Factors which affect read range for RFID

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

- Orientation

- Time to read

- Number of tags being read

- Density of tags

鉴于上述,唯一一般评论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 optimized. So please take the numbers in the way they are intended, as a general guideline.

Passive, BAP, NFC or Active RFID?

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 meters.

RFID Frequency

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.