Smart RFID tags enable reading next to metals and liquids

With the help of a RFID reader, you can scan all the clothing RFID tags inside a cardboard box from across the room. But fill that box with cans of soda, and you might not be able to read tags until you are inches away. How to solve these issues? 

Radio frequency identification (RFID) has already been adopted at a large scale in the fashion apparel industry, as in major retailer supply chain logistics. As a technology, RFID is superbly suited for scanning high product volumes and a high number of stock keeping units (SKUs), but problematic when reading next to metals and liquids. 

The big barrier to adopting the technology used to be cost. Some twenty years ago, tags cost tens and even hundreds of Euros, limiting their use to tracking expensive assets like truck chassis, tools and other equipment. Now some tags cost as little as a few cents, making widespread use highly affordable. Today, tag vendors are able to even produce low-cost tags that read properly also on metals and liquids. 


Different ways UHF RFID reacts to reading around different materials. 

Different ways UHF RFID reacts to reading around different materials. 


RFID Tags store large amounts of data

Just like with logistics, profitability in retail is closely linked with efficiency. Efficiency in retail demands intimate data knowledge and transparency. Tags can store great quantities of information specific to each individual item and build a history as they, for instance, travel from manufacturer to point of purchase or use. RFID in retail has produced tremendous results so far, and there are hundreds more use cases waiting to happen.

In addition to serial number, colour, size, brand and other product details, tags can accumulate information like ambient temperature, time in, time out, and other data via an RFID reader that has authorization to add or modify tag information.

Since RFID doesn’t require line of sight to function – like barcodes -, readers installed in ceilings and floors can read passive tags at high speed from distances of up to 15 metres. Every item on a pallet can be automatically scanned and entered to an inventory system as it is wheeled from trailer to warehouse. The biggest challenge when it comes to reliable tag reding, used to be materials such as metal or liquid

the challenge of RFID tagging

Metals detune and reflect RFID signals, while liquids absorb them. This has historically caused poor tag read range, unreliable reads, or no read whatsoever especially with Ultra High Frequency (UHF) RFID tags.

UHF RFID tags are not the only ones in use, but their advantages have made them the gold standard.
Here are some of the differences between types of RFID reading:

  • UHF Ultra high frequency reading takes place in the 800 to 900 megahertz range. Reading is quick and covers distances of up to ten metres in the case of passive (non battery powered) tags. UHF has historically performed poorly in reading metals and liquids.
  • HF High frequency reading works much better than UHF with metal objects and goods containing liquids but is limited to a read distance of maximum one metre.
  • LF Low frequency reading is appropriate with read distances of less than 30 centimetres. LF doesn’t work well with metal, but does work with beverage containers, produce and other similar items.

Active UHF RFID Active (battery-powered) UHF tags send out a signal when they detect a properly configured RFID reader. They are used to track items up to 30 metres away and more, depending on the strength of the tag signal. Active tags are often used for high value inventory such as military hardware and vehicles, aviation equipment and some logistics processes.


Accurate RFID reading near metals and liquids

Clearly, inexpensive tags that can be read quickly and accurately at a distance are the winners in most scenarios. The ultimate tag is small, thin, inexpensive, tolerates chemicals, water, reasonably high temperatures and rough handling. Furthermore, its readability is not affected by the presence of metal or water. Julian Krenge, a researcher at the Institute for Industrial Management, Aachen University in Germany, illustrates several options.

“One way to increase readability is to create a space of one centimetre or more between the tag and the object it is affixed to,” says Krenge. He notes that such tags are bulky and relatively expensive. Another experiment he conducted with some success was to use regular 2D tags on a six-pack of plastic bottles of water. “If you calculate the optimal position, choose the right type of tag and pre-determine how the product is transported, readability can be decent,” he says, though he acknowledges that such a scenario is not always practical.

A third option is to have the antenna rise vertically off the tag. This works well, Krenge believes, “but it’s not very practical in most environments where goods are stacked beside and on top of each other.”

The key: integrating antenna design with chip manufacture. Professor Heikki Seppä – a senior research professor with VTT and known in European scientific circles as Mr. RFID- is a pioneer in the area.

“In close proximity to ferrous and most non-ferrous metals, you have to use a 3D antenna in order to get a clear reading,” says Seppä. “That’s being done now – it’s quite simple – and the price of materials to build the antenna is not much higher than for a 2D metal tag.”

Seppä points to Traditional PIFA antennas (Planar Inverted FAntenna) as the obvious technical solution to the problem. However, the vias needed between the two conducting layers of the antenna make the structure complex to manufacture.”

Antenna manufacture and chip manufacture are traditionally separate industries. For cost-efficiency to become a reality, the chip must be embedded into the antenna layer in one simple, cost-effective process.


Like design, volume is crucial

In scientific research, several actors have been working on perfecting the process of manufacturing RFID tags – including thin, flexible tags that work with metal. These platform insensitive/platform tolerant and platform-adaptive RFID tags are not innovative in the sense that they make use of new technology, but rather in the way that they are put together.

Some ten years ago, VTT developed a version of the simple 3D tags called ‘PAFFA’ for Planar Asymmetrically Fed Folded Antenna, as part of their RFID research. The challenge the market faced at this time was that Traditional Planar Inverted FAntennas (PIFA) offered a technical solution to the problem, but the vias needed between the two conducting layers of the antenna made the structure complex and expensive to manufacture. The PAFFA solution replaced the vias with a special antenna substrate folding technique. This meant replacing the one critical via typically required at point of attachment to the tag by a substitute circuit element. PAFFA type antennas resulted in small tags that efficiently read over long distances. Today several tag vendors provide metal and liquid enabled UHF tags as part of their standard offering.

Economical 3D tags will be important to the advancement of many industries. In automobile manufacturing, for instance, inserting RFID into the value chain is critical for companies to be able to keep up with the increasing demand for consumer personalization. Volume is crucial to drive tag prices down. In addition, different products and applications will require specific configuration of the tag.

Tags will require different configurations because of the nature of the materials they are attached to and the way that those materials are transported and stored. Bottles of juice will require more inexpensive tags than those manufactured to track airplane wear components.

Today tracking convenience articles like water bottles and soda cans is possible at an affordable price. In June 2020 Nordic ID and Neste launched an unmanned convenience shop in Helsinki. The change in consumer behaviour lets on that the concept of always open, small self-service convenience stores will spread rapidly not only to other parts of Finland, but also to other parts of Europe. This implies that the demand for special tags for RFID reading is going to increase.

Not sure how to achieve the best coverage yet to avoid RFID reader collision problem ? Read our expert article.