When building applications with RAIN RFID, a basic understanding of the physics behind it, are going to be of great help. Especially when the application needs to read a large number of tags or read at a great distance, basic knowledge is going to help you a lot.

Principle

The reader is not only responsible for managing the tags, in most cases it is also powering the tags. The reader generates a strong signal that is transmitted via one or more antennas to the tags. The tags consist of two parts: an antenna (like with the reader) and a chip. The antenna receives the power and signal from the reader and presents it to the chip. The chip decodes the signal (using the power from the reader) and responses to the signal. This goes again via the tag antenna, over the air, back to the antenna from the reader, where the reader receives the signal and decodes it again.

You can easily compare this with two humans trying to have a conversation: person 'B' should hear person 'A' and the other way around.

Whether this whole process works (and thus whether the tags get read or not), depends on two factors:

  1. Is the chip able to be powered by the reader and does it still understand the signal from the reader? ('is person B able to hear person A')
  2. Is the reader able to receive the signal back from the tag? (the other way around: 'is person A able to hear person B')

The following factors influence this process:

  • The output power from the reader and antenna
  • The size and design of the antenna on the tag
  • The design of the chip on the tag
  • Distance between the reader and the tag
  • The orientation of the tag
  • Density of tags
  • The item the tag is applied to
  • Materials in between the reader and tags
  • The environment
  • Interference from other readers

So, that's quite a long list of influencing factors... Let's divulge them one by one.

The output power from the reader and antenna

The total output power from the reader and antenna combination is the sum of the output power from the reader ('conducted') plus the 'gain' of the antenna. The higher the output power of the reader, the longer the reading distance. In general, the bigger the antenna, the more gain it has. Most readers allow you to configure their output power. Antennas typically have a fixed gain, that depends on the design. The harder you shout, the farther the other person can be to hear you.

Unfortunately, we cannot just add more bigger antennas and higher-output readers to get a longer reading distance. If the reader and antenna combination is too powerful, it will disturb other wireless communication systems. Therefore governments around the globe have set standards and maximums for reader output power and antenna gain. You can find more information on this in the manual of your reader.

The size and design of the antenna on the tag

As with the antenna that goes with the reader, the antenna on the tag also has a gain. In general, the bigger the antenna, the better the gain and the larger the reading distance. However, the design of the antenna is also important. Some antenna designs have a better gain than other antenna designs.

The design of the chip on the tag

The chip on the tag is a semi-conductor product. As with every semi-conductor product, there performance increases year over year. So, if you want to have the largest reading distance, use the most modern chip you can get. The parameter you are looking for is the 'chip sensitivity' (this value is typically expressed in dBm's, a lower value is better).

Distance between the reader and the tag

Like you can hear a person that is standing next to you more clearly, than someone that is standing far away, the closer the tag is to the reader, the better it is able to receive the power and signal to get turned on.

The orientation of the tag

Most RFID tag antenna designs are orientation sensitive, their receiving quality is different from one side to another. That means that when the tag is presented under one angle, it can have either a much larger or much smaller reading distance compared to another angle.

Density of tags

When a lot of tags are placed closely together, they will influence each other and will have more issues receiving the signal from the reader.

The item the tag is applied to

If a tag is applied to an item of a certain material, the properties of its antenna will change (more or less gain). This depends from one material to another. Paper, wood and foams will have less influence than hard plastics and metals.

Materials in between the reader and tags

While the reader doesn't need line-of-sight connection with the tags, some materials do influence the signal. The signal is not able to penetrate through metal or liquids, but paper, wood, carton etc. are not having a significant influence.

Interference from other readers

Most readers are able to withstand other readers and not face any degradation in performance, but when faced with tens of other readers in a small space, this will definitely have an influence.


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