Demystifying UHF Gen 2 RFID, HF RFID

Before making a decision on the type of radio frequency identification technology to deploy you need to understand UHF and HF basics.

There are two components of the RF wave: magnetic and electric. Generally, HF RFID 13.56 MHz relies on the "near-field" magnetic aspect of the field, while long range UHF RFID 860-960 MHz exploits far-field radiation, which consists of both electric and magnetic components. But just which part of the RF wave a UHF tag responds to depends on two things: the tag antenna and its distance from the reader.

Since the magnetic component of the wave diminishes greatly in strength over a relatively short distance, it is characterized as near field. Its effective range is limited by antenna geometry to about one or two wavelengths. Because HF tags rely on inductive coupling to this magnetic field in order to receive power, the HF tag antenna is made up of an inductive, coil-like structure that requires more conductive material and more complex manufacturing processes than the equivalent UHF tag antenna. Fortunately, HF tags don't have a corner on the magnetic field; with the right antenna, UHF tags can just as easily harvest the same near-field energy, and do so more efficiently and cost-effectively. How? One word: Physics.

Maxwell's four equations are the basis of electromagnetic analysis and design. Faraday's law is one of these equations: "Voltage induced on a coil in a magnetic field is proportional to the intensity and frequency of the field." This translates to a very simple concept: higher frequency = greater efficiency. UHF is 60X the frequency of HF, meaning that UHF is ~60X more efficient in coupling its energy between tag and reader antennas. You can't argue with the physics!

Conventional wisdom suggests UHF is not suitable for item-level tagging: The tag is too big, UHF won't work on liquids, metals, or small items packed in close proximity to one another, and UHF has too great a range—all of which ignore the fact UHF Gen 2 can exploit the near field just as easily and much more effectively than HF. That means anything that can be read by an HF system can also be read by a UHF system, including items high in liquid or metallic content. More importantly, this means item-level applications can now leverage all the benefits the UHF Gen 2 standard brings to the supply chain.

The key is to harness UHF's near field, an aspect of the radio wave particularly well-suited to close range, item-level RFID operations. There is a growing suite of applications that rely on near-field UHF Gen 2 solutions. Some comprise Impinj's Speedway reader, Monza tag chips, and a variety of application-optimized antennas. A brief survey of UHF near-field RFID deployments in retail, textiles, and pharma illustrates the many solutions that are already producing tremendous results from the manufacturing floor to the point of sale, but first some history:

RFID and the Great Frequency Debate
In December 2004, EPCglobal ratified the UHF Gen 2 Protocol, creating the first worldwide standard for RFID. Since then, we've seen many products built to UHF Gen 2 standards. The proliferation proves the applicability in uses ranging from items to cases to pallets, for use with objects in the near field and far field, on liquids, metals, tightly stacked and packed items, and more.

Three years later, HF products developers have yet to ratify a standard. Instead, the latest HF specification has left authors disillusioned. According to Ken Laing, the editor of the proposed HF "V2" (the HF version of UHF Gen 2), their work thus far offers little, if any, improvement over existing standards and commercially available products.