Determining the Proper Frequency for Your RFID System

Most of the hype in the popular press concerning RFID centers around the U.S. Department of Defense (DoD), Wal-Mart, Target, Albertsons, METRO Group, Tesco, and other companies mandating the use of UHF RFID systems. These systems incorporate the electronic product code (EPC) numbering scheme and data construct. The EPC UHF systems have grown exponentially in popularity since 2000, which tends to make people forget that there is a whole other world of RFID out there beyond UHF and EPC. Work-in-process (WIP) systems and manufacturing automation have used high-frequency (HF) systems for decades. Toll-collection lanes, shipping containers, and similar structures have used active or semi-passive tags for nearly as long.

The contactless transfer of data and the ability to read without line of sight gives RFID numerous applications in commerce and industry. Most RFID systems operate in RF bands requiring no license. In the United Sates, the Federal Communications Commission (FCC) refers to these as nonlicensed for Instrument, Scientific, and Medical (ISM bands) uses. When designing an RFID system, the requirements of the application, trading partners who may be participating in the extended RFID network, standards and regulations governing a given geographical region, and cost can be all dictate which RF band and RFID system needs to be used.

RFID systems are commonly classified according to the properties of the data carrier, or tag. Each RFID system can be broadly classified by those tags, therefore you would refer to a system as using an active, passive, or semi-passive tag, as shown in Table 5.1.

Table 5.1: Types of RFID Systems
Open table as spreadsheet

Passive Technology	LF or Low Frequency	HF or High Frequency	UHF or Ultra-High Frequency	Microwave
Frequency Range	125 or 134 KHz	13.56 MHz	860–960 MHz	2.45 and 5.8 GHz
Approx. wavelength	87988.07˝	870.79˝	12.9˝	4.81˝
Max. Read Range (Passive)	Less than 1 foot	Less than 3 feet	Less than 30 feet	Less than 10 feet
Approx. Maximum Tag Populations	16 (w/ anticollision tags)	50	200 +
General Characteristics	Relatively expensive, even at high volumes, because it requires a longer, more-expensive copper antenna. Additionally, inductive tags are more expensive than capacitive tags.	Less expensive than inductive LF tags. Best suited for applications that do not require long-range reading of multiple tags.	Because of its adoption in supply-chain and new silicon and inlay manufacturing techniques, costs have come down. Offers a good balance between range and performance-especially for reading multiple tags.	Similar characteristics to the UHF tag but with faster read rates. Offers the most directional signal, which is ideal for certain applications.

Note 

CompTIA classifies tags that are passive but assisted by a battery as being semi-active. This is an incorrect classification, as the tags are semi-passive, in that they require a reader's interrogation zone to make them work-like every other kind of passive tag.

Here's a brief explanation of the active and passive systems:

• Active RFID Active RFID tags operate at 433 MHz or 2.45 GHz. Powered by a battery or an external power source, they have a higher energy signal and can communicate at greater distances. Because the active tag has its own transmitter and power source, the tag can be instructed to send its data at periodic intervals, often referred to as broadcasting intervals, without being interrogated by the reader. Because of an external power source, active tags can support sensors, large memory sizes, data loggers, and more. These tags have a finite working life (typically 3–5 years) because of the need for a battery. Metal can possess some nominal interference. Because of the complex tag design, the implementation costs of such systems are relatively high, with tag prices ranging from $10 to $100. Because of the battery and additional data-carrying capability, active tags also can be fitted with sensor and measurement devices for vibration, temperature, and other effects.

• Passive RFID Passive RFID tags are powered when they enter the radiated field of a reader. The reader powers the tag by providing a continuous wave after the query signal. A continuous wave is an electromagnetic wave of constant amplitude and frequency. Because the passive tag activates and communicates only when the reader interrogates and powers the tag, the read range and the signal strength of tag response is relatively smaller than for active tags. Passive tags also have limited memory size, because the memory read and write is a highly power-hungry process. Compared to active tags, passive tags are much smaller and can be manufactured in very large quantities at a very low cost. For this reason, passive technology has been largely adopted and deployed in several applications such as supply chains and manufacturing.

• Passive RFID uses the following two power-transfer techniques:

  • Inductive coupling Transfer of energy from one circuit component to another through a shared magnetic field. Inductive coupling favors transfer of lower frequencies. LF and HF technologies use inductive coupling to transfer energy between the tag and reader.

  • Capacitive coupling, or E field coupling Transfer of energy from one circuit to another by means of the mutual capacitance between the circuits. UHF and microwave technologies use capacitive coupling to transfer energy between the tag and reader.

The semi-passive tags use the same type of communication methodologies as a regular passive tag, but have a battery on board to help create a stronger signal and get a greater read distance. Having a battery on board a passive tag also allows for taking measurements from built in sensors as well.

Table 5.1 summarizes the various types of RFID technologies.

Note 

Although not exhaustive, the RFID technologies in Table 5.1 are the most popular and also the ones CompTIA is most concerned with for the test.