, the right-hand rule is what dictates this behavior in relation to the contour and the surface.
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Gauss's Law for the Electric Flux The total electric flux (defined in terms of the D vector) emerging from a closed surface is equal to the total conduction charge contained within the volume bounded by that surface.
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Gauss's Law for the Magnetic Flux The total magnetic flux (defined in terms of the B vector) emerging from any closed surface is zero.
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Maxwell's Equations With the aid of Gauss's and Stokes' laws of mathematics and the definitions
D = ε0 E + P and B = μ0(H+M)
the complete laws of electrodynamics may be expressed, when the fields are spatially continuous, in the familiar differential form
where J is the current density per unit area and ρ is the electric charge density per unit volume. These equations hold in any material and at any spatial location.
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The equations I've covered in this chapter can make physics your best friend in deploying a 100% accurate RFID network. There is one other principle that is helpful to know and that's called the dielectric effect. A dielectric is any material that is resistant to passing along an electric current. Since passive RFID tags are trying to gather electricity from a reader's field the material they are affixed to will dramatically effect how they perform. For example Lexan is a clear plastic material that is RF transparent but if you place a tag on it the dielectric properties of the Lexan detunes the tag enough to degrade its performance. A tag affixed to concrete will be detuned enough to render it useless because of the dielectric properties of concrete. The big reason behinds this is twofold-first the electric charge decreases as it passes through any dielectric material and second the velocity of the wave changes and the RFID wave will behave as if it had a shorter wavelength.
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