RFID tag performance is traditionally evaluated based on read range. But another factor plays just as big of a role and is often overlooked: bandwidth.
Most sellers and users of RFID tags only characterize tag performance by one factor: read range. There are situations where it makes since for read range to be a shorthand for tag performance. But response bandwidth that can have a significant effect on a tag’s performance in real world applications. Overlooking it can cost you read range and performance.
RFID Frequency Bands
RFID readers in North America operate in the 902-928 MHz frequency band. The readers ‘frequency hop’ within this band, changing frequencies at random. The purpose of this hopping is to avoid interference with other readers in the vicinity.
Frequency bandwidth is the ability of a component to respond to signals at a variety of frequencies. The figure below illustrates this idea. The greater the bandwidth, f2-f1 , the more frequencies to which the system can respond.
The interaction between this frequency hopping and the tag response bandwidth has an effect on real world applications. Mount on metal tags, in particular.
How Frequency Impacts Performance
Passive RFID tags are designed to gather energy and respond to query signals from a reader, known as the reader-talk-first protocol. In the context of RFID, wider bandwidth implies more uniform read distance in the operating frequency band.
Well designed, general purpose commercial RFID tags typically have excellent bandwidth characteristics when applied to cardboard or fabrics. Thus the tag response is almost uniform across the band. This fact has greatly eased the deployment of RFID systems to track movements of commercial inventory through supply chains.
The case is quite different for Mount on Metal (MoM) RFID tags. Rather than gathering energy from a propagating field in space, a MoM tag must gather energy from the reflected field close to the surface of the metal. This is accomplished by employing a resonant cavity with a dielectric core. The antenna is distributed on the surface and in the body of the tag.
However, there is a necessary trade-off that comes with a resonant cavity design. As the cavity becomes smaller, the number of modes, read bandwidth, is squeezed. This affect is significant because MoM tags are often deployed on objects with limited real-estate for mounting.
Another consideration is that MoM tags are typically used in close proximity to one another. The objects they attach to scatter radio waves and can create shadow read areas. These areas vary in space as a function of frequency. The result is that one tag may be shadowed at one frequency but not at another.
Read Range & Frequency: Real World Example
How does this relate to real world system performance? Consider the plot of read range as a function of frequency. It maps two commercially available MoM tags: a 13mm X 7mm ceramic tag and a 10mm X 5mm composite tag.
The ceramic tag shows a better read range over a few frequencies. But, it falls off for over more than half the band. At most frequencies, the ceramic tag has a lower read range than the composite tag.
In a real world environment, reading a collection of composite tags will be faster and more efficient as those tags are better configured to compensate for frequency hopping and shadowing effects.
Users that select the tag on the basis of read range, are likely to see slower and missed reads when reading high numbers of tags in high density environments.
It’s important that RFID users not over-simplify the tag selection process by an exclusive focus on read range. A tag’s response frequency affects the way the tag performs in real world applications. If the user wants to gather data reliably and quickly, it is likely that the bandwidth of the tags will be an important consideration in the tag selection process.