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Understanding RFID Part 5: RF Characteristics

Chris Corum   ||   Dec 03, 2007  ||   ,

By Jerry Banks and Les G. Thompson, co-authors of RFID Applied

Under ideal conditions, the popular Alien Squiggle RFID tag can be read at a distance of approximately 20 meters. But what happens if it is placed behind a glass of water? What about placing it to the side, but adjacent to the glass of water? How about placing it in the water?

Answers to these and other questions will appear later, but suffice it to say that the readability of a tag is affected by the placement of the tag and its ambient environment.

When we discuss passive RFID tags with an audience of people that have never seen a tag, or, if they have seen one, they didn't know what it was, we say first that reading the tags is impacted by the release of radio frequency waves in the ambient environment. We ask for examples in the room in which we are making the presentation. We ask you, the reader, to ponder this question before reading the next paragraph.

Here is a hint: Typically, we are making a presentation using PowerPoint. So, there is a computer and a projection device involved. Both of those emit radio frequencies. How about the cell phones in every attendee's possession? What about the lights and the dimmer switches? So, even in an innocuous place like an auditorium, there are ambient sources of radio frequency that interfere with the very weak signal that a passive RFID tag can generate.

So far, you have some speculation that moisture has an impact on the Alien Squiggle tag. And, you have been told that ambient sources of radio frequency waves impact RFID tags, particularly passive tags that don't have a power source.

We aren't selling the Alien Squiggle tag. But, why are there so many different RFID tag designs? For that, we turn to the subject of frequencies.

For convenience sake, the entire RF spectrum has been segregated into bands of frequencies that tend to share common characteristics. Table 1 describes these classifications. You should notice that as the frequency of the wave increases, the length of the wave decreases.

In the previous article, "The black art of RFID antennas," we discussed how to construct a tag based on a target communication frequency, but we did not discuss why a company like Texas Instruments or Alien Technologies would create a suite of tag products, each tag targeting a different frequency. The simple answer is that radio waves at different frequencies interact with their environment differently.

Imagine that you are standing in a large open. If you were to sing a variety of notes, you might eventually sing a note that seems to fill the room with sound much more than the other notes. The note that you found is produced by a sound wave that has the appropriate wavelength to resonate perfectly in the room. This is why most people think that they can sing better in the shower. If you were to change the environment, the note required to produce a resonating sound would change. For instance, if a wood table was placed in the room, the note that resonated in the room before may not still resonate like it once did, and another note may be found that resonates in the room better than the previous note. Sound waves are analogous to radio waves in this respect.

In the world of RFID, the wood table in the previous example may be analogous to another type of material such as paper, water, metal, or cloth that can change the environment. Upon further examination, the previous example is more complex than it seems. Why did the resonant frequency change when the table was placed in the room? The answer is that the table impeded the propagation of the sound wave. There are hundreds of factors that could influence why the wave was impeded, but the two most common are that 1) the sound wave was absorbed by the table, or 2) the sound wave was reflected by the table which disturbed the other waves that were bouncing around the room. Like waves on a pond, sound and radio waves can cancel each other if they collide.

The two most common environmental conditions on the minds of RFID practitioners are water and metals such as iron, lead, and aluminum. The pharmaceutical industry is worried about water because many drugs contain some type of moisture. The manufacturing industry is concerned with metal because assembly lines are usually made of metal and the products may also be made of or contain metal.

Why is water such a problem for RFID tags? The truth is that water is not a problem as long as the correct frequency is chosen. Microwave ovens are tuned to the resonant frequency of water so that they can absorb the energy from the radio waves and heat up our food. The oven produces radio waves at the 2.45 GHz frequency (microwaves). These waves have a wavelength of 12.24 cm. As the waves pass through the water in the food, the water molecules rotate to align themselves with the wave. The molecules rotate with each wavelength. This oscillation causes the increase in temperature. The structure of water molecules is perfect for interacting with this frequency. Other wavelengths would not cause the water molecules to rotate. For RFID, the absorption of energy has a negative consequence unless it is being primarily collected by the antenna attached to an RFID tag.

The microwave example illustrates why choosing a frequency in one of the higher bands such as UHF or SHF would not be a good choice for applications of RFID near water where HF bands work better. The tradeoff with employing a lower frequency is that there is a decrease in the data transmission speed between the reader and tag as the frequency decreases. HF RFID tags are most often used in close proximity to water. These types of tags have coil type antennas, which are designed to work best at lower frequencies. For more information about RFID antennas, please refer to Part 4 in this article series. Some RFID tag manufacturers, like IPico, have created dual-frequency tags to combat these issues. As the name implies, dual-frequency tags transmit at two different frequencies. These types of tags can achieve higher transmission rates when communication is possible at a higher frequency, yet the tag can always be read, even when placed in a glass of water because it can transmit at a lower frequency. These tags are more robust and more expensive.

Experimenting with RF tags and water

With this knowledge we can answer the questions posed at the beginning of this article. The Alien 9540 Squiggle tag adheres to the EPC Gen 2 standards and communicates at a frequency of 915 MHz. From what we have learned about the effects of water on radio waves in the UHF band, we can deduce that the signal and energy will be attenuated by the water. The electromagnetic field required by the tag will weaken as the tag is moved closer to the water until the tag will no longer operate unless the reader is extremely close to the tag. The exact effect cannot be determined with respect to the reduction in read range for a tag when measured outside of a controlled environment such as a laboratory. It is certain, however, that if a tag is placed in the water there will be a significant reduction in its read range. Now, what if a tag worked at the lower frequency of 13.56 MHz in the HF spectrum? We can predict that the tag will operate better than the UHF tag, but a tag that operates at 125 KHz could be read at a much further distance if it was submerged fully in the water.

At our request, the Electro-Optical Systems Laboratory at the Georgia Tech Research Institute (GTRI) conducted an experiment using the Alien 9540 Squiggle tag. As shown in Table 2, their tests demonstrated that the tag is affected by water as we would expect.

The effect on a radio wave is also influenced by some types of metal with which the wave is interacting. There are many elements on the periodic table that are classified as metals. Most of them are not used on a day-to-day basis. This discussion will pertain to the more common metals that an RFID tag may come in contact with such as iron, aluminum, and copper. Ferrous metals, such as iron are often regarded as having the worst effect on electromagnetic radiation because they are, for the most part, magnetic. Non-ferrous metals, like aluminum and copper, are not magnetic and interact better with electromagnetic radiation. Not all ferrous metals are magnetic and vice versa.

Metal (the kinds mentioned above this qualification won't be repeated every time we say the word "metal") can affect radio waves in several different ways. First, radio waves cannot penetrate these metals. If RF waves cannot penetrate a metal, the metal is said to be opaque to radio waves. It is interesting that these metals do not need to be solid to completely stop a radio wave. RF engineers work in sterile environments known as a Faraday cage. The Faraday cage has walls made of highly conductive metal mesh or screen that have holes smaller than the RF wavelength being tested. If the holes are small enough and the metal is thick enough, all radio waves will be absorbed and distributed along the surface of the screen.

Metal can detune a radio wave. Detuning occurs when the amplitude and/or wavelength of the wave is skewed if the wave comes in contact with the metal. Once the wave is detuned, it cannot couple with the RFID tag. In addition to detuning, the RFID waves form miniature RF eddies where they intersect the metal. These eddies effectively cancel out the wave such that it either dissipates completely or the wave is impeded to the point that it cannot couple with the tag.

Metal may also absorb some of the radio wave. This is known as parasitic capacitance. Just like water, the metal diminishes the strength of the radio wave by absorbing some of its energy. In active RFID systems, where energy is abundant, the metal can become a conduit for the RF energy. It is not uncommon for a gas or water pipe to channel a radio wave down a hall into another room or to another floor of the building. These types of occurrences can be very challenging for active tag real-time location systems because active tags transmit with so much more energy (wattage) than passive tags do. Any metal objects such as pipes or handrails can become a secondary antenna for the active tag's transmissions.

Understanding the characteristics of RF can aid in the successful planning and implementation of an RFID solution. The basic physical principles of RF are a necessary tool in the RFID practitioner's tool belt. It is important to remember that real world environments are much different than RF labs. Passive RFID systems are much more susceptible to harsh RF environments than active RFID systems. Even so, dynamic environments can cause even the most robust RFID systems to stumble unless they are designed correctly.

About this article:

We would like to thank Gisele Bennett and her group of researchers at the Electro-Optical Systems Laboratory at the Georgia Tech Research Institute for conducting the water readability tests referenced above.

This article is the fifth in an ongoing series that explains the principles of RFID. It was created for RFIDNews by Jerry Banks, Tecnológico de Monterrey, Monterrey, Mexico and Les G. Thompson, Lost Recovery Network, Inc., Atlanta, Georgia. The authors are two of four co-authors of RFID Applied, John Wiley, 2007, ISBN-10 0471793655; ISBN-13 978-041793656.

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