RFID Coupling: The Relationship Between a Tag and a Reader

If you’re here, you’ve probably done a bit of research and have somewhat of an understanding of what RFID is, but you might have some questions about how it works. Here at Computype we have plenty of experience providing clients with RFID solutions, so we know a thing or two about its inner workings to say the least.

In this post we will cover the process and types of coupling, along with how the relationship between a tag and a reader/antenna continues beyond coupling. There are two types of coupling, both of which will be discussed and provide an explanation to why certain types of tags work differently from one another.

What is Coupling?

Coupling refers to the method used to link or “couple” the RFID tag and reader/antenna to one another. This needs to be done in order for the reader to recognize and retrieve information from the tag, almost like greeting a friend before asking them a question.

The two primary types of coupling in an RFID system are inductive and radiative coupling. Whether inductive or radiative coupling are used typically depends on the frequency level of the tag. In case you need a refresher…

Inductive Coupling

This type of coupling is sometimes called magnetic coupling and is generally used by both low and high frequency systems. It also explains some of the reasons low and high frequency systems work the way they do.

Generally, the inductive coupling process works like this—when the reader/antenna and tag are within range of one another, the reader/antenna creates a magnetic field, that magnetic field then causes the nearby tag to couple with the reader. When the two couple, electricity is sent from the reader through the conductive antenna of the tag, sending power through the tag and allowing the stored data to be read.

This power transfer which occurs during the inductive coupling process explains a couple of reasons why low and high frequency tags work the way they do.

First, it explains the read distance. Since the power transfer can only occur within close proximity, inductively coupled tags can only be read within a short distance. This is why low and high frequency tags need to be within a fairly short range of the reader to be recognized.

Second, it also explains why low and high frequency RFID tags usually don’t require an on board battery. They receive enough power from the reader during the coupling process to power the tag.

Radiative Coupling

The other method, radiative coupling, is generally leveraged for ultra-high frequency tag coupling. It is sometimes recognized as backscatter coupling, for reasons we will discuss.

The radiative coupling process begins when the reader starts radiating electromagnetic waves. It sends these waves out to any nearby tag to capture and reflect back to the reader (“backscatter”). This process can be compared to standing in a large field, yelling, and waiting to hear an echo.

This type of coupling can allow for longer read distances since the tag and reader don’t need to be close enough to transfer electricity. However, if the reader is too far away to transfer electricity to the tag it will need an on board battery.

As with any process, there are exceptions to the rules. Some near-field UHF tags operate by using inductive coupling as opposed to radiative.

The Relationship Between a Tag and a Reader

Coupling isn’t all there is to the relationship between a tag and a reader, in fact we’ve already lightly touched upon the next step.

Reading occurs immediately after coupling and involves the transfer of data, rather than electricity, from tag to reader/antenna. Bringing this back to the analogy from the beginning, coupling is like the reader/antenna saying hello to the tag and the tag saying hello in response. Reading is when the reader/antenna follows up the hello with a question and the tag answers.

The next and final step involves the reader and the operator. People can’t understand radio waves, so the reader must translate the data received into a human readable form. The reader is either hooked up to a computer or has its own small interface where the human readable data can be displayed to the operator.

Although the focus on the final step may seem like it’s on the reader and the operator, it does add a new dimension to the relationship between the tag and the reader as well. The tag and reader are no longer friends engaging in small talk, they work together to deliver a message to the operator. The tag stores important information and the reader/antenna translates it to the operator.

Coupling is an incredibly important factor of how RFID works. Which type is used is determined by the frequency of the tag, and the type of coupling used affects how the information stored in the tag is accessed, how far away the tag can be from the reader and whether or not the tag will need a battery to run, all important things to consider when implementing RFID.

The relationship between the tag and reader also adds a deeper dimension to RFID as the processes of reading and translating occur.

Hopefully the information in this post was helpful and encouraging in your journey to learn about the powerful technology that is RFID.

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