The recent adoption to use RFID labeling for tracking packages in supply chain applications has given rise for the need of good RFID tunnel antennas. Tunnel structures placed over conveyor roller assemblies provide a convenient and efficient means for high performance antennas to be supported.
A good design for a tunnel antenna allows the reader’s energy field to be uniformly radiated. A homogeneous radiation pattern enables the reader to couple to tags regardless of their relative orientation. Most conventional conveyor roller sections are made of metal and the metal structure acts as a boundary condition to the radiated flux field. Thus consideration must be given to prevent the absorption and distortion of the magnetic and radio field energy.
Serious consideration must be given to the size of the tags used in the system. As an example, high frequency ISO RFID inlays range in length from 22 mm to 76 mm. The smaller inlays do not capture as much energy as their larger counterparts and as a consequence do not perform as well in the tunnel.
It appears that as the energy level captured goes through the minimum levels of detection it causes collision interference with their adjacent tags. Once a solid detection level is assured of all of the tags, the anti-collision function can separate a fairly large population.
A tunnel that can accommodate packages larger that 12 inches high will likely require an array of antenna elements to cover the internal volume. It may also become necessary to have antennas at varied elevations to assure complete coverage in the linear plane.
One method of powering multiple antenna elements is with the use of power splitters. The transmitter power source feeds a transmission line terminated with two or more driven antenna elements. The advantage gained is improved potential coverage in areas that a single antenna would have null or dead areas in the radiation field pattern. A disadvantage of using power splitters is the insertion loss they introduce. One dB, or more, is lost with the introduction of each splitter. Measurable losses, typically ½ dB, are also added when connector adapters are utilized, typically implemented to convert BNC to SMA sizes.
The utilization of High Performance RFID Readers provides intelligence so that the reader controls the processing. Otherwise, when using a lower performance reader it may require an external host for the processing and will most likely slow down the processing throughput. The processing of multiple tags in the tunnel also slows down the response.
High performance readers may have featured separate transmit and receive ports, internally synchronized in the reader. This performance feature allows a second or third antenna element to get its induced power parasitically from an adjacent driven element and detect tags that are responding without the use of splitters. The integration of this multiple element configuration creates a highly efficient detection model.
The Atrait Ultimate Gateway RFID Tunnel, designed by Tomas Grajales, Atrait' VP R&D, can be seen here. It assures relative high speed dectection from all orientations of multiple smart labels.
The Atrait Ultimate Gateway RFID Tunnel antenna uses a pair of 180 degree, out-of-phase driven elements directly across from each other. This optimizes the detection of tags oriented orthogonal to the antenna. Additionally, another element located forward of this pair is connected to the receive-only port of the reader. Finally, an additional parasitic element is suspended over this array to provide depth to the field energy pattern. Our goal was to create uniform radiation of the available energy so that all tags moving through the tunnel, regardless of orientation, would have a high probability of detection by the reader. This multiple antenna array offers such a result.
A close up view shows the multiple elements that comprise the tunnel. The parasitic element is across the top of the tunnel and unfortunately does not show too well in this photo.
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