Electromagnetic radiation in the frequency ranges used by RFID systems cannot be seen, heard, tasted, smelled, or touched, so you might be a little skeptical that electromagnetic radiation exists. For those unbelievers among you, you are invited to turn on your car radio or use your mobile phone to talk with someone on the other side of the world to experience the wonders of electromagnetic radiation. RFID, of course, uses electromagnetic radiation for communication and, in the case of passive RFID systems, for transferring power to the tags.
Because we cannot experience electromagnetic radiation directly, we must understand the physics of electromagnetic radiation to gain insight into how electromagnetic radiation works and how to make it work effectively for our RFID systems. Through study and the intuition that knowledge brings, we will be able to understand why various environments and environmental fixtures create better, or worse, RFID communication reliability. And, we will be able to correct any deficiencies that may be found.
In my book RFID for Dummies (Wiley Publishing, 2005), I spent a lot of time explaining the detailed physics of RFID. The reason is simple: if you can picture the invisible, solutions to complex problems become that much easier. In preparation for the CompTIA exam, I'll give you information about some of the physics surrounding RFID and help you get a clear picture in your mind of what is happening when you plug a reader into a socket and start making waves.
You are probably familiar with the many modern conveniences that utilize electromagnetic radiation, but have you ever wondered why the particular radiation used by these devices-radios and cell phones, for example-are called "electromagnetic"? The simple answer is that the radiation consists of both an electric field and a magnetic field.
Quite a few movies contain a scene in which the bad guy tries to kill the hero in an agonizingly slow death by using a scrap yard and a car crusher. Well, the big crane with the magnet attached to it is doing some of what an RFID reader does. It is using electricity to charge a piece of metal with a magnetic signal. In this case, it's strong enough to pick up a car.
We are all familiar with magnets and the magnetostatic, or static magnetic, fields that they generate. Magnetic fields flow from the north pole of the magnet, out into space, and back into the south pole of the magnet. The magnetic field describes a volume of space near the magnet, where a change in energy attributable to the magnet can be detected. Magnetic fields are strongest close to the magnet and diminish in strength as one moves away from the magnet. Magnetic fields generated by magnets are static power storage fields. They do not vary over time. This stasis lets your refrigerator magnet firmly hold your child's artwork on the refrigerator without it falling off periodically.
Magnetic fields do interact with moving electric charges. The magnetic field will change the direction of motion of a charged particle, but it cannot change its speed. Thus, a current-carrying wire experiences a force upon it when it is placed within a magnetic field. Time-varying magnetic fields, possibly caused by physically moving a magnet closer to a wire, will perform work upon a charge, thereby creating an electric current within a wire. We will examine this interrelation between dynamic electric and magnetic fields when we investigate Maxwell's equations later in this chapter.
Magnets are not the only sources of magnetic fields. Electric currents flowing through wires also generate magnetic fields. The strength of the magnetic field around the wire is proportional to the current carried by the wire. Just as with magnets, the magnetic fields generated by flowing currents are strongest near the wire and diminish in strength as one moves away from the wire. Unlike with magnets, the magnetic fields created by electric currents do not flow into and out of the wire carrying the electric current. Instead, they flow around the wire in concentric circles. The direction of the magnetic field generated by an electric current is easily determined by using the right-hand rule. If you grasp the electric current-carrying wire in your right hand with your thumb pointing in the direction of the current, then your fingers will circle the wire in the direction of the magnetic field.
The strength of the magnetic field that is created by electric currents carried in a wire can be increased by forming the wire into a loop or coil (that is, multiple overlapping loops). The magnetic field created by each loop of wire combines with the fields from the other loops to produce a concentrated magnetic field in the center of the coil. The magnetic field generated by a solenoid, a tightly constructed coil, is similar to that of a bar magnet. The magnetic field is relatively uniform and of high strength within the center of the coil. The magnetic field flows from the north pole of the coil (this can be determined by using the right-hand rule with the fingers pointing in the direction of the north pole) out into space and returns to the south pole of the coil.
Magnetic fields generated by electric currents vary as the current through the wire varies. Thus constant, or direct current (DC), electric currents generate static magnetic fields (in the steady state) just as do magnets. Alternating electric currents, which is to say time-varying currents with a constant period, or frequency, such as that used to power the lights in our homes and to communicate information across telecommunication wires, generate time-varying magnetic fields that follow the periodicity of the electric currents. Time-varying magnetic fields, in turn, generate time-varying electric currents that follow the periodicity of the magnetic fields. In this alternating fashion, the electrical energy flowing through the wire is transformed into a series of alternating magnetic fields and electric fields that are radiated into space.
By utilizing alternating electric currents, coils and many other shapes of metals become efficient radiating elements that send electromagnetic radiation into our world, which is when they become far field.
Although a coil will radiate all frequencies based on the frequency of the electric current traveling through it, it does not radiate all frequencies equally well. The coil's resonant frequency is the frequency at which it most efficiently generates magnetic fields. When viewed as an antenna, a coil is a tuned inductance (L)-capacitance (C), or simply LC, circuit. The antenna is at resonance when the inductive impedance is equal to the capacitive impedance for a particular frequency. The resonant frequency f is then f = 1/(2π) . Consequently, low frequencies such as 125 kHz require a large number of loops to achieve proper resonance, whereas high frequencies such as 13.56 MHz require few loops to achieve resonance. Large-diameter coils also require fewer loops because of the natural radio LC (RLC) within the metal wires.
Thus far we have investigated primarily the power storage fields (that is, magnetic fields) generated by electromagnetic radiation, but we cannot ignore the power propagation fields that result from this radiation. RFID systems operating in the low-frequency (LF) and high-frequency (HF) ranges couple to the power storage fields generated by the reader's antenna. RFID systems operating at higher frequencies such as ultra-high frequency (UHF) couple to the power propagation fields (that is, the electromagnetic waves) that result from the RFID reader's radiation.
Just as you learned in high-school physics that light rays can be modeled as a sinusoidal wave traveling in a straight line, electromagnetic radiation in the power propagation field (that is, the far field) can be modeled as electromagnetic waves that travel in straight lines. Electromagnetic waves are a model of how electromagnetic radiation travels in the far field (the region beyond the near field).
An electromagnetic wave carries energy from one point to another with a velocity equal to the speed of light in a vacuum, c= 3 × 108 meters per second (m/s). Electromagnetic waves exhibit a property called linearity. Linear waves do not affect the passage of other waves as they intersect. Thus, the total of two linear waves at their intersection is simply the sum of the two waves as they would exist separately.
The electromagnetic waves utilized in RFID systems are continuous harmonic transverse electromagnetic (TEM) waves. Continuous harmonic waves are typically sinusoidal in nature; thus, they are characterized by frequency, amplitude, and phase. They are also characterized by their three-dimensional shape. For RFID systems, linear waves are common, with their polarization used as the primary shape characteristic. TEM waves are characterized by having electric and magnetic fields that are transversal to the direction of the wave's propagation.
An electromagnetic wave consists of two sinusoidal signals traveling in perpendicular planes at the same frequency (f), with one signal corresponding to the electric field (E) and one signal corresponding to the magnetic field (H). The line corresponding to the intersection of the two planes defines the direction of travel of the electromagnetic wave. At every point along this line, the ratio of the amplitudes of the electric field and the magnetic field is a constant equal to the characteristic impedance of the medium the wave is traveling through.
The amplitude of an electromagnetic wave is proportional to the energy being propagated by the wave. As the electromagnetic wave travels through a medium which creates signal loss, such as air, some of the energy being propagated by the wave is absorbed by the medium. This absorption diminishes, or attenuates, the amplitude of the electromagnetic wave. Thus, the farther a wave travels away from its source, the lower is its amplitude. The attenuation factor is a characteristic of the medium through which an electromagnetic wave travels.
The polarization of the electromagnetic wave is defined by convention by the motion of the electric field (E). If the motion of the electric field is confined to two dimensions (that is, a plane), linear polarization results. If the motion of the electric field is allowed to be spread in three dimensions, the polarization of the wave is defined by the path the electric field takes. Circularly polarized waves have electric fields that follow a corkscrew shape as they propagate forward in time. When looked at head-on, this corkscrew collapses into a circle, hence the name.
Remember that electromagnetic waves are simply models for the power propagation fields. When we remember that the electromagnetic wave is propagating energy away from an antenna, we understand that polarization impacts propagation distance. Circularly polarized electromagnetic waves spread their energy over three-dimensional space, whereas linearly polarized electromagnetic waves spread their energy over a plane. Because the linearly polarized electromagnetic wave maintains its energy over a smaller volume at each point in space, it is able to be detected at a greater distance than a circularly polarized electromagnetic wave emitted with the same amount of energy. The total energy propagated by each wave is the same; however, the contours of the volumes through which the energy propagates differ.
Polarization of the waves becomes important in tag antenna designs and deployments. Antennas may be designed such that they efficiently capture and communicate with energy in one or a few different polarizations. If a reader antenna is linearly polarized and the tag antenna is linearly polarized, then the tag and the reader may communicate only when both antennas are oriented in the same linear direction. Circularly polarized antennas reduce the orientation requirements, but do not completely eliminate the orientation dependence for optimal performance.