RFID gates are an effective way of tracking assets automatically without manual labor. But what are the technical aspects that need to be taken into account at deployment and how do we make the most out of the system?
What materials affect the performance of RFID readers? How do
the amount of tags and the reading purpose affect antenna and tag
positioning? What RFID antennas are optimal for what reading
environment? And, how do we reduce the power consumption of
automated systems? These questions among others will be answered in
RFID gates - What are
An RFID gate is a physical gate with a fixed RFID reader and
antenna(s) attached to it. The system registers everything that
moves through the gate as long as it is equipped with an UHF RFID
tag. RFID gates use the same technology as RFID mobile computers,
except for that everything works automatically and no manual labor
is required. The automated operations make RFID gates a faster and
more accurate way of checking goods - especially compared to
checking them manually.
Material choice effects antenna
The positioning of the RFID readers and antennas depend a lot on
the material used on the loading cages, pallets or containers as
well as the material surrounding the gates. Different materials
call for different positioning solutions.
When it comes to items loaded on wooden pallets and plastic
crates, the positioning of the antennas isn't that demanding. Wood
and plastic do not interfere remarkably with the RFID radio waves.
But when we are talking about metal roller cages there are some
issues to be considered. Firstly, the metal roller cages often have
at least two sides covered with "metal net". If this net is very
thick, reading the tags through the net can be a challenge. This
can be solved by using a material that RF signals can pass through
in the bottom plate of the metal roller cage and by installing one
antenna on the top of the gate and another under the gate. This is
the most reliable solution when it comes to reading accuracy with
"metal challenges". When registering movement of people or other
"non-metallic and non-liquid elements", the antennas can preferably
be placed on both sides of the gate for optimal reading.
As with RFID in general, liquids and metals pose challenges on
reading accuracy and may call for creativity for finding a
well-performing solution. The reflective nature of metal can be
used as a benefit: for instance one manufacturer of large drink
containers has placed the RFID tags in a location where they are
read well by using the reflection of the metal around them. Tags on
liquid items are read most successfully by optimizing the placement
of the tag so that the radio signals can reach them without passing
through the liquid. They can e.g. be placed high up on a bottle's
neck if read from above/side, or underneath the bottle it if the
antennas are in the floor. The more distance between the bottles,
the better passage for the radio signals.
The more tags - the more
Generally the amount and density of goods, i.e. tags, on a
transport platform define both the needed amount and the
positioning of the antennas, as well as the optimal output power
level of the reader. The more tags that pass through the gate
simultaneously, the more antennas are needed - which should be
taken into account when planning the system.
A high density of tags may cause a so called "shadowing effect"
(the tags shadow each other) or/and a "coupling effect" (the tags
mesh as a result of being in to close contact with each other) that
decreases the reading accuracy. The solution to these problems is
simply to use a higher output power. "Increase the volume and the
tags shall hear you!"
The intended "reading level" - whether the purpose is to read
tags on item-level or box/container-level - also affects the
positioning of the tags and antennas. Maximum reading capacity is
achieved by optimizing the positioning of the tags so that their
radiation hit the antenna from the right angle. If the tags are
placed in random angles and positions in a box, reading accuracy
can be improved by installing several antennas, preferably with
circular polarization, that read from as many angles as
A tag passed by - but where did it
come from and where did it go?
RFID gates can tell us whether a tag is getting closer to or
moving away from an antenna. The strength of the signal (RSSI
value) and the "phase difference" tell the reader how far away the
tag is. By interpreting the phase difference and measuring whether
the strength of the signal goes stronger or weaker, the reader
knows whether the tag is approaching or retreating from the
antenna. It can roughly tell the distance, but it can't
necessary tell whether the tag is approaching from the right or the
left. If the system administrator wants to know exactly to and from
where the target is moving, two antennas are required. This ensures
reliable information about the direction as well as the
- RSSI value (strength of signal): the changes
in the RSSI value are examined over time. When a tag approaches,
the RSSI value increases. When the tag retreats from the antenna,
the value decreases. There is one problem related to RSSI based
systems, and that is the fact that the RSSI value can change as a
cause of other factors than just distance. For instance, when
placing a hand between the tag and the antenna the strength of the
signal decreases and the system interprets the event as the tag
moving away from the antenna.
- Phase difference: This method is a bit more
"high tech", but it's less prompt to be affected by environmental
influences. The system compares the phase difference between the
reader-to-tag signal to the tag-to-reader signal. The phase
difference between the signals lets the system know how far away
from the antenna the tag is. By examining the phase difference data
over time, it's possible to determine whether the tag is moving
towards or further away from the antenna. The signal leaving and
returning to the antenna can be compared to waves on the water
surface with their "peaks". At the reader antenna, the gap between
the peaks of the reader-to-tag and the tag-to-reader wave gets
bigger the further away the tag is from the antenna.
Specific tag reading
In some spaces it isn't possible to have a long distance between
the gates and the tags (e.g. in warehouses with shelves next to the
gate). In these areas the reading distance and angle should be
limited to ensure correct reading. If lowering the output power is
not an option, the use of attenuating materials and directing the
antennas optimally at the gate might solve this issue. This helps
the reader pay attention to the right tags and ONLY the right
What antenna for what
When it comes to the preferred type of antenna for an particular
application, circular antennas provide more accurate results in
situations when the tag position and angle is unknown, for instance
when the tags are higgledy-piggledy thrown in a sealed boxes. A
linear or a cross-dipole antenna is preferred when the tag position
and angle is known AND the same for all tags.
Constant tag reading or
Automatized RFID reading systems may raise questions about the
reader constantly being on sending "RF pollution" to the
environment and consuming power. However, different solutions do
exist to solve this challenge.
The RFID reader can be triggered to switch on with the help of
clapping or light sensors. Or, a "light curtain" can be used to
activate the reading of tags in large areas. E.g. the curtain
recognizes a forklift approaching and tells the reader to wake up.
This avoids blocking the nearby air waves when nothing interesting
is happening at the gate.
Fixed RFID readers should be supplied with constant electricity
at installation and they should have a fixed Internet connection to
ensure a smooth and well-performing system. There are also fixed
RFID readers with Ethernet on the market, where the PoE feature is
supported. This enables powering the reader via the Ethernet
A reliable system calls for
qualitative connections and cables
The antenna cabling is something that should be taken into
account when planning the system. It's not uncommon that half of
the output power from the reader is lost due to the attenuation
properties of the cable in certain applications. Therefore, to
insure the best possible performance, the cables have to be of good
quality and the connection from the antenna to the RFID reader
should be as short as possible.