Alien “Insider” Blog

The use of pRFID (Passive Radio Frequency Identification) in the DoD (Department of Defense) supply chain has demonstrated the potential to provide real benefits for Inventory Management, Asset Visibility, and Interoperability in an end-to-end integrated information technology environment. pRFID enables data accuracy opportunities inherent in all types of automatic identification technology (AIT). Additionally, pRFID is a non- intrusive methodology for data capture (pRFID does not require human intervention) as it is a non-line of sight technology with both read and write capabilities.

The DoD, like its commercial counterparts, is benefiting from the rapid advancement of pRFID technologies. Specifically, the performance, costs, technical requirements, and physical footprint of pRFID have dramatically decreased. pRFID tags have matured over the past five years resulting in significantly increased performance, and as production volumes increase, economies of scale allow for a competitive landscape in pricing. Evolution of the hardware and tags, combined with enhancements made to the enterprise data systems that accommodate the tracking of items through a growing network of read-points, is allowing the vision to unfold.

pRFID addresses many opportunities, but a key challenge identified at every node within the DoD supply chain is the lack of visibility of item data. pRFID has become a key technology enabler for the DoD’s supply chain and logistics operations by providing:

• Near-real time in-transit visibility for supplies and material
• “In the box” content level detail for supplies and material
• Quality, non-intrusive identification and data collection enabling improved inventory management
• Confidence in the supply chain by improving visibility to item level visibility and consumption data

Case Tagging Tips
Regardless of the tag application method used, case tagging success increases when:

• The RF polarization of the tags is vertically oriented on the case
• Tags are placed as high on the case as possible to take advantage of inherent air gaps within the case
• Tags are not folded over corners
• Tags are placed on double corrugate flaps
• Tags are positioned furthest from absorbent materials (e.g. aqueous liquids) or reflective foils
• Tags are asymmetrically placed on the long “service” side of the case

Achieving case reads on a pallet generally requires a greater investment in time, planning, and the implementation of an RFID system. Be sure to glean on the lessons learned across the industry before jumping into a compliance effort.

Summary
Since the first retail industry compliance initiatives, retailers and suppliers alike have influenced significant advances in RFID. Today’s hardware systems are much smaller and more easily integrated into the manufacturing and distribution process; tag performance has increased dramatically as the price for tags continues to fall; and the benefits of RFID are being realized as suppliers leverage the RFID infrastructure for internal applications. Pallet and case level tagging initiatives will surely evolve with these advances and in the near future could begin to include the use of RFID for brand authentication, security, e-retail, and more.

Most retail industry requirements have evolved to include the tagging of cases. Case tagging provides an additional layer of visibility and benefit as it captures the movement of products shipped directly to the store, as well as movement between the store backroom and the sales floor. Visibility of product moving to the sales floor is also of benefit to the supplier.

Unlike the tagging of pallets, the tagging of cases may be either a manual or an automated process.

Manual Application
For many retail suppliers the apparent entry into case tagging involves printing or acquiring tags which are then applied to cases manually. This process, commonly referred to as “Slap & Ship”, may be cost justified for low initial volumes. However, manual tag application has several disadvantages, including:

• Not easily scalable as the number of cases to be tagged increases
• Lack of tag placement control may result in poor performance with minimal repeatability
• Limited corporate system integration results in minimum data collection
• Induces secondary operations reducing opportunity for internal Return on Investment (ROI)

While the manual tagging method does have long-term disadvantages, it does minimize inline process changes and allows the supplier an opportunity to grow into the technology, expanding as they become more experienced or as demand requires.

Manual tagging most often occurs for a small, select number of products. Rather than tagging along the packaging line, tagging typically occurs at the point of shipment. Tags are either acquired pre-programmed by a third-party provider or they are generated using an RFID-enabled desktop printer driven by software which generates unique tag identification numbers.

Before settling on the manual tagging approach, be sure to do a thorough business analysis.

Integrated In-line Application
An alternative to applying tags manually is to incorporate tag application into the packaging process. This process is most appropriate for higher volumes and carries with it several long-term benefits including:

• A more controlled, repeatable, and scalable approach
• Improved results from an automated process that consistently places tags on the product
• Increased opportunity for ROI as tags are applied early in the process

The automated tagging method does have long-term advantages, but it sometimes requires in-line process changes and a higher initial capitol equipment investment.

In-line tagging typically occurs along the packaging line. Blank tags are programmed with a unique identification number, often printed with human readable information, and applied directly to the case as it passes down the conveyance line. Photo eyes are commonly used to trigger the start of the process and enterprise software is used to drive the data produced on the tag.

Though in-line tag application requires greater up-front process analysis and investment, its scalability, repeatability, and minimal human intervention allows for a greater ROI.

Like pallet tags, most case tags are temporary, one-time use tags applied to the outside of the case. However, selecting a case tag along with its position and orientation, takes a little more consideration as the tag’s performance may be altered through interaction with the material inside the case. When placing a tag on the case, it needs to be located so that it can be easily
accessed for reading with either fixed or hand-held readers.

Most retail industry tagging requirements begin with tagging at the pallet level. Retailers,
when receiving tagged pallets at the distribution center benefit from the visibility of inbound
receipts and of outbound shipments. For the suppliers, benefit comes from real time visibility
of inventory in the supply chain, improving forecast accuracy, dampening the inventory
bullwhip effectively and assisting with freight claims processing.

Pallets are usually tagged at either the point of aggregation, at a stretch wrapper, or at the
point of shipment. Tags may either be pre-commissioned (i.e. purchased from a third party
with unique identification numbers) or printed and programmed as needed using a “desktop”
printer. In either case, tags are a temporary, one-time use item durable enough to survive the
supply chain environment.

Some adopters of RFID have affixed “permanent” tags directly to the pallet. These tags,
typically enclosed in robust packaging, are meant to be reused, monitoring the pallet through
the supply chain.

When placing a temporary pallet tag, the tag needs to be located so that it can be easily
accessed for reading with either fixed or hand-held readers. An EPCglobal Accredited
Performance Test Center is an excellent resource to assist suppliers verify appropriate tag
placement.

Pallet Tagging Example
For typical pallet tagging, the EPC tag must be placed on a side of a pallet that can be
determined to be the “front” of the pallet (40 inch side) upon arrival at the Distribution
Center. The EPC tag must be placed in the middle (horizontally) and approximately 30
inches from the floor (vertically). The EPC tag must be applied on the outside of the
stretch-wrap.

Achieving pallet tagging compliance generally requires minimal RFID system investment.
However, one should carefully consider the benefits beyond compliance when designing a
pallet tagging solution. Investing in as little as a dock door portal to capture and track shipping
information could pay off down the road.

Building a Foundation
First time RFID compliance initiatives can be intimidating. Fortunately, the technology has matured into easy to use solution sets that, in many cases, have become virtually transparent. Nonetheless, whether starting a compliance effort or working to expand one’s first steps towards compliance, nothing is more beneficial than establishing a solid foundation of knowledge through training.

With several years and hundreds of installations now behind the industry learning curve, lessons learned are contributing to an expanding body of best practices. Well designed and delivered educational programs are in place for both those needing to gain a fundamental understanding of the technology (e.g. a systematic overview of the capabilities and limitations) and for those seeking more in-depth, real-world implementation experience.

The first step to a successful compliance initiative is to develop foundational RFID knowledge and practical, hands-on experience through education.

Seeking Expertise
A second method of leveraging the expanding body of knowledge is to call upon the expertise of solution providers with proven integration experience.

Third-party organizations come in many shapes and sizes with specialties ranging from niche services to complete system design and integration. Engaging such an organization allows one to learn from the organization’s experience while ensuring a successful path to compliance.

In 2003, Wal-Mart made the retail industry’s initial steps with a RFID (radio frequency identification) supplier requirement when it announced a pallet and case level tagging initiative. Currently, over 600 suppliers are participating in the initiative. In November 2007, Sam’s Club built upon this and other similar retail requirements by announcing that its suppliers should begin EPC (Electronic Product Code) tagging according to a provided specification and schedule. Through these defining initiatives, the retail industry continues to realize the tremendous gains derived from RFID technology and its application throughout the retailer/supplier distribution chain.

Today, most retail industry RFID compliance initiatives require tagging at the case and pallet level. Case read requirements are typically limited to conveyor reads, where tags are singulated and in motion. Pallet tag read requirements are commonly limited to placard tags (e.g. a single tag on a SSCC or SGTIN label adhered to the stretch wrap) which are read as the pallet passes through a dock portal.

This paper serves to address some of the more common questions raised when preparing to meet current retail supply chain RFID compliance initiatives, including:

• Where do I start?
• How do I successfully begin tagging at the pallet level?
• What options do I have for tagging at the case level?

What is the difference between Near-field, Magnetic field, B-field, and H-field?
These are fundamentally interchangeable terms. They describe the relatively short distance where coil or loop antennas can couple a magnetic field.

What is the difference between Far-field, Electric field, and E-field?
These are interchangeable terms. They describe the properties of an antenna where electric fields are typically coupled between dipole antennas.

What’s the relative difference in the rate of decay of the RF field for a near-field vs. far-field antenna?
The near-field dramatically decays at a rate inversely proportional to the distance of the transmitter antenna cubed (1/d3) whereas the far-field decays at a much slower rate, proportional to the distance of the transmitter antenna squared, (1/d2).

Are Near-fields restricted to HF?
No. All antennas comprise both the near-field and the far-field component. However, the lower frequencies (e.g. HF) are practically restricted to near-field coupling. UHF on the other hand can, and does use both near-field and far-field components of the radiated electromagnetic field.

Does HF have both a near-field and a far-field component?
Yes, but the far-field component is not practical. With a ¼ wavelength of 218”, a dipole antenna would be ridiculously large. Hence, in practice, HF relies solely upon near-field coupling.

Does UHF have both a near-field and a far-field component?
Yes, and both are used in practice, as their geometries are very practical. A typical UHF loop is typically under 1/2” in length and with a ¼ wavelength of 3.2”, a dipole antenna is very practical for use in standard label applications. Smaller far-field antennas are also available, with reduced range, but even this range is very substantial.

How can one distinguish between a near-field and far-field antenna?
Generally speaking, near-field antennas comprise of a coil or a loop. For HF, there are generally upwards to 10 turns in the coil. For UHF, there is typically only one turn in the coil, hence a simple loop.

What might one expect in read range when comparing a 1.5” HF near-field tag to that of a 0.9” UHF far-field tag?
In Practice the 1.5” HF tag would be expected to provide a read range between 1” and upwards to 3 or 4” – this would of course be near-field. A standard 0.9” UHF far-field tag would be expected to have a read range between 6 and 10 feet. Both read ranges stated as free space measurements.

What might one expect in read range when comparing similarly sized HF and UHF near field tags, say both being slightly under 1”?
The HF tag would be expected to have a read range of approximately 1-3 inches and UHF tags typically show read ranges in the 9-12 inch range. Both read ranges stated as free space measurements.

Are HF tags affected by the materials for which they are placed?Absolutely – in fact more so than UHF. HF tags are comprised of a resonant “tank” (L / C) circuit with a high quality factor (Q) inductor (the coil) and capacitor. This creates a “tuned” circuit whereby the tag must be exactly tuned at 13.56MHz to have any decent read range. This circuit is depicted below. Note the read range peaks only at 13.56MHz. Off that frequency (slightly lower or higher), and the effective read performance (vertical axis) drops like a rock.

When HF tags are placed against dielectrics (e.g. on product), “parasitic capacitance” is coupled to the tag. This has a tendency to shift the resonant tag frequency downward, hence the degraded read performance. One way to resolve this is to tune the tags for particular products, shifting the free space resonant frequency upward in anticipation of the downward shift due to the parasitic capacitance. Note the inverse relationship between capacitance (C), and the resonant frequency: when tags are placed on product, parasitic capacitance results in a downward shift in the tuned frequency.

Where L is the coil inductance (fixed) and C is the resonant capacitance (affected by the parasitic capacitance of the product for which the tag is placed)

Are UHF tags affected by the materials for which they are placed?
Yes, but there are two advantages with UHF. The first is that the tags are typically designed to operate over a very broad spectral frequency band, e.g. 840MHz to 960MHz vs. the single frequency of an HF tag. With such a broad band available to operate, a slight frequency shift does not affect the performance nearly as much as with a narrow band, high Q resonant HF circuit. A typical UHF frequency response curve may be expected to look as shown below – so if the tag shifts around due to parasitic capacitance (e.g. a shift from 925MHz to 920MHz for example), the effective performance is not as affected as with HF where the frequency needs to stay fixed at 13.56MHz to obtain decent performance.

What about aqueous materials, such as bottled water – how do HF and UHF tags compare and can UHF tags be read adjacent to liquids?
Magnetic fields are not subject to the same absorptive properties as electric fields, hence HF tags can operate when placed on aqueous materials. The same holds true for UHF. Near field coupled UHF tags can also be placed on aqueous materials – in fact, we’ve demonstrated UHF tags reading “in” water. Remember the UHF read range starts much further then HF, so even if UHF tags have a dramatic reduction in read range due to RF absorption, the end result still often exceeds that of HF.

Newer vintage UHF readers and silicon with very sensitive circuitry, and tags with broad band capability, have tremendously improved UHF readability near aqueous materials. This is no longer the challenge which existed with Gen 1 vintage UHF technology.

What about metals or foil based product – how do HF and UHF tags compare?
Whether HF or UHF, or near field or far field, tags do not perform when positioned directly on metal surfaces. But the magnetic field does not benefit from reflective surfaces, whereas the electric field can. Properly placed UHF tags can result in greater read distances than their free space measurements.

From a cost perspective, is there a difference between a HF and a UHF tag antenna?
Absolutely. A HF tag is substantially more complex and hence more costly to construct. UHF tags have a definite cost advantage. See below.

Is there an advantage with HF over UHF for aqueous materials?
Prior to about 1.5 years ago, UHF tags were not practical when placed adjacent to aqueous materials, so for those applications, HF was the only choice. Today, this is untrue. UHF tags have been shown to read next to, and within, aqueous materials.

Given that UHF is generally required for case and pallet level reads, and now that a solution is available for item level reads, it makes sense to hone in on a single technology to address item, case and pallet read requirements vs. splitting frequencies between HF for item and UHF for case and pallet.

Is it true that shadowing is a problem with UHF, but not with HF?
Shadowing/shading issues have dramatically diminished with UHF. Vintage Gen1 UHF tags were dramatically affected if an adjacent tag was less than about ¼ wavelength away (~3” or less). Today, this is no longer the scenario – in fact, tags have been shown to read when placed in very close proximity to one another (e.g. RFID enabled medical and legal file folders).

What about singulation?
Because HF tags have a very limited read range (e.g. typically 1-4”), it is relatively easy to singulate product – for example bottles on a conveyor.

By contrast, UHF has the potential to cover tremendously wide read zones – this makes it difficult to isolate tags of interest – for example the bottles on a conveyor. Remedies to help alleviate this issue include: use of low gain, directional antennas, shielding, attenuation of transmit power, and even smaller tags. But often, even these remedies may not allow enough resolution to isolate tags in close proximity.

For these applications, Alien is developing a small, near-field tag. Given the focused energy required to activate a near-field tag, this helps address the singulation issue. Performance can be further dialed in by adjusting reader power.

The near-field tags can easily be activated with the conventional far-field patch antenna, as they emit an adequate near-field component in addition to the far-field for which they were designed. The challenge is that even though the conventional antennas can read the near-field tags, and that distance can be limited to something on the order of 1 foot, any stray dipole tags in the vicinity are activated by the far-field component. Also, the near-field emissions from the conventional patch antenna are often too wide to be practical.

Alternate small patch antennas have proven to be very successful in limiting the near-field coupling, and allow near-field tags to be singulated even when they are as close as 1 or 2 inches from one another. This is very encouraging, however, they emit too strong of a far-field component, so stray dipole tags continue to be read.

What can be done to further refine the singulation challenge, and minimize the far field reads?
Aliens’ Intelligent Tag Radar (ITR) is one very key component for success in this application. Two features of this platform are crucial: the first is the Tag Singulation feature, whereby densely populated tags can be effectively singulated and the second is Zone Control whereby a read zone perimeter of interest can be set, and all tags beyond that perimeter are ignored. This is much different from attenuating power – when reducing power, this generally compromises read performance margins, especially reducing the opportunity to read all tags in a challenging application. Zone Control allows the reader to operate at full power, and ignores those outside the intended boundaries.

Upcoming near-field tags will inherently assist with singulation by minimizing the read distance, however without Zone Control, stray dipoles (e.g. on nearby cases or pallets) could still be read.

Alien is investigating new reader antennas with smaller footprints. Consideration is being given to near-field antenna options whereby the far-field is minimized further than anything currently available on the market. Again, Zone Control can play a vital role in this system solution.

Doesn’t Impinj have a near-field antenna, e.g. the Brickyard?
Yes. But remember, by their very nature, all antennas radiate both electric and magnetic fields. The Impinj Brickyard antenna is rather large, with about a 1 foot diameter footprint. It radiates a very substantial far field component, and dipoles can be read at ranges well over 6 feet away.

Can Alien’s readers attach to Near-Field antennas?
Yes. The only requirement is that the reader have a 50 ohm impedance, which is standard in the industry for both far-field patch and near-field antennas. The Alien ALR-9900 will work with the Impinj Brickyard antenna, however, it has not been submitted for regulatory compliance with that antenna, hence, it is not a viable option at this time.

What are good application examples for Near-field systems?
Applications where tag read performance needs to be strictly controlled, such as access control (insuring the person requesting access into a building is the one next to the reader, not one 20 feet away), SmartCards (where financial transactions need to be well controlled), and perhaps applications where short range is acceptable, and the tags are to be placed on aqueous materials.

Should near-field tags be promoted for item level products, e.g. pharmaceutical drugs?
It depends upon the product and the use-case. Some items are too small to accommodate a dipole antenna, and UHF near-field loops can work well under these circumstances. Another example would be jewelry tags, whereby the tags are very small and the application calls for very short read distances. However, once the Zone Control and Tag Singulation platform is released, customers such as in the pharmaceutical sector would likely benefit with a higher performing dipole antenna. By using a tag which can be singulated on a conveyor (e.g. Tag Singulation and Zone Control) bottles (for example) on a conveyor can be easily isolated, and stray tag reads can be alleviated. Further downstream, the Zone Control periphery can be extended such that all bottles within a case can easily be read.

Further down the supply chain, if the product lends itself to such, there may be an opportunity where all bottles in cases and in a pallet could potentially be read (e.g. on a stretch wrap turntable) – this would not be feasible with a near-field solution where the read range is restricted to a few inches.

Lastly, pharmacies are looking to potentially minimize implementation costs by virtue of covering larger read zones for inventory vs. requiring high density, low read range smart shelves. In the future, with further development in the smart shelf area, this will be a valuable tool for the retailer / pharmacy.

What is NFC?
NFC stands for Near Field Communication. This is a relatively new term, and should not be confused with traditional Near-Field HF or UHF (magnetically coupled) passive RFID. This technology is typically used for transactions which allow consumer devices (e.g. mobile phones, digital cameras, Kiosks, SmartCards, etc.) to interact with one another and share information or enable fast / secure payment transactions. The coupling is similar to that of HF near-field passive systems in that it is a wireless connectivity technology with an operational frequency of 13.56MHz, and its interaction is limited to short distances, e.g. < 4cm. Other than that, it’s not considered a passive RFID term.

Unlike the loop or coils used in the near-field (magnetically coupled) tag antenna, conventional UHF tag antennas typically use dipole antennas to transmit or receive electric-field (hence a voltage) vs. the coil which conducts a current through the loop.

The schematic representation of a Far-Field dipole antenna is as shown below, where the dipole (e.g. two poles) on the left represents the powered transmitter of the reader, and the dipole on the right represents the receiving tag antenna. In this representation, the transmitting (powered) antenna on the left induces a potential, or voltage, signal to be coupled to the tag dipole antenna on the right. Here we are showing a static D.C. power source (the battery) for simplicity, when in reality this would be an A.C. signal at a frequency in the UHF spectrum (e.g. 915MHz).

Far-field antennas are very commonly used – they are the principal coupling mechanism of car radios, cellular phones, wireless networks and even older TV dipole (rabbit ear) antennas. They are a popular means of transmitting signals over long distances, unlike near-field where the distances are very limited.

Similar to the magnetic coupling example, imagine electrically turning the electric field on and off by virtue of switching the power source on and off. When “on”, an electric-field is established and a nearby receiving coil can detect this electric signal. Now imagine that the receiver queries the state of the electric field at uniform time intervals (e.g. time 1, time 2, time 3, etc.) and let’s say if it detects an electric field, we assign a “logic 1” to that sampled time, if no field is detected, we assign a “logic 0”. From that, the circuit decodes the “1’s” and “0’s” and determines what action to take. This in essence is how far-field (electric) coupling is achieved. See example below.

It should be noted that, unlike the read distance limitations of near-field, far-field signals can be used where tags and antennas are in very close proximity (e.g. millimeters), or at a very significant distance apart (10’s of meters). For applications where the read range needs to be limited, either the tag can be made less efficient (or smaller), the reader antenna can be made with less gain, or the transmit power can be reduced (e.g. similar to dimming the power of a light bulb) where one can attenuate (reduce) the transmit power.

Alien’s conventional dipole antennas are actually comprised of both a resonant loop and a dipole antenna. Let’s take a closer look at an exemplary construction.

Note the resonant loop where the dipole antenna connects? Though not necessarily optimized for the near-field, this tag can effectively couple to either the near-field (via the loop), or the far-field (via the dipole). If it were in a pure magnetic field, the dipole would not contribute – the primary coupling would be via the loop. To optimize the near-field coupling, the loop would typically be more circular or square than oblong and the dipoles would not be necessary.

Again, it should be noted that ALL electromagnetic antennas comprise both electric-field and magnetic-field components (hence the term electromagnetic). However, in many instances, only one coupling mechanism is of interest (e.g. either the electric or the magnetic field) so the antennas are typically optimized for one field, and the other is generally de-emphasized.

Previously we described the practical read distance rule of thumb as being approximately the same as the longer axis of the antenna. There are more factors involved, such as the allowable transmit power, the efficiency of the passive tag silicon, and the construction of the tag. Looking at the proportions of the UHF loop and the HF loop, we noted that the UHF loop was relatively smaller – nothing prohibits us from designing smaller HF loops or larger UHF loops, but it is interesting to note that even though the UHF loop was smaller, in practice it achieves at least the range of the larger HF tags – and typically quite a bit more.

In addition to the loop diameter, the wavelength of the frequency comes into play in determining the near-field and far-field calculations. For simplicity, let’s level the playing field and assume the antenna diameter for both the HF and the UHF antennas is 1 inch, so D=1” in this example. Next, let’s define the wavelengths: For HF, the wavelength is 870 inches long, and for UHF, it is 12.9 inches long. This is a very substantial difference.

Far-field dipole antennas are typically designed to approximately ¼ to ½ of a wavelength (λ). From the chart above, it’s now easy to understand why the far-field is not used for HF, given that a ¼ wavelength would result in a dipole antenna with a length of 218 inches (436 inches for a ½ wave dipole) – this of course is not practical. For this reason, HF exclusively relies on the near-field coupling.

For UHF, the ¼ wave dipole would result in an antenna length of approximately 3.2 inches. This is very practical. This offers UHF the advantage of combining the available near-field and far-field components to power a tag.

If we compare the calculated near-field and far-field distances for HF and UHF, using the formulas below, we observe the following. Here the diameter, rN, represents the radiated near-field distance. Beyond that point is where the far-field, rF, dominates.

Mulling through the calculations, the results show the UHF near field having over an 8X advantage in near-field read distance over that of HF.

Hopefully these analogies have helped to explain the relative differences between near-field (aka magnetic-field, or B-field) coupling and far-field (aka electric-field, close-coupled, or E-field) coupling systems.

Exemplary Near-Field Tag Antennas (note Coil / Loop construction)

Note the simplicity of the UHF coil over that of a typical HF coil. This has a large impact on the construction cost of the tag – hence the reason UHF tags are less expensive.

The schematic representation of a Near-Field loop or coil antenna is as shown below, where the loop on the left represents the powered transmitter of the reader, and the loop on the right represents the receiving tag antenna. In this representation, the transmitting (powered) coil on the left causes current flow from the “+” to the “-“, much like water would flow in a hose. This current flow induces a magnetic field, which couples to the adjacent tag coil on the right. Here we are showing a static D.C. power source (the battery) for simplicity, when in reality this would be an A.C. signal at a frequency in the HF or UHF spectrum (e.g. 13.56MHz or 915MHz).

Let’s draw an analogy of the relative field strength of a near-field (magnetic) coupling antenna to something we can easily relate to: a conventional permanent magnet.

Imagine two permanent magnets in close proximity to one another – as in the depiction below. Assume the polarities are such that the same polarities (e.g. “N”) of both are magnets are adjacent to each other. As you would expect, the permanent magnets would repel. As they approach one another, the repelling force increases – you would feel this resistance increase as they get closer and closer together. But as they are moved apart, the interactive force dramatically reduces with distance. In other words, the magnetic field decays rather rapidly.

When current flows in a coil, a magnetic field is established – this is similar to the magnetic field of a permanent magnet.

Now, instead of two magnets interacting with one another, we replace the two permanent magnets discussed above with two coils. As with the permanent magnets, the RF field strength is a function of distance. Also affecting the field strength is the driving power (e.g. current through the coil) and the diameters of the coils.

This is analogous to magnetically coupled RF coils (near-field loops), but with antennas, the permanent magnets are replaced by an electromagnet where a transmitter (e.g. within the reader) drives current through the coil, which in turn induces a magnetic field. See depiction below.

When current flows in a coil, a magnetic field is established – this is similar to the magnetic field of a permanent magnet.

Now, instead of two magnets interacting with one another, we replace the two permanent magnets discussed above with two coils. As with the permanent magnets, the RF field strength is a function of distance. Also affecting the field strength is the driving power (e.g. current through the coil) and the diameters of the coils.

For near-field (magnetic) systems, the coupled energy decays quite rapidly, at a rate proportional to the cube of the distance (1/d3) – as suggested in the field strength chart above, one can envision the rapid decay of available coupled power over distance, much like the rapid decay of the force of the two magnets previously discussed.

Now imagine electrically turning the magnetic field on and off by virtue of switching the power source on and off. When “on”, a magnetic-field is established and a nearby receiving coil can detect this magnetic signal. Now imagine that the receiver queries the state of the magnetic field at uniform time intervals (e.g. time 1, time 2, time 3, etc.) and let’s say if it detects a magnetic field, we assign a “logic 1” to that sampled time, if no field is detected, we assign a “logic 0”. From that, the circuit decodes the “1’s” and “0’s” and determines what action to take. This in essence is how near-field (magnetic) coupling is achieved. See example below.

With a coil, the electric field (far field) is minimize and the magnetic field (near field) is maximized. HF does not offer a viable far field solution – the technology exclusively relies on the near field coupling component (more about this later). Therefore loop antennas are the predominant coupling mechanism for HF.

As a general rule of thumb, the “practical” distance of the near-field magnetic coupling is said to be “approximately” the same as the length (or diameter) of the long axis of the smaller coil (generally the tag antenna coil). Hence the reason why typical HF RFID tag coils, which are approximately 1.5” in their longer axis, generally are observed to have read ranges in that same vicinity. In practice this range can be extended with additional tag coil turns or lower loss antennas (e.g. higher Q’s) etc.

First, we will use the term “coupling” to refer to wireless interaction between two antennas (e.g. one on the reader, and the other on the tag) – consider “coupling” as an invisible wire, connecting the two circuits.

Second, let’s associate equivalent coupling terms. There are really only two categories of concern:
MAGNETIC-FIELD COUPLING (NEAR-FIELD COUPLING, B-FIELD COUPLING)
ELECTRIC-FIELD COUPLING (FAR-FIELD COUPLING, E-FIELD COUPLING)

There is a third term which is often confused in the industry. Often, uninformed UHF RFID users insist they are using “Near-Field” technology when in fact they really are using conventional Far-Field tags and antennas. After digging deeper, we sometimes find they associate short distances between an antenna and a tag with the technical “Near-Field” coupling. They contend that they have the system set up to read tags at short distances, often by virtue of using either low gain antennas or by substantially attenuating the reader’s power level. Where short distances are characteristic of true near-field (magnetic) coupling, the proximity is not indicative of the coupling itself. So in such instances, especially where standard dipole tag antennas are used, this is more correctly referred to as a “close-coupled E-Field”. Again, close-coupling simply referring to low reader power levels, often in conjunction with low gain antennas.

For reference, the following are some typical examples of near-field (aka magnetic- or B-field) and far-field (aka electric- or E-field) tag antennas.