NFC Antenna basics

As with any antenna, and NFC antenna follows the basic rules of any antenna system. The antenna is basically a form of tuned circuit. Power is fed into the antenna and much of it is radiated. As all passive antennas are perform in an equivalent manner in reception as they do in transmission, it is often easier to look at them as a radiating element as it is often easier to look at the concepts in terms of radiation.

There are a number of parameters and definitions for antennas that are useful when looking at NFC antennas:

Radiation resistance: The resistance that equates to that which would be required to dissipate any power that is radiated.

Resistive losses: The losses that occur as a result of the resistance of the antenna elements – these losses plus the radiated power equate to the total input power.

Bandwidth: The band over which the antenna will operate satisfactorily. Normally antennas operate as resonant elements and therefore their performance falls either side of the centre frequency. This must be accounted for in the design of the NFC antenna, or any other antenna for that matter.

Feed impedance: The current and voltage will vary along the length of the antenna element. Voltage rises towards the ends and the current falls and is also dependent upon the length of the antenna, etc. As impedance is the ratio of current and voltage this means that the feed impedance varies. TO ensure the maximum power transfer the source and load impedances must match, and therefore the feed impedance of the antenna is particularly important to ensure efficient operation.

These and many other parameters are used when designing antennas and in this case NFC antennas.

RFID reader elements

The RFID reader can be broken down into a number of major elements or sub-systems:

Antenna:The antenna is an integral element within the RFID reader. The antenna must obviously be tuned to the frequency of operation. It must also be mechanically incorporated into the overall packaging or case design for the RFID reader. For lower frequencies, the RFID reader antenna may comprise a coil, whereas for higher frequencies it may be a form of dipole element. With RFID up to 30 MHz typically using inductive coupling this means that most antennas would comprise a coil, but above this where radiative systems are used and the wavelengths are much shorter, then forms of dipole offer good performance.

Controller:The controller is the area within the RFID reader that provides the control for the system. It will enable the read or read-write processes to be actioned correctly and any protocols that need to be observed will be initiated in this area.

The complexity of the controllers for RFID readers varies considerably dependent upon the application and the system. Some are particularly simple, whereas others will be considerably more complicated.

Network interface: Once information has been gained from a tag, the RFID reader needs to ensure that the required actions are taken. To achieve this, the RFID reader will normally need to communicate with a central controller. Traditionally RS232 or RS422 interfaces have been used, but now there is much greater use of Ethernet, or wireless systems including Wi-Fi, Bluetooth, or Zigbee.

In addition to these elements there is also software as most items these days have processors which are software driven.

RFID tag storage and processing

One important area and function of the RFID tag is the area that handles the information storage and processing. RFID tags range vastly in their capabilities as some do not have their own power, relying on the received signals to provide any power and this limits their abilities. Other RFID tags with their own battery power are able to carry out far more sophisticated tasks.

There are several types of RFID tag that may be used:
One-bit EAS RFID tags: EAS (Electronic Article Surveillance) tags are commonly found in shops and stores to prevent theft. EAS tags are often termed “1 bit” tags. The reason for this is simply that they are only designed to communicate one bit of information, i.e. their presence. They are widely used in anti-theft measures in shops and stores. If the RFID tag is present and active, then it means that the item has not been through the checkout. If they have been passed through the checkout the RFID tag is either deactivated or removed.

Because of their use, EAS tags are used in their millions and possibly the most widely used form of RFID tag. They do not have any memory or other chips as these would make them too expensive. Coupling used for these tags is generally inductive or backscatter. The tags simply consist of a resonant circuit, and the reader is able to detect their presence. A further point to note about EAS tags is that the readers have to sweep across a small frequency band, because the manufacturing tolerances of these RFID tags is such that there is a spread in the resonant frequencies of the different tags.

RFID smart labels : Smart labels are simple RFID tags that are embedded in a an adhesive paper label. The advantage of this form of tag is that they can be used by RFID and barcode readers as well as having the option for human readable characters. They can be used in areas where the end product may enter one of a number of scenarios where the form of reader is not known – for example retail outlets a product may be shipped to may have either a barcode reader or an RFID reader, and outlets will have different options. Therefore to cover all eventualities a combined RFID and barcode tag is printed.

SAW RFID tag: SAW – Surface Acoustic Wave tags form a half way house between the very basic 1-bit RFID tags and the more advanced tags that are available. The SAW RFID tags operate in the microwave region using backscatter techniques, and although they do not have a processor, they can be encoded at point of manufacture with a number. This number is limited by the technology but may be up to 32 or 64 bits.

Smart card tags: Smart card tags are different to smart labels. Advanced smart card tags are used for many applications, and in particular where secure communications is required, for example for transactions involving finance. These cards may have complicated processors on board along with sufficient memory. When using these cards there is a balance to be made between functionality and cost – this needs to be taken at the outset of the design and needs to be carefully balanced.

Although RFID tags may appear to be the more straightforward or simple element within an RFID system, this may not be the case as considerable ingenuity and careful design is required to ensure the RFID tags perform correctly while being capable of being manufactured to a very low cost and within constraints of size, weight, form factor and also reliability. While most RFID tags are very cheap to manufacture, this hides the design behind them.

Read only and read-write RFID tags

RFID tags may be able to either perform as a read only RFID tag, or they may be a read-write Radio Frequency Identification tag. In view of the cost of manufacturing different types against the quantities made and the differences between the two, most RFID tags today are the read-write variety, and for applications where only a read function is required, the write ability is not used.

Read only radio frequency identification tags are typically programmed either in the factory. Data included will be a unique identifier and other specified data that cannot be changed.

Read-write RFID tags normally contain an area where data cannot be altered – this is often a segregated secure read-only area in the memory. Again this will include a unique identifier, and other data that may be required. The writeable area can then be used to contain data that may be required. For example if the RFID tag is used with a container, it can contain details of the container contents, etc. This area of memory within the RFID tag can be re-written many times.

RFID backscatter coupling

RFID backscatter coupling or RFID backscattering uses the RF power transmitter by the tag reader to energise the tag. Essentially they “reflect” back some of the power transmitted by the reader, but change some of the properties, and in this way send back information to the reader.

Using RFID backscatter or RFID backscattering, some tags achieve their data transmission by changing the properties of the tags themselves, while others switch a load resistor in and out of the antenna circuit.

RFID backscatter coupling operates outside the near field region, and the radio signal propagates away from the RFID reader. When the signal reaches the RFID tag, this interacts with the ingoing signal and some energy is reflected back towards the RFID reader. The way in which the signal is reflected back depends upon the properties of the tag (or any other object for that matter). Factors such as the cross sectional area, and the antenna properties etc within the tag all have an effect. In particular the antenna will pick-up and re-radiate energy, and the way this energy is re-radiated is dependent upon the antenna properties – by changing factors such as adding or subtracting a load resistor across the antenna, the re-radiated signal properties can be changed.

Over short ranges, the amount of power reaching the tag from the reader is sufficient to allow operation of small low current circuits within the tag. This can be used to drive an electronic switch, e.g. a FET that can switch an antenna load resistor in and out of circuit. This will effectively modulate the returned signal and allow data to be passed back to the reader.

In order to allow transmission and reception of a signal at the same time, a directional coupler is often used to allow the received signal to be separated from the transmitted on. Additionally the reader must be able to detect the modulation in the presence of a host of other reflections, although these will normally be stable and not modulated in any way.

Active RFID

The original tags that were used in the shopping electronic surveillance tags was purely passive – the next step in the RFID history was to develop active tags.

As happens in the development of technology, several people were working on similar types of development around the same time, each with their own approach or result. In one development a US patent was granted for an active RFID tag with a rewritable memory in January 1973.

Also in the 1970s the Los Alamos National Laboratories started to develop a system to track the transportation of nuclear materials securely and safely. The system included a variety of readers and transponders attached to the vehicles carrying the materials. These would then enable the truck to be identified at various points along its route.

In another development in the RFID history, again at Los Alamos, but in the agricultural department needed to devise a system that would allow individual cows to be identified. A system was devised that enabled a passive transponder was injected under the skin of the cow. The RFID was based at a frequency of around 125 kHz transponder drew power from the reader, reflecting back a “backscatter” signal that was modulated with the cow identification information.

While the low frequencies of 125 kHz were initially used, systems around the 13.56 MHz license free frequencies were also developed. The use of the higher frequency allowed for higher data rates and longer ranges to be achieved.

UHF RFID history

While RFID had previously been focussed on lower frequencies where the technology was cheaper, the advantages of the UHF frequency spectrum started to be employed in the early 1990s. Experiments were undertaken by IBM who then trialled them with Wal-mart. However the technology was sold on to Internec and they were able to commercialise the technology.

The main drawback to large scale commercialisation was the lack of standards. Several processes started to come together to ensure proper standardisation that would allow the grown and widespread use of RFID.

In 1999 a number of organisations set up the Auto-ID Center at the Massachusetts Institute of Technology. This enabled common standards to be set up.

Also the International Standards Organisation, ISO introduced standards for the different elements of RFID from tags to readers and writers, etc.

Another milestone in RFID history occurred when suppliers started to take RFID seriously and in January 2005, Wal-Mart required its top 100 suppliers to apply RFID labels to all shipments.

The history of RFID has shown a steady development in RFID technology. Having its routes in the earliest days of electrical science and then radio, RFID history has come out of developments such as radar and IFF. Now RFID is a technology in its own right which is widely used and showing massive benefits to industry and society as a whole.

NFC

Using NFC data is exchanged by two inductively coupled coils — one per appliance — generating an magnetic field with a frequency of 13.56 MHz. The field is modulated to facilitate data transfers. For the communication one device acts as the initiator (starting the communication) whereas the other device operates in target mode (waiting for the initiator). Thus not more than two devices can be evolved in the communication.
The rolls of the devices — initiator and target — are assigned automatically during the listen-before-task concept which is part of the mode switching of NFC. In general each NFC device acts in target mode. Periodically the device switches into initiator mode in order to scan the environment for NFC targets (= polling) and then falls back into target mode. If the initiator finds a target an initiation sequence is submitted to establish the communication and then starts exchanging data.
NFC distinguishes two operation modes for communication: passive and active mode.
Passive Mode
In passive mode only the device that starts the communication (the initiator) produces the 13.56 MHz carrier field. A target introduced to this field may use it to draw energy but must not generate a carrier field at its own. The initiator transfers data by directly modulating the field, the target by load-modulating it. In both directions the coding complies with ISO14443 or FeLiCa, respectively. This mode enables NFC-devices to communicate with existing contactless smart cards. The term load modulation describes the influence of load changes on the initiator’s carrier field’s amplitude. These changes can be perceived as information by the initiator. Depending on the size of the coils, ranges up to 10 cm and data rates of 106, 212, and 424 kBit/sec are possible.
Active Mode
When in active mode, both appliances generate an RF field. Each side transmits data by modifying its own field, using an Amplitude Shift Keying (ASK) modulation scheme. Advantages compared to passive mode is a larger operating distance (up to 20 cm) and higher transmission speeds (eventually over 1 MBit/sec). To avoid collisions only the sending device emits a electromagnetic field; the receiving entity switches off its field while listening. If necessary these roles can change as often as needed.
Usecases and Applications
An NFC compliant device offers the following modes of communication:
Reader/Writer Mode: In Reader/Writer mode an NFC system acts as an ordinary reader for contactless smart cards. If two or more cards are present in the reader’s carrier field one is selected using an anti-collision algorithm. NFC also takes care of sensing whether the chosen card is ISO 14443-A/B or FeLiCa compliant. The method used for anti-collision is dependent on the type of card detected. This mode causes the NFC device to act as an active device. From an application’s view there is no difference between a conventional and an emulated terminal, accesses to the contactless token proceed equally.
Operating in this mode, the NFC device can read and alter data stored in NFC compliant passive (without battery) transponders. Such tags can be found on e. g. SmartPoster allowing the user to retrieve additional information by reading the tag with an NFC device. Depending on the data stored on the tag, the NFC device takes an appropriate action without any user interaction. If e. g. an URI was found on the tag the handset would open a web browser.
Card Emulation Mode: Tag emulation mode is the reverse of reader/writer mode: A contactless token is emulated. Now the device acts soley in passive mode. Due to the fact that the card is only emulated it is possible to use one NFC wdevice to act on behalf of several „real“ smart cards. Which card is presented to the reader depends on the situation and can be influenced by software. Additionally an NFC device can contain a secure element to store the information for the emulated card in a secure way.
In this case an external reader cannot distinguish between a smart card and an NFC device in card emulation mode. This mode is useful for contactless payment and ticketing applications for example. Actually, an NFC enabled handset is capa-ble of storing different contactless smartcard applications in one device.
Peer-to-Peer Mode: This mode is specific to NFC. After having established a link between the two participants (the method is equal to ISO 14443-A) a transparent protocol for data exchange can be started. The data block size can be chosen freely, with an MTU (maximum transmission unit) limited to 256 bytes. Main purpose of this protocol is to enable the user to send his/her own data as soon as possible (i. e. after a few milliseconds). In a peer-to-peer session either both initiator and target can be in active mode or initiator in active and target in passive mode. This helps the target to reduce its energy consumption and is therefore especially useful if the initiator is a stationary terminal (e. g. a ticket counter) and the target a mobile device (e. g. a mobile phone).
The NFC peer-to-peer mode (ISO 18092) allows two NFC enabled devices to establish a bidirectional connection to exchange contacts, bluetooth pairing information or any other kind of data. Cumbersome pairing processes are a thing of the past thanks to NFC technology. To establish a connection a client (NFC peer-to-peer initiator) is searching for a host (NFC peer-to-peer target) to setup a connection. Then the Near Field Communcation Data Exchange Format (NDEF) is used to transmit the data.

RFID Technology Standards

Depending on the standard use, these cards are either proximity cards (according to ISO 14443) or vicinity cards (according to ISO 15693). Both standards use the so popular RFID Technology, so I’ll talk about that first. RFID standard for Radio Frequency Identification. The technology bases in the inductive coupling of two coils/antennas. Depending on the antennas and the power used to generate the electro mangentic field, distances starting from some centimeters up to several meters can be briged by using RFID Technology. There are several different frequencies used in the RFID Industry, popular ones are: 125 kHz, 13,56 Mhz (used by ISO14443, ISO15693, ISO18092) or 900 Mhz. In an RFID System there is usually an active reader, that generates the electro mangentic field and a passive transponder (tag, smartcard) that is powered by this field (and thus needs no battery). After the passive part is powered, the communication is established and both parties can exchange data. The active part (reader) emitting the field is also called initiator where as the passive one (waiting for the initiator) is called target. So far for the electro magenetic stuff (from a very basic, theoretical side) and back to the smartcards.
As already mentioned both proximity and vinicity card use 13,56 Mhz and therefore are compatible on the very low (physical) layer of communcation. ISO 15693 actually is not considered by the NFC Forum – the standardization body for NFC – thus, I will not give further details on this standard. During the standardization of ISO 14443 the two major players (Infineon, Philips; now NXP) could not agree on a modulation schema, thus there are two different one: ISO 14443-A (Philips, now NXP & Co.) and ISO 14443-B (Infineon & Co. ). There are several popular products using ISO 14443, like the RFID Passport. One of the most widely used RFID Card is Mifare. Mifare is not compliant to ISO 14443 on all levels (only 1 – 3) as it implements a proprietary cipher. Several public transport Systems, like London (Oyster) or Hongkong (Octopus) use Mifare. The cipher actually was broken in 2008. In Japan Sony has introduced its one contactless Smartcard: Felcia; there it is used for Payment & Public Transport (Suica). Felcia is neither ISO14443-A nor –B. NFC Devices in the further should be able to overcome all this different smartcard standards and should be able to talk to any of these contactless smartcards or reader.

NFC Tag Types

The NFC Forum has agreed on the following four NFC tag types.
Type 1:Type 1 Tag is based on ISO/IEC 14443A. This tag type is read and re-write capable. The memory of the tags can be write protected. Memory size can be between 96 bytes and 2 Kbytes. Communication Speed with the tag is 106 kbit/sec. Example: Innovision Topaz
Type 2: Type 2 Tag is based on ISO/IEC 14443A. This tag type is read and re-write capable. The memory of the tags can be write protected. Memory size can be between 48 bytes and 2 Kbytes. Communication Speed with the tag is 106 kbit/sec. Example: NXP Mifare Ultralight, NXP Mifare Ultralight
Type 3:Type 3 Tag is based on the Japanese Industrial Standard (JIS) X 6319-4. This tag type is pre-configured at manufacture to be either read and re-writable, or read-only. Memory size can be up to 1 Mbyte. Communication Speed with the tag is 212 kbit/sec. Example: Sony Felica
Type 4:Type 4 is fully compatible with the ISO/IEC 14443 (A \& B) standard series. This tag type is pre-configured at manufacture to be either read and re-writable, or read-only. Memory size can be up to 32 KBytes; For the communication with tags APDUs according to ISO 7816-4 can be used. Communication speed with the tag is 106 kbit/sec. Example: NXP DESfire, NXP SmartMX with JCOP.)
Mifare Classic is not an NFC forum compliant tag, although reading and writing of the tag is supported by most of the NFC devices as they ship with an NXP chip. The specifications for the tag types are available for free from the NFC-Forum website.
NFC data exchange format (NDEF)
The NFC forum has defined a structure for writing data to tags or exchanging it between two NFC devices. The format is called NDEF. A so called NDEF record can contain multiple different RTD. A RTD is an information set for a single application, as an RTD may only contain an isolated information such as text, a URI, a business card or pairing information for other technologies. The different RTD specifications are available from the NFC Forum website.