Problem Description
UHF RFID has made tremendous strides in its widespread adoption,
especially in the supply chain. Its combination of low cost
of the transponder, long read distance, and strong standards
around the technology have resulted in considerable interest in
secondary markets, such as asset tracking.
Technically, most passive UHF RFID transponders use some form
of a dipole antenna. When a dipole antenna is placed near metal
or water, the tag becomes "de-tuned." (The actual mechanisms are
quite a bit more involved, but it's sufficient to know that the
performance degrades considerably.) Since many assets are made
of metal, or could have liquid contents, passive RFID struggles.
While ideally a passive RFID tag may be read 30 feet away, when
placed near metal the read distance may be reduced to, for example,
less than 5 feet, and when placed directly on metal, the tags
may be come completely unreadable.
There have been a number of technical solutions to solving the
RFID tag-on-metal problem. However, none have yielded the combination
of size, performance, and ease of manufacturing.
KUTag – A low-profile, planar, microstrip UHF RFID tag
The above represents a fundamental advancement in microstrip
RFID technology. Traditionally, microstrip antennas are
viewed as unbalanced devices, having a signal (feed)
and ground (ground plane). The IC is a two-terminal device
with a characteristic impedance. The IC can be integrated into
two antenna terminals of a balanced device, such as a dipole,
or one terminal may accept a signal and the other grounded,
which is the case with practically all microstrip designs.
Since the signal is one plane and ground is on another,
microstrip RFID tag designs traditionally require some way of
crossing planes, such as a via, which makes them relatively
difficult and expensive to manufacture.
Versions of the above tag are readable over 30 feet away,
and work equally well in air, on metal, or water containers.
How It Works
We have developed a simple balanced feed approach.
Like a traditional edge-feed microstrip designs, we attach a
microstrip transmission line at the desired impedance point.
The difference, of course, is that we use two feeds.
Note that the microstrip antenna is a half wavelength, and
there is a line of odd mode symmetry through the middle of
the antenna. The two feeds are then 180 degrees out of phase,
i.e., they form a balanced feed.
As can be seen in the picture, we can add a transmission line
connecting these two feeds. These serves as a shorting stub,
with the (virtual) short achieved exactly in the middle.
The following picture illustrates the analysis.
Note that the dashed line through the diagram represents
a virtual ground, or short circuit. Because we have the
flexibility of where to attach the feed, the width of the
transmission lines, we have several degrees of freedom to
design the matching circuit to match to practically any IC.
Attributes
The "big deal" about this tag is that it has three very
important attributes.
- It works on anything. Because we integrate a ground
plane into the antenna design (the bottom layer is
a metal tape), the tag performance is independent of
the material to which it is attached. That means our
tag works equally well in air, on metal, or on water.
- It is completely planar. Thare are no structures that
connect one layer to the next, so it affords a simple
manufacturing process. Also, you can see that now
we can use traditional inlay technology that has
been extensively developed for other antennas, thus
leveraging a considerable industry infrastructure.
- It is low profile (i.e., thin). Total thickness
is approximately 2 mm. If your application demands
narrower tags, we can always trade width for height.
- Low cost. Because of the planar nature of the design
and the fact that it leverages existing inlay
technology, we anticipate that the total cost of
manufacturing will be small compared to other metal
tags currently on the market.
- High performance. The prototype tags we make in the
lab consistently have high performance and are readable
30 feet or more.
Commercialization Status
The KU-Tag technology is currently under license. We anticipate
a press release shortly to describe the current relationships and
products that will be available.