What You Need to Know about Radio Frequencies for RFID / EPC

For global supply chain applications, UHF (ultra-High Frequency) RFID/EPC is the standard frequency range used for tracking pallets, cases, and items. For closed-loop applications such as work-in-progress management (WIP) and asset tracking, there really isn't a standard. The best practice is to use the technology that is best suited for your requirements. That could mean using the same UHF standard designed for supply chain, another RFID technology, or maybe a completely different tracking technology. There are companies that will tell you "UHF RFID does not work well with metal and liquids". Yes, these materials can present challenges, but they can be overcome if you know what you're doing.

Magnetic Coupling and Passive Backscatter

It’s important to know that the fundamental way most UHF equipment operates is different from HF equipment. HF equipment uses a near-field technique called magnetic coupling. UHF equipment typically uses a far-field method called passive backscatter. Although these characteristics exist at both frequencies, readers on the market typically support one or the other. (Editor's note: we include the word "typically" because at the time of this writing, there is only one RFID reader on the market known to support both magnetic coupling and passive backscatter for UHF in the same device: The Impinj Speedway, reviewed by our team here.)

This is largely in part because the antenna geometry for both tags and readers needs to designed differently. Near-field will give you read ranges between a few inches and a few feet at the most. Far-field can give you much longer read ranges - a typical stationary reader can read a passive UHF tag up to 25 feet away - depending upon the tag characteristics, physical and environmental conditions. Battery-powered tags can be read at greater distances.

For in-depth technical information on UHF RFID Tags and how they work, read our expert article Passive UHF RFID Tags

Obviously, longer read ranges are better suited for identifying pallets and cases in supply chain applications.

Using Passive UHF Tags Directly On Metal

Passive UHF tags CAN work directly on metal items. If you need far-field performance, the secret is you have to have at least 1/8 inch distance between the tag and the metal, otherwise the metal will short out the tag and it will not read. There are numerous UHF tags on the market specifically designed for mounting on metal.

The pictures below show two UHF metal mount tags installed directly on steel vehicle carriers. There is one tag on each side of the carrier (see photo below left). A single antenna (a read point) is mounted in each work cell (see photo below right). Since the read points could not consistently be installed on the same side of the cell, and the UHF passive backscatter can not read through metal, two tags per carrier are required. Cost is not an issue as you'll soon see.

Our requirements defined the minimum tag quality that was acceptable is 200+ reads per second when the tag was in a horizontal orientation at a 6 foot distance from the antenna. These tags read up to 13 feet away.

The $2.50 tags are used over and over, thousands of times, so the per-use cost is less than 1¢. We installed this solution in April of 2005 and it is still in production today. As of July 2010, it has never missed a tag.

UHF RFID tags in water

UHF Tags and Water

It’s a fact; water detunes RF waves and reduces their range when using the UHF far-field spectrum. For products such as bottled beverages and shampoos, where far-field range is required, work with air gaps in the packaging. Build pallets with tags on packaging facing out as much as possible.

For short-range, near-field applications such as item level tracking, magnetic coupling may be the best solution. Impinj and other companies have designed UHF tags that use the near-field magnetic coupling component. As a result, the UHF tags can be applied directly to liquid-filled containers or even submerged. See photo above.

There are several other reasons why UHF is the frequency of choice for B2B supply chain.

1. The largest retailers and the DoD are already tagging pallets, cases, and items using UHF.
They have been for several years now. These are not just U.S. based initiatives. Companies using UHF RFID for supply chain in North America, Central America, South America, Europe, Asia, Australia, and the Middle East. The most common UHF tagged items in retail are apparel and electronic products such as televisions, DVD players, and digital cameras.

2. Companies do not want to double-tag products.
The predecessor to EPCglobal, the Auto-ID Center, was conceived with the mission of generating a single technology to replace Electronic Article Surveillance (EAS) tags. Manufacturers did not want to have to put two different EAS tags on every product, which is what they had to do. Retailer A used Checkpoint and retailer B used Sensormatic. EPC was designed to make a single standard, which they did: EPCglobal Class 1 Gen 2, which later became ISO-180006-C.

3. Improvements to UHF technology.
No one can change the laws of physics; however, some incredibly bright minds in the RFID industry are focusing full time in improving UHF tags and readers. There are also battery-assisted UHF tags that help overcome moisture and condensation that forms on tags.

4. Changes in packaging
Some of the biggest champions in RFID are the packaging engineers. These forward thinking folks are not just slapping a tag on the side of a box and saying, “we’re compliant.” They are determining what is the best tag for their product; the ideal placement for the tags; and how to build a pallet so the tags are positioned for the greatest reading accuracy. Many have re-designed the packaging around their products with RFID in mind.

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