The farther, the better!
There are many reasons why RAIN (UHF) RFID technology is successful - one of the more important reasons is the ability to read tags at a distance of several meters.
All former passive RFID technologies like LF and HF/NFC use magnetic fields to transfer energy and information. To obtain this result, both the reader antenna and the tag antenna are formed by coils that needs to stay in proximity in order to obtain the so called “inductive coupling” that permits the energy transfer between the antenna of the reader and the tag antenna.
This is the reason why the reading distance of LF and HF/NFC technology is typically limited to few centimeters and, only using very high-power levels and very big antennas, can it can reach a distance of a meter.
RAIN technology uses instead the electromagnetic field to transfer energy and information, the same used by cell phones, radio and TV communication.
In this case, the antenna of the reader and the tag antenna do not need to “couple” so they do not need to stay in proximity, and the energy and information exchange can happen at longer distances. The reading distance is anyway limited because the tags are passive and need to harvest the energy from the reader signal. This energy decreases with the square of the distance, but it can reach a value of some meters, up to 20 meters in some situations, a distance that opens up a new range of applications.
We typically talk about reading distance, but it is more the volume covered by the electromagnetic field that is important in many scenarios, especially for inventory applications, because it permits bulk readings, i.e. the reading of hundreds of tags within a few seconds. This is also relates to another advantage of the RAIN technology: using a higher frequency, it permits the exchange of information at higher speeds.
All these improvements are possible from the beginning of this century because of the availability of very low-power demanding chips (the energy transferred with the electromagnetic field is much lower than the energy transferred using magnetic field) and the continuous evolution in the technology on both readers and tags is improving the performances every year.