UHF RFID Tag Antennas
UHF RFID Tag Antennas
A UHF RFID tag is a device that identifies objects or people based on their unique frequency. There are several types of tags that exist, such as microstrip, folded, and planar patch type antennas. These types of antennas are useful in applications that require optimal power transfer between the chip and the antenna, such as the identification of metallic objects.
Microstrip folded tag antennas
The aim of designing ultra-high frequency RFID tag antennas is to maximize their gain. To accomplish this, the impedance network is parametrically optimized. Several factors play an important role in this. For instance, the dimensions of the C-shaped resonators must be optimized to ensure that the resonance frequency is appropriate.
Another key element is the U-shaped feeder structure. This is important in providing a perfect match between the chip and the tag antenna. Also, the length and width of the feeder determine its input impedance. If there is mismatch between the chip’s impedance and the antenna’s impedance, the performance of the tag antenna can be deteriorated.
In this study, a foldable dipole tag antenna is designed for the UHF European band. An embedded matching loop, metallic vias and additional tuning elements are used to achieve gain enhancement. These antennas are fabricated on a simple PTFE dielectric slab, which is cheap and easy to produce.
The simulated and measured results demonstrate that a miniaturized folded dipole tag antenna can realize a gain of -0.53 dB at resonance frequencies of 865 MHz to 960 MHz. Additionally, the maximum detectable range is 3.36 m. However, 10% discrepancy is found between the simulated and the measured results. Some of the discrepancies may be due to fabrication defects.
Miniaturization is the key to achieving this. Moreover, the overall size and the reading range of the tag antenna also affect its performance. Therefore, it is imperative to minimize the amount of materials used for its fabrication. Moreover, the use of outer strip lines to prevent metallic vias is essential.
The input impedance of the tag antenna is computed by measuring the S-parameters of an equivalent two-port network. It is then compared with the impedance of a CST simulator. Finally, the realized gain is calculated using the equation: =xt.
Using this method, it is possible to compute the real and imaginary conjugate chip impedances. R-Chip is the real conjugate impedance of the chip and X-Chip is the imaginary conjugate impedance.
Moreover, the C-shaped resonators are tuned to match the impedance network of the IC chip. As a result, the resonance frequency is brought down to 886.5 MHz, which is suitable for use over the UHF band.
T-matching technique for optimal power transfer between the chip and the antenna
One of the most important aspects of power transfer between the chip and the antenna is the optimal matching of the circuit. This can be achieved by implementing an adaptive impedance matching scheme. The purpose of the scheme is to increase power transfer efficiency as much as possible.
An adaptive impedance matching system is composed of three elements. These elements include an input voltage measurement unit, an impedance matching unit, and a control circuit. In addition to the usual transistors and capacitors, the system uses a fuzzy logic algorithm to control the taps of a variable transformer and inductor.
An optimal matching network design should be able to find a good trade-off between efficiency and cost. This is not always the case. It is also important to consider the amount of time the equipment is actually in use. For example, the optimal matching of an antenna will not be as efficient if it is only used in a laboratory environment.
Using a graphical chart, one can measure the antenna’s power transfer performance. This chart is called the Volpert-Smith chart, and it shows the measured impedance at a single frequency. While it is not as precise as the corresponding mathematical function, it does illustrate the point.
A simplified antenna impedance chart is illustrated in Figure 5-3. In this example, the arcs represent capacitive reactance and the circles indicate active resistance.
The Volpert-Smith chart isn’t the only chart that can show the impedance-matching performance of an antenna. LF RFID Tag Also, it doesn’t necessarily have to be a simple graph. There is a more sophisticated technique based on the S11 scattering parameters matrix, which is not only a lot more graphical, but also more useful.
Optimal impedance matching can be achieved using a single L-shaped network, though it can only achieve impedance matching over a narrow Smith Chart region. However, a global optimal matching network design can improve power transfer efficiency. Compared to a simpler L-shaped network, a multi-element, lumped-element network is more complex, and requires a larger number of components.
Moreover, a matching system that incorporates off-chip components can add LF RFID Tag weight and cost to the overall system. Similarly, an on-chip active component can replace a variable inductor.
Planar patch-type UHF-RFID antenna for metallic applications
A planar patch-type UHF RFID antenna for metallic applications has been recently proposed by Mo Linfei and Son H. W. These antennas have the potential to meet the UHF tag’s performance requirements and to help reduce the cost of manufacturing and installing the tags.
The design is based on the use of a microstrip antenna to provide a wide impedance bandwidth. Its output is enhanced by the use of a ground plane beneath the radiator. This can mitigate the influence of conductive objects on the antenna’s radiation performance.
One interesting feature of the planar patch-type UHF RFID antenna is the use of an open stub feed. This provides the opportunity to have large scale impedance tuning. Another feature is the incorporation of a slit in the unit-cell. Slitting is not only beneficial for achieving a compact structure, but it also changes critical external coupling.
Another impressive feature is the integration of a metasurface absorber to provide superior performance. Several studies have been carried out to enhance the conventional patch antenna’s performance. Various antenna configurations have been proposed for metal hosts, including inverted-F and bow-tie slots. Moreover, several metamaterial structured antennas have been designed to increase structure complexity and alleviate affection of metal surfaces.
An electromagnetic band-gap structure may be too complicated to mass-produce. Nevertheless, it has the potential to realize efficient, low-profile antennas. In addition, it can alleviate multiplex reflection interference in UHF RFID systems.
There are many other antennas that have been proposed, such as loop and shorted printed strip lines, and the aforementioned EBG cells. But none of them come with the novelty of a UHF RFID tag’s most important feature. For example, the microstrip patch antenna is the perfect solution for tagging small items.
However, the aforementioned EBG cell has a high operational cost and a complicated structure. On the other hand, the microstrip patch is cheaper and easier to produce. Plus, it can isolate the radiator from the backing platform.
In summary, the planar patch-type UHF RFID tag antenna for metallic applications is a nifty little gadget that could reduce the cost of manufacturing and installing the tags. Combined with the right hardware, it could be an ideal RFID reader, particularly in a multipath environment.
Label antenna designed to identify metallic objects in the UHF European band of 865-868 MHz
In order to meet the requirements of identifying metallic objects in the UHF European band, a new low cost tag antenna has been developed. It uses an inductive and capacitive part with nested slots. This design minimizes the size of the device and achieves good impedance matching on high permittivity materials. It also achieves good power transfer. The conjugate impedance of the RFID tag antenna was tuned to realize good power transfer and to increase the reading range.
A low cost tag antenna with an overall dimension of 33.5 x 30 x 3.1 mm has been designed for the UHF European band. It has a read range of up to five meters. For the central frequency of 866 MHz, it has a theoretical reading range of 10.4 m.
A prototype of the tag antenna has been manufactured, and its performance has been simulated. It is capable of achieving a reading range of 9.5 m when bent to an arching angle of 95 degrees. Its maximum gain in the + direction is 6 dB. Full wave simulations have shown that the proposed tag antenna can be operated in the 865-868 MHz range.
An additional feature of the tag antenna is the use of a metallic box. While this box has a lower gain, it has a peak read range that is shifted toward the higher frequencies. These features can affect the performance of the tag.
An improved version of the tag is proposed. During the final impedance optimization step, other parameters are fixed. However, the mechanical self-alignment of the device is still a critical factor for its operation.
After the design was optimized, numerical calculations were carried out over the cylinder’s parameters. Several factors affect the impedance of the device, including the polarization of the magnetic field. To eliminate the possibility of polarization mismatch, the resonator has a dipole oriented at a perpendicular to the ground.
As a result, the tag antenna is capable of detecting and identifying metallic objects in the passive UHF RFID band of 865-868 MHz. By reducing the antenna’s size, the tag can be incorporated into standard-sized tickets and documents.