The size of the tags is not the only difference between far-field and near-field RFID technology. This article aims to look closer at how parameters and antennas can be modified to optimize the reading performance of near-field tags.
Author: Toni Heijari
You can roughly divide UHF RFID into two categories based on the
mechanism, in which the energy is transmitted from the reader
antenna to the tag antenna. These mechanisms are far-field and
near-field UHF RFID. Far-field UHF RFID uses electromagnetic
radiation, meanwhile near-field UHF RFID relies solely on a
THE THEORY BEHIND THE CURTAIN
In near-field UHF RFID the strength of the magnetic field will
decrease very rapidly as the distance between the reader antenna
and the tag antenna increases. This makes the read range of a
near-field tag quite limited.
A rule of thumb is that near-field UHF RFID is sufficient up to
a distance of about one wavelength, which in UHF frequencies equals
about 30 cm or 12 inches. If the reading distance is longer than
that, the tag is required to have an antenna structure that is
capable of capturing electromagnetic radiation. The antenna
structure required makes far-field tags quite big. For this reason,
the main visible difference between far-field and near-field tags
is the size. Whereas typical far-field tags have a length of about
5-10 cm (2-4 inches), near-field tags can be even smaller than a
The extremely small form factor of near-field tags has opened
new markets and opportunities to use UHF RFID. It is now possible
to tag even the smallest of items. Especially the cosmetics and
jewelry sectors in retail are leading the way in near-field RFID
technology. Their products were earlier quite impossible to tag on
item-level, because of the big size of the tags.
OPTIMIZING THE ANTENNA PARAMETERS
When designing the UHF RFID reader antenna, it is possible to
optimize certain parameters, for example gain, SWVR, radiation
pattern, polarization etc. to fit the chosen tag type.
Most of these parameters are used for describing and optimizing
the performance of far-field transmission. When we want to optimize
the antenna for near-field performance, the parameters to be
changed are usually the strength of the magnetic field, and
especially, how evenly the magnetic field is distributed along the
surface of the antenna.
When the magnetic field is evenly distributed, it makes it
possible to read near-field tags in all locations along the antenna
surface. This will improve the read reliability when reading large
near-field tag populations and especially when the tags are located
very close to each other.
MODIFYING THE ANTENNA TO AVOID DEAD SPOTS
IN NEAR-FIELD READING
Picture 1 below shows the magnetic field distribution of a
traditional loop-antenna. As seen in the picture, the field is
strong where the loop wires/tracks are located (the green areas),
but weakens as we go to the center of the antenna surface (the blue
areas). The same kind of distribution is shown in picture 2, which
shows the field distribution of a patch-antenna. It is good to
remember that patch antennas are the most common antenna types in
fixed RFID readers. So, if you have ever wondered why you are
unable to read small near-field tags when they are positioned in
the middle section of the antenna surface, this is the reason.
Picture 1. Magnetic field
distribution of a traditional loop-antenna.
Picture 2. Magnetic field
distribution of a patch-antenna.
There are several ways to improve the near-field performance and
make the magnetic field more evenly distributed. A widely used way
is to modify the traditional loop antenna. In a traditional loop
the fading of the magnetic field in the center of the structure is
caused by this: The direction of the current flow in the conductor
lines changes to the opposed direction every quarter of a
wavelength. And, in the same way, the direction of the magnetic
field vector in the center is generated by the currents is
changing. These opposed field vectors in the middle of the antenna
cancel each other, causing the fading in the center. Picture 3
shows the direction of the current flow in a traditional
Picture 3. Direction of the current
flow in a traditional loop-antenna.
What we need to do is to modify the loop structure, so that the
direction of the current flow will not change the loop conductors.
We can either shorten the loop to be shorter than a quarter
wavelength or add series capacitance to the loop conductors, since
adding capacitance makes the loop shorter from an electrical point
We can add the capacitance by cutting the conductors evenly. Two
metal plates create a capacitor. When capacitance is correctly
added, the loop will be electrically shorter than its physical
length. Picture 4 shows the current flow in this kind of segmented
loop-antenna. Note that the direction is not changing along the
Picture 4. Direction of the current
flow in a segmented loop-antenna.
Now that the direction is not changing, the magnetic field in
the center is not being canceled and for that reason it will be
quite evenly distributed along the surface of the antenna
structure. This allows near-field tags to be read in every location
of the surface. No more dead spots.
The magnetic field distribution of a segmented loop-antenna is
shown in picture 5. As you can see, there is a significant
difference in the field distribution when compared to the magnetic
fields of traditional loop and patch antennas shown in pictures 1
Picture 5. Magnetic field
distribution of a segmented loop-antenna.
FROM THEORY TO PRAXIS
We have tested the near-field antennas inside our own products;
ID Sampo and Nordic
ID Merlin Blade. We noticed that the theory described in this
article does work in real-life situations as well.
By using the modified antennas, while reading small near-field
tags, inventory could be performed faster and with more accurate
results. We also gained verification that there were no longer dead
spots, where near-field tags were impossible to read.
For more information about UHF near-field and antennas, contact
me at firstname.lastname@example.org.
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