In the RF device, when the operating frequency is increased to the microwave region, the matching problem between the antenna and the tag chip becomes more severe. The goal of the antenna is to transmit the most energy into and out of the tag chip. This requires careful design of the antenna and free space and the matching of its associated tag chips. The bands considered in this paper are 435MHz, 2.45 GHz and 5.8 GHz, and are used in retail products.
The conditions that the antenna must meet: • Sufficiently small enough to be attached to the item you need;
• directionality with omnidirectional or hemispherical coverage;
• The chip that provides the most possible signal to the tag;
• The polarization of the antenna matches the interrogation signal of the card reader regardless of the direction of the item;
• Robust;
• Very cheap.
• the type of antenna;
• the impedance of the antenna;
• the performance of the RF applied to the item;
• RF performance when there are other items surrounding the labelled item.
There are two ways to use it:
a) The tagged item is placed in a warehouse, has a portable device, may be hand-held, asks for all items, and requires them to give information feedback information;
b) Install a card reader at the door of the warehouse, ask and record the entry and exit. Another major choice is whether it is an active tag or a passive tag.
There are several optional antennas for RFID systems that are used at 435 MHz, 2.45 GHz and 5.8 GHz. The gain of such a small antenna is limited, the magnitude of the gain depends on the type of radiation mode, the omnidirectional antenna has a peak gain of 0 to 2 dBi, and the directional antenna has a gain of 6 dBi. The magnitude of the gain affects the range of the antenna. The first three types of antennas in the table below are linearly polarized, but the microstrip antennas can be circularly polarized, and the logarithmic helical antennas are only circularly polarized. Since the directionality of the RFID tag is uncontrollable, the card reader must be circularly polarized. A circularly polarized tag antenna can produce a strong 3dB signal.
Impedance problemFor maximum power transfer, the input impedance of the chip behind the antenna must match the output impedance of the antenna.
For decades, the antenna has been designed to match the impedance of 50 or 70 ohms, but it is possible to design the antenna to have other characteristic impedances. For example, a slot antenna can be designed with an impedance of a few hundred ohms. The impedance of a folded dipole can be 20 times the impedance of a standard half-wave dipole. The lead point of the printed patch antenna provides a wide range of impedance (typically 40 to 100 ohms).
It is critical to choose the type of antenna so that its impedance can match the input impedance of the tag chip. Another problem is that other objects that are close to the antenna can reduce the return loss of the antenna. For omnidirectional antennas, such as double dipole antennas, this effect is significant. Changing the double dipole antenna and listening to the pitch of the ketchup made some actual measurements, showing some changes.
Other objects have similar effects. In addition, the dielectric constant of the object, rather than the metal, changes the resonant frequency. A plastic bottle of water reduces the minimum return loss frequency by 16%. When the distance between the object and the antenna is less than 62.5mm, the return loss will result in a 3.0 dB insertion loss, while the free-space insertion loss of the antenna is 0.2 dB. The antenna can be designed to match the situation of the approaching object, but the behavior of the antenna varies for different objects and different object distances. It is not feasible for omnidirectional antennas, so antennas with strong directivity are designed and they are not affected by this problem.
Radiation patternThe antenna pattern was tested in a non-reflective environment, including various objects that required labeling, and performance was severely degraded when using omnidirectional antennas. The performance degradation caused by cylindrical metal hearing is the most serious. When it is 50mm away from the antenna, the signal of the reverse return is more than 20dB. When the center distance between the antenna and the object is separated to 100-150 mm, the back-return signal drops by about 10 to 12 dB. A few bottles of water (plastic and glass) were measured at a distance of 100 mm from the antenna, and the back-return signal was reduced by more than 10 dB. Similar results were obtained from tests on liquids in wax cartons and even on apples.
Local structureWhen using a hand-held instrument, a large number of other adjacent objects severely distort the radiation pattern of the reader antenna and the tag antenna. This can be calculated for an operating frequency of 2.45 GHz, assuming a representative geometry that shows a 10 dB reduction in the return signal compared to free space, and more when the dual antennas are used simultaneously than in the expected mode. In the warehouse environment, there is a problem with having one label in one item box, and several labels are attached to one box to ensure that a label is visible at all times. The use of portable systems has several antenna problems. The two antennas of each box are sufficient for the access control device to be detected, so that the influence of the local structure becomes less important because the reader antenna of the access control device is fixed at the entrance and exit of the warehouse and directly points to the tagged object.
distanceThe gain of the RFID antenna and whether or not to use an active tag chip will affect the distance the system is used. Optimistic consideration, when the electromagnetic field's radiation intensity meets the relevant standards of the UK, in the passive case of 2.45GHz, the full-wave rectification, the driving voltage is not more than 3 volts, the optimized RFID antenna impedance environment (impedance 200 or 300 ohms), the use distance It is about 1 meter [3]. If the WHO limit [4] is used, it is more suitable for global use, but the distance is reduced by half. These limit the electromagnetic field power of the reader to the tag. The range of action decreases as the frequency increases. If you use active chips, the distance can be 5 to 10 meters.
to sum up Omnidirectional antennas should be avoided in the tag, however directional antennas can be used with less radiated mode and return loss interference. The antenna type must be chosen such that its impedance matches the free space and ASIC. Using an antenna in a warehouse does not seem to be feasible unless an active tag is used, but in any case, the antenna radiation pattern in the warehouse will be severely distorted. The use of an access control system will be a good choice, and passive tags with short range of action can be used. Of course, the access control system is more expensive than the handheld instrument, but the handheld instrument staff needs to use it to search for items in the warehouse, and the personnel costs are equally expensive. In the access control system, each item box requires only 2 instead of 4 or 6 RFID tags.
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