RFID: Optimizing Label Placement

Most people know that metal and liquids can pose problems for radiofrequency identification (RFID). But did you know that an RFID tag on a shipping container full of metal parts might perform better than the same tag on a carton of photocopy paper?

The same product that has several packaging configurations may result in different performance with the same RFID tag (label). It's important, therefore, to evaluate every product and packaging variation for proper tag selection and performance.

Basic UHF RFID Physics
If you're concerned about RFID in the supply chain, you're concerned about the EPC system. EPC operates in the ultra high frequency (UHF) range of 860-920 MHz. UHF is a compromise between the effective range and speed of data transfer offered by microwave RFID (2.45 GHz) and the ability of high frequency RFID (13.56 MHz) and low frequency RFID (approximately 320 kHz and below) to penetrate obscuring material and see around corners. UHF was also chosen because it was the frequency band that had the best chance of being acceptable under all the national regulations around the world. It was, in fact, the best available choice at the time, and still is.

However, no RFID frequency range and system is perfect and UHF RFID is no exception. Liquids absorb or attenuate (detune) UHF RF energy and metal can either absorb or reflect it, depending on the amount and shape of the metal. While it may seem obvious that certain products will have an adverse effect on RFID, the truth is not always so clear-cut.

Liquids. Even a carton of photocopy paper may prove problematic for RFID labeling. Why? Because the liquid that affects RFID performance does not have to be an actual liquid. Paper typically has high moisture content, and it does absorb RF energy.

Wood pallets made with green wood— and certainly bunks of anything but kiln dried wood—present the same challenge because of the moisture content. Fresh fruits and vegetables and frozen items will also pose liquid problems for RFID.

It's important to evaluate whether your items have the potential to hold or attract moisture when considering liquid effects of RFID.

Metals. Metal is perhaps more of a challenge because it may either reflect or absorb RFID. In some situations, the presence of metal can actually improve the performance of an RFID tag.

There are special RFID tags designed to be placed directly on flat metal surfaces. These tags employ a relatively thin layer of dielectric insulation between the tag and the metal surface. This effectively turns the metal surface into part of the antenna and can significantly improve performance by using the metal to reflect the RF signal back to the interrogator that would otherwise radiate into the item.

The amount of space required between the tag and the flat metal surface is a matter of millimeters—and could be no more than the thickness of the corrugated container in which the item is packaged.

Irregular metal, on the other hand, will either absorb the signal or reflect it in random directions.

As with liquids, there is more to metal than may be immediately obvious.

Metalized foil bags and even anti-static bags can act as metal. Some materials have metallic contents or coatings that you may not consider. Rice, for example, has a high mineral iron content that does actually affect RFID performance. As another example, many automotive windshields now contain metallic compounds to improve reception of integrated radio antennas.

Void Fill. The choice of void fill can affect RFID. Bubble wrap and loose Styrofoam void fill have very little effect on RFID whereas dense foam will absorb some RF energy. Crushed or formed paper/corrugate void fill will have little effect unless it's very densely packed (and then may pose the potential of the liquid problem).

Tag Selection, Placement and Workarounds
At this point, different manufacturers' tags can perform quite differently. Once EPC Gen2 UHF tags become available, this performance difference should begin to diminish, but it will never go away completely. Therefore, testing must be done with various vendors' products.

Another significant factor is the tag's antenna design. Beyond proprietary differences among vendors, there are omnidirectional antennas and dipole antennas. Omni-directional provide relatively orientation-insensitive coverage, depending on the product and packaging, whereas dipole antennas are orientation specific. That is, dipole antennas produce a rather cigar-shaped field that must be aligned with the reading antenna. While this may seem like a limitation for dipole antennas, it can work to your advantage (and most reading antennas are designed to accommodate both directional and omnidirectional tags).

Let's look at some examples: a box of gears, a case of quart-size oil cans, and a case of copy paper. These products present rather consistent issues. Nonetheless, testing under a variety of real-world conditions will be required to determine whether a particular tag and placement will produce adequate results.

First, the copy paper. Regardless of where you place the label, you'll encounter the same moisture issues. Will this be a problem? It depends. Some types of paper are packaged in plastic wrappers and will maintain a consistent moisture content. Some types of paper are packaged in paper wrappers or are simply packed tightly in the case. Here the contents may absorb or lose moisture depending on the environment. Packaging has a great deal to do with determining performance.

A case of quart oil cans sounds like a real problem, but that isn't necessarily true. There are two possible approaches. First, you can place the label on the top of the case so that the flat metal surface of the top of the can reflects the signal. Second, you can use a tag with a dipole antenna on the side of the carton, placed vertically between the cans. The air space that's the result of the curved surfaces can provide enough distance that the metal cans do not significantly affect the RF signal.

A box of gears is more problematic. The metal will both absorb and reflect the RF energy, reducing range. Gears may also be packaged with dense void fill that may further absorb RF energy. One simple workaround for this is to place a metalized foil sheet on the inside of the carton behind the label. The foil will reflect the signal and nullify any adverse effects of the contents.

But what about items that are not so consistent, like a gallon glass jar of pickles? The jar is full of liquid that will absorb RF. The lid, however, is a piece of relatively flat metal that will reflect RF. The solution to this is the same as for the case of oil cans.

Pallets, Drums, Cages and Non-packaged Items There are times when the transport device is problematic, such as green wooden pallets, metal bins, cages, totes, drums, kegs, or non-packaged items. Again, it may be possible to use a specialized tag that uses a dielectric insulating layer directly on a metal item. In other situations, however, it may be more advantage to use a tie-on tag—assuming the tag can be held away from the metal sufficiently. Here the caution is not to use a metal wire to tie on the tag since the wire will affect the RF signal. A nonconductive tie should be used.

There is no simple answer to tag selection and placement. Testing is the only real answer. For a more complete understanding of the issues, the RFID Experts Group (REG) of AIM Global, the worldwide trade association for AIDC technologies, has produced a guideline on RFID labeling that is available as a free download. While the current document is proposed guidelines for the U.S. Department of Defense, it is the best information available to date.

RFID Label Placement Options

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