When One Reader Antenna Is Not Enough To Identify A Tag

By Times 7, Nov 05, 2018

Times-7 designs and manufactures world-leading RFID antennas. These are the linkage between RFID-tagged items and RFID-readers. As specialists in the antenna technology, we have deep knowledge in the efficient use of radio frequency electromagnetic waves as a tool for better business.

A question many people have is whether a single antenna will always identify that tag?

Let’s assume you have selected the correct tag and reader antenna for your environment in terms of gain, physical and electrical specifications.

Now, for the reader antenna to be able to communicate with the tag antenna three things are significant: The tag needs to be in the same plane as the reader antenna, and it needs to match the antenna’s polarisation and orientation.

If your application set up provides these prerequisites to support the tag and reader antennas’ communication, then the answer would be: Yes. One antenna will be able to identify a tag. 

In practice, things look different!

In most UHF RFID applications, though, the tag may be presented to a fixed reader antenna in a number of orientations and planes and is out of our control.

In that case, the answer to the question will be: One antenna will not be sufficient to assure tag detection at all times.

Here is the reason why and how to solve it.

The tag antenna

RFID tags also incorporate small antennas to capture the RF signal transmitted by the reader antenna. This reader antenna receives the backscattered signal from the tag, and that signal is fed back into the RFID reader itself. Tag antennas are usually linearly polarised.

Caution: Orientation mismatch

If the tag’s presentation to the fixed reader antenna is unpredictable, the use of a linearly polarised reader antenna will more likely lead to orientation mismatches, and the tag won’t be identified.

Why is that? Linearly polarised antennas, whether tag or reader antenna, are either horizontally linearly or vertically linearly polarised.

(Fig. 1 a) shows a vertically linearly polarised reader antenna and the identified tag, which is vertically orientated, thus matches the antenna’s polarisation. 

(Figure 1 b) shows the same vertically linearly polarised antenna, which cannot identify the tag as it is horizontally orientated.

A tag can be matched with the reader antenna’s linear polarisation by placing the tag in the correct orientation, either horizontally or vertically. 

The advantage of circularly polarised fixed reader antennas

The orientation mismatch is less of a problem with circularly polarised antennas as they can identify the tag in both orientations, horizontal and vertical if the tag is presented in the same plane.

Mind the plane!

The tags’ and reader antenna’s polarisation and orientation are matching, but the antenna still doesn’t identify the tag? Make sure it is in the same plane as the fixed reader antenna.

For example, in (Fig. 2 a) the tag is horizontally polarised along the Y-plane with respect to the pink reader antenna’s cable, which is circularly polarised.

In (Fig. 2 b) the tag is vertically polarised along the X-plane.

In both (Fig. 2 a) and (Fig. 2 b) the tag is in the reader antenna’s line-of-sight, in the same X-Y plane. The circularly polarised reader antenna can detect the tag regardless of its orientation in the same X-Y plane as the reader antenna.

However, if the tag is oriented in the Z-plane as in (figure 3 a), it is not in line-of-sight of the reader antenna’s radiation. The mismatch in orientation means the tag cannot be detected.

A second antenna fills the blind spot

But, if a second reader antenna is included in the Y-Z plane - the blue side antenna in (Fig. 3 b) - the tag can now be detected.​

Recommended antenna set-up

From a customer operational point of view, if the UHF RFID application doesn’t offer control over the tag orientation being presented to an antenna reader, then two circular polarised reader antennas in different planes should be deployed to ensure all tags are accounted for.

In this way despite the orientation, polarisation or plane of the tag antenna, it will always be able to be detected.

Times-7 are RFID antenna experts, with deep knowledge across the tag-antenna-reader continuum. We can help you with your particular RFID challenge. We are located in New Zealand and globally connected. Get in touch today at sales@times-7.com.