Read range for an RFID tag is affected by many factors, including:
- Passive, BAP, NFC or Active
- RFID frequency - LF, HF or UHF
- Surrounding materials
- Type of tag
- Type of reader
- Time to read
- Number of tags being read
- Density of tags
Given all of the above, the only general comment that we can make is that read range can be anywhere from 1cm to a hundred meters. We address each of these issues in the following sections. Note that we have given general information assuming a typical working environment. The numbers we quote below are conservative, and reflect the different factors that affect read range. In all cases the numbers could be improved considerably if each of the factors above was optimised. So please take the numbers in the way they are intended, as a general guideline.
Passive RFID is the most common. Power to the tags is provided by the reader, and the tag has no power source. Advantages include low cost, indefinite life, small size.
BAP tags (Battery Assisted Passive Tags) have an internal battery. They are more tolerant of surrounding materials, but they are more expensive, slightly larger, and have a finite life, typically a few years.
NFC, for near field communication is essentially using passive RFID Tag technology to communicate in both directions. Low range is a benefit in this case, as there is less chance for confusion as to which tag is communicating with which reader.
Active RFID gives greater range. Roles are reversed in this technology. The tag is powered, and emits a signal on a pre-determined schedule, which might typically be one beacon per second. The readers are receivers, typically static, powered and connected to a network. Read range is typically one hundred metres.
A little history here. The first RFID in common use was LF - Low Frequency. Used for animal tracking, it offered a low read range, typically an inch or two, which was, in this application, sometimes beneficial, as the person using the system had a lot of confidence in knowing which tag was being read. Disadvantages of LF technology included the cost and size of the tag. The tags required many turns of fine wire to transfer energy from the reader to the tag, and this made them expensive to produce, and bulky in appearance.
The next generation of tags were HF. These operated at a higher frequency, and required less turns. Tags could now be printed using conventional printed circuit board techniques, but as a complete circuit was required, tags had to use double sided laminate, with a through connection to complete the circuit. Tags were less expensive and less bulky than LF. Range was extended to perhaps 25cm or so. Typical uses included door entry mechanisms.
The next generation of tags were UHF - these tags use a simple antennae. This makes the design much simpler, though the antennae still needs to be long enough to give a reasonable read reliability. Often, also, the tags are encapsulated, and may be artificially thick, to enable the tag to be mounted on a metal surface. In this case the actual tag mechanism is stood off from the metal by the thickness of the tag. Read ranges can be up to 6 meters or more, but the tags are more affected by metal or liquid. Placing your hand over a tag can reduce its readability.
All RFID tags are affected by the materials around them, but the degree to which they are affected depends on the type of tag. Straight passive tags are affected most. These tags have no on board power source, so they rely on energy that they harvest from the reader. If they are close to conductive material, the transfer of power from the reader is disrupted, and the communication with the RFID chip is lost. Conductive materials in this case include metal and water, particularly salty water. The human body contains lots of salty water, so a passive RFID chip held close to the body (or in a pocket or wallet) may be difficult to read.
For all frequencies of tag there are different physical configurations. These usually make
trade off between size and sensitivity, particularly for UHF tags. We use small
button tags for some laundry type applications. These have a read range of a few inches. The
squiggle type tags are about four inches long and offer a much better read range, two
or three feet. Encapsulated tags, such as the long blue tags from Intermec are more reliable still
longer reads. And some tags have even longer antennae, specifically designed for use in
car windscreens. We have read these reliably at a range of sixteen feet with a handheld reader.
Tags can be read with mobile or fixed readers. For UHF tags, power is provided by the reader, and the trade off between range, battery life and antenna size for a mobile reader means that read range is reduced. A typical working range for a mobile handheld reader is up to one meter, again, depending on tag configuration. A fixed mount reader has more power available, and a larger antenna. Read range in normal practical conditions can be up to 6 meters.
The read range around a tag is not spherical. Tags are least sensitive
end on. The
readability is also affected by the antenna of the reader, with circularly
polarised antenna providing a shorter range but less reliance on orientation. A linear antenna can
give a more impressive read range for a given tag, but turn the reader through ninety
degrees, and the tag becomes invisible.
The longer the read time, the better prospect for a read at the extremes of range. This is especially a challenge for portals, that try to read many tags on a skid that passes quickly through the portal. A particularly elegant solution is to place the reader on the fork lift truck, so that it has more time to read.
Not usually a serious problem unless the tag count is very high, but it can have an effect.
This can particularly be a problem if small, thin items are to be tracked. Tracking
file folders is a common application of RFID. This can work well on a pile of folders
in use, where the contents spread the tags. But a pile of empty folders, which result in tags being
placed back to back, can make the tags at the bottom of the pile difficult to read, as
they are in the
shadow of the tags above them.
RFID is is a powerful technology for the right application. The above is intended to give some idea of the various factors that can affect the read rate, and which should be considered for any given application.
We would be happy to discuss this with you in details.