RFID reader antennas have both a near field and a far-field. As stated previously, UHF “squiggle” type RFID antennas often are capable of working in both the near and far-fields even though they are generally optimized for far-field communication. UHF RFID tags are generally considered to be the standard for inventory tracking/management so we will focus on UHF RFID tags with UHF RFID reader antennas.
Most UHF RFID reader antennas on the market can be considered far-field antennas since, while they still have a near field by definition, are meant for medium to long-range RFID communication using backscatter coupling. If a reader on the market does not specify that it is a near field antenna, it can usually be assumed that it is not a near field antenna (e.g. it is designed for backscatter coupling but is technically capable of both).
Near field antennas are optimized for short read ranges and while they are typically used for LF and HF RFID, there has been significant work done in recent years to design and manufacture near field antennas for UHF RFID.
While general-purpose RFID reader antennas reign supreme when it comes to reading many tags in a large read area, near field RFID reader antennas have the advantage of confined read regions that don’t pick up unwanted nearby tags. By virtue of this confined read region, one can imagine many possibilities for object locating and identification. For example, several near field antennas could be placed on a shelf and identify objects based on their location. This would be much more difficult for general purpose RFID reader antennas because general-purpose RFID antennas will pick up nearby unwanted tags (without a significant decrease in their output power and/or additional external shielding).
General Purpose RFID Antenna Range vs Near Field RFID Antenna Range
General Purpose RFID Antenna Shelf vs Near Field RFID Antenna Shelf
Null Spots and Antenna Diversity
When radio waves are radiated from an antenna, they interact with the surrounding environment. This interaction can come in the form of absorption (when some or all of a wave’s power is absorbed by a material, for example, liquid), refraction (when a wave’s direction upon passing through one medium to another), and reflection (when a wave bounces off some material, for example, metal). This is analogous to light being absorbed by plants/trees, refracting through water causing visual distortion, and being reflected on metallic materials. Furthermore, when waves are absorbed, refracted, and reflected, they will interact with one another as they propagate. These interactions can be constructive or destructive, meaning that when the radio waves interact with one another, they can combine to create a ‘hot spot’ within which RFID tags are easily read, or they can cancel and create a ‘null spot’ within which RFID tags may be impossible to read. This 3-dimensional RF power map can be difficult and expensive to accurately characterize (especially in environments that are constantly changing) and since we have other tools at your disposal, it may be unnecessary for your application. The characteristics of an RF power map are probabilistic in nature (and time-dependent in a changing environment) so null spots can mean that certain tags locations/orientations are impossible to read or that it may take several seconds (or minutes) for that tag to be “seen” by the reader.
Hot Spots and Null Spot Visualization
A well-planned antenna layout can mitigate the unpredictability of hot spots and null spots in your RFID application. For example, by using multiple antennas at different locations within the environment you can mitigate null spots since each antenna will have a different hot/null spot pattern within the environment.
A combination of general-purpose (far-field) or shelf type (near field) antennas can also help mitigate this issue. Placing tagged items on a shelf type near field antenna with surrounding general-purpose antennas can minimize the number of null spots in the area of interest without the height limitation that would be introduced when only using a near field antenna.
Intelligent planning when it comes to the type of antenna as well as the location and orientation of those antennas has a major impact on the performance of your RFID inventory system. Read range, reliability, and speed are all affected by the selection of your antennas and their placement. RF environment modeling can be used when cost and time permits but high performing systems can still be achieved with proper antenna selection, intentional antenna placement, and testing.