RFID tags are classified by the frequency they are intended to function in. There are four main frequency bands that are assigned to be used in RFID systems, by different government agencies.
RFID Frequency Ranges
RFID frequency ranges are classified as:
- Low Frequency (LF)
- High Frequency (HF)
- Near Field Communication (NFC)
- Ultra High-Frequency (UHF)
The above RFID frequency ranges were initially confined to non-commercial usage and comprised of frequency ranges called as ISM (Industrial, Scientific and Medical) radio bands. Based on the purpose of the RFID tags, they are designed to work in one of these bands. The frequency is determined by two key elements – the materials of the object to be tagged and the read range that is required.
LOW FREQUENCY (LF) RFID TAGS
- LF Frequency Range – The low frequency range comprises of frequencies ranging from 30 KHz to 300 KHz. However, only 125 KHz or 134 KHz (to be precise 134.2 KHz) are utilized for RFID. This frequency range is used for RFID tags to track animals and it is available for RFID use across the world. The RFID tags used in this frequency range are typically referred to as LF tags.
- Power Source and Read Range – They utilize near-field inductive coupling to get power and connect with an interrogator. The LF tags function as passive tags and they have a small read range of just a few inches.
- Transfer Rate and Storage – Among all RFID frequencies, the LF tags have the lowest transfer rate and typically can store only a limited amount of information.
- Multiple Tag Reading Capability – The LF tags are not equipped with or have limited anti-collision features; thus the simultaneous reading of multiple tags within the interrogation zone is either impossible or extremely difficult.
- LF Tags Readability and Usage – The Low Frequency (LF) tags are easily read when they are attached to items that contain animal tissues, water, metal, wood and liquids. LF tags are the most widely used tags and they are used for asset tracking, access control, animal tracking, automotive control, healthcare and POS (point-of-sale) applications. LF tags are used the most in automotive industry, especially in vehicle immobilizer systems where the LF tags embedded in the ignition key is used to start the car.
HIGH FREQUENCY (HF) RFID TAGS
- HF Frequency Range – The high frequency spectrum comprises of frequencies ranging between 3 MHz and 30 MHz, but there is only one frequency of 13.56 MHz that can be utilized for RFID applications. The frequency is now accessible for RFID applications across the globe with the same power. Tags and interrogators that use 13.56 MHz frequency are typically referred to as the HF tags and HF interrogators.
- Power Source and Read Range – Similar to LF tags, the High Frequency (HF) tags also utilize near-field inductive coupling to get power and connect with an interrogator. The HF tags function as passive tags and they have a read range of under 3 feet.
- Transfer Rate and Storage – The HF tags have lesser transfer rate when compared to UHF frequency, but they have the capability to store more data than LF tags. Certain HF tags can store data up to 4K.
- Multiple Tag Reading Capability – Unlike LF tags, HF tags are equipped with anti-collision capabilities that allows reading several tags simultaneously within the interrogation zone. However, since the read range for HF tags and interrogators are limited, they don’t implement anti-collision which simplifies the process and reduces the cost.
- Design and Affordability – HF tag antennas are typically made up of copper, silver or aluminum coil that has three to seven turns. HF tags are very simple to make and they are thin and two-dimensional. The HF tags are less expensive than LF tags because of the simpler antenna design. They are available in various sizes, even less than half-inch in diameter.
- HF Tags Readability and Usage – The HF tags can be easily read when they are attached to items that contain water, tissues, wood, metal and liquids but can be affected by metal objects that are in the vicinity. Inductive coupling used by High Frequency interrogators utilizes magnetic flux to get power and connect with the tag. Magnetic flux is not directionally oriented but Omni-directional, which means that it evenly covers the entire region around its source and there are no holes in its density. This feature makes HF tags the preferred choice for applications such as smart shelves where the magnetic flux covers the entire shelf so that all the items in the shelf are scanned. Some of the other areas where HF tags are used are in asset tracking, credit cards, smart cards, airline baggage and library books. The lack of restrictions on the usage of HF frequency and the growing popularity of smart cards has enabled HF tags to be the most commonly used tags across the globe.
Near Field Communication (NFC) Tags
- NFC Frequency Range – NFC tags operate in the high frequency range of the RFID band at 13.56 MHz.
- Power Source and Read Range – NFC tags function as passive tags and operate without a power supply of their own and communicate using the ISO 14443 type A and B wireless standards. NFC tags provide secure data transmission within a distance of approximately 10 centimeters.
- Design – NFC tags are designed to store information and transfer a wide range of data types to other NFC enabled devices. They are designed with the basic architecture of RFID tags and contain three components – a tiny microchip or integrated circuit, an antenna and a material or substrate layer to hold all the components together. NFC tags are available in a wide range of shape, size and form factors with a variety of storage capacity and transfer speeds.
- Usage – NFC tags are ideal for use in access control, transport, consumer electronics, healthcare, payment, information exchange and collection, ticketing applications and coupons.
ULTRA HIGH FREQUENCY (UHF) RFID TAGS
- UHF Frequency Range – The ultra-high frequency range comprises of frequencies ranging between 300 MHz to 1,000 MHz, however only two frequencies 433 MHz and 860-960 MHz, can be used for RFID applications. The 433 MHz frequency is used for active tags whereas the 860-960 MHz frequency range is used mostly for passive tags as well as some semi-passive tags. The 860-960 MHz frequency range is generally termed as one frequency, either 900 MHz or 915 MHz. Tags and interrogators within this range are known as UHF tags as well as UHF interrogators.
- Power Source and Read Range – The semi-passive and passive tags within this frequency range use far-field radiative coupling or backscatter coupling. UHF tags have an approximate read range of 15-20 feet.
- Multiple Tag Reading Capability – All the protocols in the UHF frequency range have some form of anti-collision ability that allows reading several tags simultaneously within the interrogation zone. The new Gen 2 protocol for UHF tags is designed to read hundreds of tags every second.
- Design and Affordability – UHF antennas are typically made of copper, silver or aluminum that is deposited in the substratum. Their length of effectiveness is around 6.5 inches, roughly half the wavelength of the 900 MHz radio waves. The optimal length of UHF antennas is one-half of the wavelength of the carrier wave; however, with the right design, the length could be decreased. The UHF antennas are thin and they are simple to make which allows tags to be extremely small, just 100 mM, and pretty much two-dimensional. UHF interrogators are expensive than HF interrogators; however, UHF tags are getting more cost-effective.
- UHF Tags Readability and Usage – Since water absorbs UHF signals, UHF tags can’t be quickly read when they are being attached to objects with animal tissues and water. The UHF tags, when attached to metal objects, become detuned. Therefore, to improve UHF tag efficiency, it is important to separate UHF tags from objects made of metal or that contain liquid. UHF tags are not readable when water or any other conductive material is put between the tag and interrogator antenna. Radiative coupling that is used by UHF interrogators utilizes radio waves for power and to interface with the tags. Diffraction, reflection and refraction of radio waves provide the multipath effect that allows radio signals to arrive at the receiver through different routes. Signals that come from multiple paths diminish the strength of the original signal. This results in an interrogation zone with diverse signal strength. Sometimes, the UHF tags cannot be read in areas with low signals, which eventually causes random tag readability issues. The UHF antennas are directional that enables you to create an interrogation zone with clearly define boundaries, even though the zone could contain holes. UHF tags, because of the requirements of large corporations to use them within the supply chain, are getting a huge boost. This along with the introduction of Gen 2 protocol have created an enormous momentum in the RFID industry to make UHF tags at a low cost in high quantity.
- Government Regulations – UHF frequency regulations aren’t similar to the frequency regulations for HF tags. Governments across the globe had allocated UHF frequencies of approximately 900 MHz, which was long before RFID, for applications that were not RFID-related. Therefore, there isn’t a common frequency band of around 900 MHz that can be used for RFID applications. Different countries have different frequency ranges with different allowable maximum powers and duty cycles. To solve this issue, Gen 2 protocol was developed to operate on any frequencies in the 860-960 MHz frequency range and also with various max power levels. Government regulations have split the allowed frequency range into different smaller frequency bands. These smaller bands are called as channels. Countries have different number of channels within their allocated bandwidths. Regulations also require that interrogators don’t make use of one channel all the time, but rather randomly hop between the channels that are available.
The following table displays the RFID frequency ranges by country and the allocated band sizes, maximum power and number of channels assigned.
|North America||902-928||0.5-4 W EIRP||Varied|
|Europe (302-208)||918-928||4 W EIRP||50|
|Europe (Lower Band)||865-868||2 W ERP||15|
|Japan||908.5-914||4 W EIRP||16|
|Singapore||866-869, 923-925||2 W ERP||20|
|Korea||950-956||0.5 W ERP, 2 W ERP||10|
|Australia||865-868||4 W EIRP||12|
|Argentina, Brazil, Peru||902-928||2W ERP||20|
|New Zealand||864-929||4W EIRP||50|