Understanding RFID Part 1: Hardware Basics
By Jerry Banks and Les G. Thompson, co-authors of RFID Applied
The most basic radio frequency identification solution is made up of three main hardware components. These components are the RFID tag, the RFID reader, and the antenna. This is, of course, an over simplification of what it takes to apply today’s RFID technology to a real world problem, but these are the fundamental building blocks. Understanding the fundamentals of RFID is the key that allows practitioners to be successful in their application of the technology. Even though this article does not discuss the software required to interpret and make use of the RFID data, its role in a complete RFID solution is vital.
The components of the basic RFID tag are an integrated circuit (IC), an antenna, and the substrate that holds it all together. The IC is responsible for controlling the tag; much like a CPU controls a desktop computer. The IC controls what is broadcast from the tag, processes commands received from the reader via the antenna, and manages any peripherals such as temperature and pressure sensors. The antenna plays multiple roles in most RFID tags. It is responsible for receiving and transmitting data from and to the reader, and, in the case of passive type RFID tags, they collect the energy required to power the tag. Passive tags power themselves off of the energy they collect from high gain antennas that are connected to the RFID reader; therefore, they must be in close proximity to the RFID reader’s antenna in order to collect enough energy to function.
RFID tags with onboard batteries are known as active tags. Unlike passive tags, they transmit their data even when they are not in close proximity to an RFID reader. In most cases, active tags can be read at a longer distance than passive tags. There is a hybrid tag known as the semi-active tag. It has an onboard battery just like the active tag, but it will only transmit when it is in close proximity to the reader.
RFID tags may transmit many different pieces of data, but the most fundamental piece of data is the tag’s unique identifier. The unique identifier is, in most cases, associated with a real world asset that is to be tracked. The unique identifier is used as the key that identifies information about an asset in a database in most applications. Tags may also transmit state information or telemetry such as temperature or humidity if they have the sensors to collect this type of information. Most passive tags do not have peripheral functionality due to the power limitations of not having an onboard battery.
The RFID reader is sometimes referred to as the interrogator. The reader receives all of the data that the tags are transmitting. The data is then passed on to software that makes use of the data. The tags that are in close enough proximity to a reader are referred to as the reader’s “tag population.” As a reader’s tag population grows, the density of tags around the reader also grows, and the reader may require more time to read all of the tags in its vicinity. This is due to the fact that if all the tags transmit at the same time, the reader will not be able to separate their data into discreet transmissions, so it is important that the tags do not transmit all at once.
Passive tag readers select subsets of the population to query over time until beacons from all of the tags in the population have been received. Most active tag readers do not control the sampling of the tag population like passive readers do. Active tags beacon at a pseudorandom interval to avoid transmission collision with other tags. Anti-collision algorithms such as the ALOHA algorithm determine when the tag will beacon. The ALOHA algorithm assigns transmission time slots to each tag. The name ALOHA is not an acronym, but was given its name because it was developed at the University of Hawaii. The ALOHA algorithm is a common anti-collision algorithm that is used by many RF applications, not only RFID. Over time, the randomization of the tag transmissions will ensure that the transmissions from all the tags are eventually received. There exists a threshold where the tag density is so great that it cannot be guaranteed that all the tags will be sampled in a timely manner. The tag density maximum is different for each RFID tag and reader manufacturer. Some manufacturers even allow the anti-collision algorithm to be changed based on the needs of the solution.
The importance of the antenna that is connected to the reader cannot be underestimated. In a passive RFID solution, the antenna must be sensitive enough to receive the RFID tag transmissions and it must also be powerful enough to power the tags. Passive tag reader antennas may be deployed in many different configurations depending on the application. A portal configuration is the most common type. Portals place an antenna on each side of the tag’s path (i.e., at a loading dock door or on an assembly line). Sometimes, a portal configuration may also affix antennas on the top and bottom of the pathway to completely surround the tag’s path, thus increasing the chances of reading the tag as it passes through the portal.
Antennas used in active tag applications must solve a different set of problems. Many times, active tags are used in a real-time location system (RTLS). An RTLS is used to track tagged assets as they move through a building, yard, or supply chain. Active reader antennas are usually installed in the middle of the desired coverage area. For example, an antenna could be placed in the ceiling in the middle of a room. This antenna could then read all of the tags in the room. Because of the increased transmission power of most active RFID tags, when compared to passive tags, the antenna may also read tags outside of the room. Transmissions from tags in adjacent rooms, hallways, or in a room immediately above the antenna in a multistory building may be inadvertently received by the antenna. This is known as “bleeding coverage.” Most RTLS’s require that the coverage be well defined to a single room or to a zone within a room. To resolve this issue, the correct antenna must be selected that meets the needs of the RF environment. Antennas must provide smooth and consistent input to the RFID reader in order for it to efficiently decode the tags’ transmissions. Bad input will yield bad results, especially in RTLS’s.
In the words of Scotty from Star Trek, “You can’t bend the laws of physics, Captain!” Even though RFID practitioners are bound by the laws of physics, they can make smart decisions about what components they choose and how they are deployed.
This article is the first in an ongoing series that will explain the principles of RFID. The series is developed for RFIDNews by Jerry Banks, an Independent Consultant working in Atlanta Georgia and Les G. Thompson, Chief Technical Officer for Lost Recovery Network, Inc., Atlanta, Georgia. They are two of four co-authors of the book RFID Applied, John Wiley, 2007, ISBN-10 0471793655; ISBN-13 978-041793656.
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Understanding RFID Part 2: Investigating active tags
In the previous article, “The principles of RFID: Hardware Basics,” we discussed the key hardware components found in most RFID solutions. This article will focus on the active tag, an RFID tag that has an onboard power source. This power source is, in most cases, a coin cell battery that can be found in many different types of electronic gadgets. The battery extends the functionality of the RFID tag so that it can be used in a multitude of new applications due to the boost in transmission power and the ability to integrate and power circuitry in the tag beyond that in a passive tag.
The EPC specification defines battery-assisted tags as Classes 3, 4, and 5. EPC Class 3 tags are battery assisted, but they communicate using a method called backscattering. Backscattering is the same method used by passive tags.
Backscattering will be covered in more detail in a future article on passive tags, but, in short, it is a method of communication by which the tag transmits data to the reader through a transmission that originated from the reader. EPC Class 3 tags are also known as semi-active tags because they do not use a battery to power their transmissions. They only use the battery to power the tag’s circuitry.
EPC Class 4 and 5 tags use active transmission communication methods. This means that the transmission originates from the tag. Tags can transmit much farther using active transmission communication methods as opposed to backscattering. The most common type of active tag is the EPC Class 4 tag. EPC Class 5 tags can also act as an RFID reader, and may even be powered through a wall outlet as opposed to a battery.
EPC Class 4 tags usually transmit on a regular cycle. This is known as the beacon rate of the tag. The beacon rate can be sub-second to several minutes. The application governs the beacon rate. The period of the cycle is set when the tag is manufactured and, for some tags, can be set dynamically in the field. Regulatory organizations, such as the Federal Communications Commission (FCC) in the United States, control how a tag’s transmission must behave. Most regulatory agencies have four basic parameters that can be tweaked to meet the RF transmission guidelines for the respective agency. The four parameters are frequency, power, duration, and cycle. In most cases, increasing one may require a tag manufacturer to decrease another. For example, a tag that transmits for a longer period of time may need to decrease its transmission power to meet the regulatory rules. Also, note that a tag that beacons every two seconds will have a battery life that is as much as twice as long as a tag that beacons every one second.
It is important to mention a subclass of EPC Class 4 tags called the semi-active tag –which is not to be confused with the semi-passive tag mentioned earlier. The semi-active tag communicates using active transmission methods. The difference between the semi-active and the standard EPC Class 4 tag is that it only transmits when it is queried or interrogated by a reader. Once queried, the semi-active tag can transmit the same distance as the standard EPC Class 4 tag. Semi-active tags may have longer battery lives when compared to other types of active tags because they are not constantly transmitting on a regular cycle. The drawback is that semi-active tags must be close enough to a reader to be queried before they will transmit. This stipulation results in a loss in transmission distance in many cases.
How do you pick out an active tag that is right for your project? First, create a list of all of the requirements for the tag. The following is a list of some tag characteristics to keep in mind:
- Tag size
- Carrier frequency
- Target deployment environment
- Tag density
- Definition of RF coverage
- Beacon rate
The tag size matters. If the tag is as large as a book, it will not be a viable tag for a piece of clothing, but it could be perfectly fine for tagging a train boxcar.
In most cases, tag frequency is paramount when choosing a tag for a specific application. The tag frequency will determine where the tag can be deployed based on governmental regulations, and it will also enhance or retard the propagation of the radio waves through certain substances. A future article in this series will describe how RF behaves at different frequencies and in close proximity to items such as fluids and metal. In real-time location systems (RTLSs), being able to accurately locate a tag using methods such as triangulation can be affected by the frequency of the tag. Certain frequencies are more accurate than others when it comes to determining the position of a tag.
The target deployment environment not only influences the frequency at which the tag should transmit but it also determines how the tag should be packaged. Harsh environments require enhanced durability enclosures which may be custom made for a particular environment. Extreme cold, heat, vibration, or exposure to corrosive chemicals are a few of the environmental conditions that should be taken into account.
Tag density refers to how many tags can be placed in close proximity to each other before a reader cannot receive all of the transmissions. Most systems will eventually read all of the tags, but it may take several minutes. In high security applications, a single missed read may be significant. Most manufacturers will publish their expected maximum tag density, but it is important to do some testing to validate the manufacturer’s numbers when high tag density can occur.
When RFID systems are installed, there are usually physical areas that must be covered with an RFID reader. The reader my cover a 25 square foot choke point or portal, or it may cover an entire store. It is important to pick a tag that has transmission characteristics that fit the application. Fine grain coverage does not require a tag that can transmit a mile, but if the coverage area is a parking lot, a tag that can transmit a mile may be just right. Even though most active tags get praised for their range, most applications require some level of fine grain coverage. In these cases, long transmission distances do not matter.
The beacon rate of the tag governs how fast the tag population can be sampled. For solutions where tagged items move relatively often or where they may move through a reader’s defined coverage area quickly, a faster beacon rate would be preferable. Longer beacon rates can be used in applications were items tend to be idle for extend periods of time, such as in storage warehouses. In the case of security applications, shorter beacon rates are required. For example, if an expensive item, such as a laptop, leaves the building without being cleared to leave, the on site security team may want to know within seconds that the asset has disappeared and the last known position of that laptop.
The protocols that active tags use to transmit are varied. Most corporations keep their protocols secret because each corporation’s protocol attempts to conserve as much battery power as possible without sacrificing distance, functionality, or tag density. Some RFID tags use known protocols such as WiFi (802.11x) or ZigBee (802.15.4). WiFi tags tend to be large, short on battery life, costly, hard to manage, and are relatively hard to pin-point in an RTLS; however, WiFi tags are easy to deploy because most businesses already have a WiFi backbone installed. In addition, any WiFi enabled device can be tracked.
ZigBee was originally designed as a communication protocol for remotes, appliances, and tools, but it was quickly adopted by the RFID community. ZigBee gets its name from the way in which bees zig and zag as they fly from flower to flower. Analogous to how bees transfer small amounts of pollen between flowers, the ZigBee protocol transmits small packets of data between the wireless nodes. ZigBee tags look more promising than WiFi tags because the ZigBee protocol was created to transmit smaller amounts of data. This equates to longer battery life and less software complexity in the tag. ZigBee tags can also operate at all of the standard active tag frequencies such as 303 MHz, 433 MHz, and 2.4 GHz. ZigBee requires the installation of access points in order to provide RTLS functionality, but the good news is that once a ZigBee wireless backbone is installed, other ZigBee enabled devices can make use of the access points.
Ultra wideband (UWB) tags are taking advantage of a fairly new revolution in RF communications. UWB protocols transmit tiny bursts of electromagnetic radiation across a very large bandwidth at specific time intervals in order to communicate. Traditional RF communications modulated a carrier wave with a narrow bandwidth. UWB is sometimes referred to as zero carrier radio because it does not use a carrier wave. The transmissions are so weak that they are equivalent to the spurious RF transmissions that a computer’s hard drive gives off when it is powered up. The benefits for UWB tags are:
- Extremely high data transmission rates compared to other wireless protocols
- High accuracy when used in an RTLS (decreased multi-pathing)
- Longer battery lives
- No interference with legacy wireless transmissions or other UWB communications
UWB systems refer to their readers as sensors. The downside for UWB systems is the price. For a system to obtain the highest accuracy, four sensors must be installed in every location where tags will be tracked, and the tags and sensors are very expensive due to the high quality components required to build them. Also, the highest data rates are only available at a range of up to 10 meters. The size of the UWB tags is comparable to WiFi tag which is usually fairly large. Looking forward, UWB is one of the most promising protocols for active RFID solutions.
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Understanding RFID Part 3: Passive Tags
In the previous article, “The principles of RFID: Active Tags,” we discussed a classification of RFID tag called the active tag. Even though the active tag has many applications, it does not get as much press as its sister, the passive tag. It isn’t hard to understand why this is the case. Passive tags are the darling of the retail industry and the U.S. Department of Defense (DoD) because they are cheaper than active tags and disposable. Passive tags cost less because they do not require a battery to operate and are generally less expensive to manufacture. The most inexpensive passive tag is used in electronic article surveillance (EAS) systems. EAS tags are found in books, attached to clothes, and sealed inside DVD and compact disc cases, among many other applications. These types of tags only transmit an “I am here” signal when they are activated. They do not have the unique identifier that is usually associated with RFID technology but they do discourage would be shoplifters.
The EPC specification classifies passive tags as Classes 0 through 3 because they communicate via a method called backscattering. EPC Class 3 tags do have an onboard battery, but the battery is not used for active transmission. The battery is only used to power the tag’s circuitry and any onboard sensors or peripherals. Class 3 tags are also known as semi-passive tags. The vast majority of passive tags on the market today are the second generation EPC Class 1 tags.
Basics of passive tagging
Passive tags do not require a battery to operate because they can extract energy from the electro-magnetic radiation with which they come in contact. In 1948, Harry Stockman discovered that radio waves can be used to power a remote transmitter. This is the principle of operation for passive tags. Passive tag manufacturers design their tags to be efficient energy collectors. This requires the manufacturers to become very creative with the antenna designs that they attach to the tag’s central processing circuitry. The antenna is the key component in the physics behind how energy is collected from the electro-magnetic radiation generated by the antenna connected to a reader. This interaction between the tag and the reader’s antenna is also known as coupling. Our next article will delve into the details of different antenna types and configurations. For now, it is important to know that the antenna is a key component in the function of the passive tag.
Most passive tags use a method called backscattering to communicate with the reader. Backscattering refers to the way in which the tag communicates with the reader through a carrier wave that originated from the reader’s antenna. The tag “tugs” on the carrier wave to create minute fluctuations in the wave’s amplitude. The amplitude changes are used to encode digital information to transmit to the reader. The reader must be able to detect these tiny changes while, at the same time, provide enough energy to power the tag. Backscattering works much like a transformer does except that it occurs in free space. The reader and tag play the parts of two coils in the transformer. As the tag shunts the coil through a transistor, the reader’s side will detect a tiny drop in voltage. The tag simply shunts the coil to encode the data that is to be transmitted.
Passive tags can be purchased in many different form factors. The naked tag is called the inlay consisting of the integrated circuit and antenna only. Usually, the inlay is deposited onto a substrate using a chemical process (but, not electroplated). The substrate can be paper, polystyrene, or some other material. Most “slap and ship” RFID tags are nothing more than the inlay in a paper envelope. The paper provides some protection and is ideal for tags that have a short life expectancy. The paper is usually printed with a barcode that contains the RFID tag’s unique identifier so that the item can be identified with an RFID reader, barcode scanner, or by simply reading the number printed below the barcode. If a more durable passive tag is required, the tag may be encased in rubber or enclosed in plastic.
Passive RFID tags come in a wide range of sizes. Some tags’ dimensions may be measured in millimeters while others could be measured in feet. The selection of antenna affects the size of the tag. Larger antennas allow tags to be read at a greater distance.
Once the passive tag is powered and the coupling between the tag and reader has occurred, the transponder, the tag, and the reader, can now have a conversation as long as the tag stays in close enough proximity to the reader. The conversation that the two components have is known as the air interface protocol. There are several organizations that are in the process of standardizing RFID protocols. Some of the more notable standards organizations are GS1/GS1 US and the International Organization for Standardization (ISO). Many of these organizations work closely with each other so their standards are compatible at some level. In fact, it is not uncommon for these organizations’ members to actively participate in more than one standards organization. Some tag standards of interest are the ISO 18000 series of air interface standards. The ISO 18000 series also includes amendments that ratify other standards such as EPC Class 1 Generation 2 (ISO 18000-6C) air interface protocol.
Key commands for passive tags
No matter which standard the tag implements there are some basic commands that all passive tags must implement to be effective. Readers must be able to control the tag population in an organized manner. If all tags transmitted at the same time and without any order, the reader may never be able to receive a single uninterrupted transmission from a tag when the tag population is high. Tags are usually selected by the reader and given directions on when they are allowed to transmit. The EPC standard refers to a Q value which represents a seed number used to select subsets of the tag’s population for interrogation. Other protocols have other methods to reach the same end.
Once a tag has been selected, the reader must be able to read the data from the tag. If the tag has onboard memory, the reader can request that the tag transmit a certain number of bytes from a given address in the tag’s memory. Some tags do not have extra memory, so they only transmit their unique identifier. Some protocols have access restrictions for different address regions in the tag. To access these restricted regions, the reader must authenticate with the tag. Authentication may be associated with only certain commands, or may be required of them all
Some tags allow readers to write to the tag’s onboard memory. If the tag supports writing, the reader transmits the starting memory address, the number of bytes to write, and the data to write beginning with the starting memory address.
The ‘kill' command can be issued by the reader to stop the tag from ever transmitting again. This command usually has multiple parts because the tag manufacturers want to make sure that the request to kill the tag is deliberate. These are the basic types of commands that can be found in most standardized air interface protocols.
Interrogating tags in real world environments
The deployment of a solution using passive tags has some interesting considerations. As mentioned in our first article, passive tag antennas are usually set up in a portal type configuration. In this configuration reader antennas are placed on each side of the path through which a tag may travel. In loading dock scenarios, the antennas may be placed above and/or below the path in addition to the sides. This configuration ensures that no matter which way the tag is oriented, there will most likely be an antenna that can power it. In some cases, tags at the very center of a palette or container of goods may not be able to receive enough energy from the antennas in any configuration.
Misreads may require changes in how and when the RFID tags are interrogated. Most RFID enabled systems with these types of issues require the tag to be read at multiple locations and in different configurations. For example, an attempt may be made to read all the tags in a container when the container is taken off of the truck, and a second attempt is made when the container is unpacked. If there are boxes in the container, a third attempt may be made when the items are removed from each box. When these interrogation attempts are correctly integrated into the inventory process, it is possible to obtain almost 100% read rates using passive tag technology.
Assembly lines have an added advantage because they can control the orientation of the tag and placement of the tag on the goods being tracked. Real time location systems (RTLSs) place portals at each entry and exit way into a zone that is to be tracked. As tags move in and out of these portals, the system can assume the tag’s current physical location.
Passive tag prices have not yet reached the US$0.05 per tag goal set by the retail industry, but there is still a great return on investment based on today’s prices. We may not be able to tag every can of soup, but container and pallet tagging is a valid goal with real monetary rewards.
This article is the third in an ongoing series that will explain the principles of RFID. It was created for RFIDNews by Jerry Banks, Independent Consultant, Atlanta Georgia 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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Understanding RFID Part 4: The black art of RFID antennas
The design of the antennas used by RFID tags and RFID readers is one of the most critical pieces in RFID infrastructure. This is because the antennas facilitate the communication between an RFID reader and a tag through free space. Both the reader and the tag utilize an antenna to transmit and receive data. In the case of passive RFID systems, the antennas have the added burden of being able to efficiently collect and radiate energy, respectively. Without well designed antennas there can be no efficient communication between the tags and readers, and, in the case of passive tags, there may not be enough energy to power the tag.
Characteristics of antennas
Most antennas are made of a highly conductive material such as copper so it is sensitive to electrical and/or magnetic currents found in radio waves. When an antenna is receiving, the conductive material comes in contact with radio waves and converts the radio wave’s electrical and/or magnetic currents into signals and energy that can be consumed by some type of circuit. The opposite is true for a transmitting antenna.
In most cases, antennas possess a property called reciprocity. This means that if an antenna can transmit well at a certain frequency, it can receive equally well at the same frequency.
Most antennas are tuned to operate within a certain bandwidth. This means that the properties of the antenna, such as construction materials, length, and structure, are all precisely chosen to work efficiently in a specified, usually very narrow, range of frequencies. The frequency at which the antenna works best is known as its resonant frequency. When an antenna works well at multiple frequencies, the antenna is said to have harmonic resonance. In most cases, antennas that are designed to work at multiple frequencies are not as efficient as antennas that are designed to work in a very specific frequency spectrum.
As discussed, the construction material of an antenna enables the antenna to convert electric and/or magnetic currents into usable forms of energy. The two components of electromagnetic radiation are the electric component, E field, and the magnetic component, H field. Based on an antenna’s design, it will make use of one of these components.
Near field vs. Far field antennas
RFID tags that operate within one wavelength’s distance from the reader’s antenna make use of the magnetic component. If an antenna is designed to operate at 13.56 MHz, its antenna should be designed to operate in the “near field” and utilize the magnetic component. This is because the wavelength of a 13.56 MHz wave is approximately 22 meters. This is not to say that the tag will function at a distance of 22 meters. It won’t. The tag’s antenna would not be able to derive enough energy from the electromagnetic field at that range. It is important to recognize that the wavelength of the target frequency directly influences how the tag’s antenna is constructed.
Any RFID tag that is designed to operate at 13.56 MHz will function best in the near field and will most likely have an antenna that is shaped like a coil (see Figure 1). A coil antenna configuration works best for making use of the magnetic component of electromagnetic radiation.
Figure 1 - Texas Instruments 13.56 MHz Tag-ItTM Inlay
It is important to note that the wavelength of a radio wave is inversely proportional to its frequency; thus, a radio wave with a frequency of 13.56 MHz will have a wavelength (approximately 22 meters) that is much longer than that of a radio wave with a frequency of 915 MHz (approximately 32.8 centimeters). The wavelength of any radio wave at a certain frequency can be calculated by the equation λ = c/f, where λ is the wavelength and c is the speed of light in meters per second.
Figure 2 - Radio Wave
Far field antennas operate most efficiently outside of one wavelength’s distance of its target frequency. Far field antennas are usually manufactured as straight lines, as opposed to the coils because far field antennas utilize the electric component of the electromagnetic radiation. The most common type of far field antenna is the half wavelength dipole antenna (see Figure 3). The half wavelength dipole consists of two antennas, with each antenna being a quarter of the target frequency’s wavelength. Most RFID tags place the tag’s processor between both antennas. Based on the previous example, a half wavelength dipole antenna that is designed for the 915 MHz spectrum would require each dipole to have a length of 8.2 centimeters which is one quarter of the 32.8 centimeter wavelength. This antenna would be 16.4 centimeters in total length.
Figure 3 - Symbol Technologies Single Dipole Inlay
The principle of Gain
An antenna’s gain is a key characteristic for RF engineers. Gain refers to the radiation pattern of an antenna. Low gain antennas radiate their energy equally in all directions, while a high gain antenna is direction biased.
High gain antennas have a strong direction and a weak direction. Gain is a very important factor in RFID systems because it allows system designers to “shape” RFID coverage areas. Antennas can be arranged in a portal style configuration (as described in Part 3 of this series) where they are directionally biased toward the inside of the portal. Passive systems benefit from this type configuration because the reader’s antennas can concentrate most of their energy on the location where the tag is most likely to be; thus providing more energy to power a tag as it passes through the portal.
Active tag systems benefit from gain as well. In a real-time location system (RTLS), a directional antenna allows system designers to tweak their coverage areas and strictly define the boundaries of the “zone” to be covered by an antenna and reader. For example, if an RTLS zone is defined by the RF layout designer to only cover a single room, the engineers can use the antenna’s gain characteristic as one of the tools to make sure that the antenna does not read tags beyond the borders of the room.
Enhancing performance through antenna design
There are two basic rules for designing a passive tag antenna. First, the longer the antenna, the more energy it can collect. There is a point of diminishing returns with regards to the length of the antenna, but, for the most part, longer is better. With respect to coil type antennas, increasing the number of rings of the coil is equivalent to increasing the length of the antenna. The second antenna characteristic that is beneficial to collecting energy is the surface area of the antenna. The Alien “M” tag is a great example of a passive tag with a large surface area as shown in Figure 4.
Figure 4 - Alien Technology Corporation "M" Passive Tag
As the width and height of the antenna increases so does its energy collecting efficiency. As discussed earlier, the layout of the antenna depends on how the tag is intended to be used and at which frequency it will operate. Tags that operate at lower frequencies and work in the near field have antennas that are coils. The higher frequency tags that work in the far field have straight edge antennas.
RFID tags are very rarely placed on stationary items (there is usually no point in tracking something that does not move unless you want to make sure it does not move!). RFID tags tend to move regularly and may be placed in many different orientations as the object they are attached to is shipped, carted, or carried. The laws of physics dictate that an antenna will achieve its best reception when its element is oriented orthogonally to the radio wave. This means that the antenna works best when it intercepts the wave at a 90 degree angle; therefore, orientation is crucial if a tag is to achieve its maximum range and transmission data rate capabilities.
Passive RFID tag antennas sometimes look strange because they may be offset at abrupt angles. These angles allow the tag to present some part of its antenna to the radio wave at an angle most conducive to coupling. The half-dipole antenna mentioned earlier is extremely efficient when its orientation is correct, but can be completely useless when it is not. Dipole antennas should only be used in applications where the antenna’s orientation can be ensured, so this is why many RFID antennas are “squiggle” type as shown in Figure 5.
Figure 5 - ALN-9440 Gen2 Squiggle from Alien Technology
Active tag antennas
Most of this discussion has focused on passive RFID technology, but it is important not to forget about the active RFID world as it has its own set of antenna challenges. Active tag antennas do not have the added burden of collecting energy from radio waves to power the tag, because the active tag’s battery provides all the power required. This additional power gives active tags some flexibility on how the antenna is constructed. The laws of physics have not changed, but the ability to blast a signal at a relatively high wattage when compared to passive tags can nullify many of the primary design considerations associated with passive tag antennas.
Our next article will discuss the characteristics of radio frequency (RF) and why certain frequencies and antennas are chosen for certain applications.
This article is the fourth 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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Understanding RFID Part 5: RF Characteristics
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.