Rfid antenna main problems

tmily

Rfid antenna main problems [Copy link]

RFID Antenna Issues 1
The type of RF antenna must be chosen so that its impedance matches that of free space and the ASIC. Directional antennas have less interference with radiation patterns and return losses. Access control systems can use passive tags with short ranges.

Introduction
In RF devices, as the operating frequency increases into the microwave region, the matching problem between the antenna and the tag chip becomes more severe. The goal of the antenna is to transfer the maximum amount of energy into and out of the tag chip. This requires careful design of the matching of the antenna to free space and to the tag chip to which it is attached. The frequency bands considered in this article are 435MHz, 2.45 GHz and 5.8 GHz, which are used in retail goods.

The antenna must be:
small enough to be attached to the desired item;
have omnidirectional or hemispherical coverage;
provide the maximum possible signal to the tag chip;
have the polarization of the antenna match the interrogation signal of the reader regardless of the orientation of the item;
be robust; and
be very inexpensive.
The main considerations in selecting an antenna are:
type of antenna;
impedance of the antenna;
performance of the RF applied to the item;
performance of the RF when there are other items surrounding the tagged item.

RFID Antenna Problem 2
Possible Choices
There are two possible uses here: 1) Tagged items are placed in a warehouse and a portable device, possibly handheld, interrogates all items and requires them to give feedback; 2) Card readers are installed at the door of the warehouse to interrogate and record items entering and leaving. Another major choice is whether to use active tags or passive tags [1], [2].

Optional antennas
are used in RFID systems at 435 MHz, 2.45 GHz and 5.8 GHz. There are several optional antennas, as shown in the table below, which focus on the size of the antenna. The gain of such a small antenna is limited. The gain depends on the type of radiation pattern. Omnidirectional antennas have a peak gain of 0 to 2dBi; directional antennas can have a gain of up to 6dBi. The gain size affects the range of the antenna. The first three types of antennas in the table below are linearly polarized, but microstrip antennas can be circularly polarized and logarithmic spiral antennas are only circularly polarized. Since the directionality of RFID tags is uncontrollable, the reader must be circularly polarized. A circularly polarized tag antenna can produce a signal that is 3dB stronger.

RFID Antenna Issues 3
Impedance Issues
For maximum power transfer, the input impedance of the chip behind the antenna must match the output impedance of the antenna. For decades, antennas have been designed to match impedances of 50 or 70 ohms, but it is possible to design antennas with other characteristic impedances. For example, a slot antenna can be designed with an impedance of several hundred ohms. The impedance of a folded dipole can be 20 times that of a standard half-wave dipole. The lead-out point of a printed patch antenna can provide a wide range of impedances (usually 40 to 100 ohms). It is critical to choose the type of antenna so that its impedance matches the input impedance of the tag chip. Another issue is that other objects in close proximity to the antenna can reduce the return loss of the antenna. For omnidirectional antennas, such as dual dipole antennas, this effect is significant. Some actual measurements were made by varying the distance between a dual dipole antenna and a can of ketchup, showing some changes, see Figures 4 and 5. Other objects have similar effects. In addition, it is the dielectric constant of the object, not the metal, that changes the resonant frequency. A plastic bottle of water reduces the minimum return loss frequency by 16%. When the object is closer than 62.5mm from the antenna, the return loss will result in an insertion loss of 3.0 dB, while the free space insertion loss of the antenna is only 0.2dB. The antenna can be designed to match the proximity of the object, but the antenna behavior will be different for different objects and at different distances from the object. This is not feasible with omnidirectional antennas, so antennas are designed to be highly directional, which are not affected by this problem.
RFID Antenna Problem 4
Radiation Patterns
The antenna patterns were tested in a non-reflective environment, including various objects to be tagged, and performance was severely degraded when using omnidirectional antennas. The performance degradation was the most severe for a cylindrical metal tagged object, with the return signal dropping by more than 20dB at a distance of 50mm from the antenna (see Figure 6). The return signal dropped by about 10 to 12dB when the center of the antenna was separated by 100-150mm. Several bottles of water (plastic and glass) were measured at a distance of 100mm from the antenna, see Figure 7, and the return signal dropped by more than 10dB. Similar results were obtained when testing liquids in wax paper boxes and even apples.

Effects of Local Structure
When using handheld instruments, the presence of a large number of other nearby objects can 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, see Figures 8, 9, and 10, showing a 10 dB reduction in return signal compared to free space, with a greater reduction than expected when both antennas are used. Figures 11 and 12 are contour plots of the received signal in a cross-section plane in front of one antenna, showing the severe distortion. In a warehouse environment, where a box of items has one tag, there are problems with several tags attached to the box to ensure that at all times a tag is visible. Portable systems have several antenna issues. Two antennas per box are sufficient for access control detection, so the effects of local structure become less important because the access control reader antenna is fixed at the warehouse entrance and pointed directly at the tagged object.
RFID Antenna Issues 5
Distance
The gain of the RFID antenna and whether an active tag chip is used will affect the system's operating distance. Optimistically, the range for passive operation at 2.45 GHz, full wave rectification, drive voltage no greater than 3 volts, and an optimized RFID antenna impedance environment (impedance 200 or 300 ohms) is about 1 meter [3]. If the WHO limits [4] are used, it is more suitable for global use, but the range is reduced by half. These limit the electromagnetic field power from the reader to the tag. The range decreases with increasing frequency. If active chips are used, the range can be 5 to 10 meters. Summary of

RFID Antenna Issue 6 Omnidirectional antennas should be avoided in tags, however directional antennas can be used, which have less interference with the radiation pattern and return loss. The type of antenna must be chosen so that its impedance matches that of free space and the ASIC. Using the antenna in a warehouse seems to be unfeasible unless active tags are used, but in any case the antenna radiation pattern will be severely distorted inside the warehouse. Use in an access control system would be a good option, using short range passive tags. Of course, access control systems are more expensive than handheld instruments, but the staff of handheld instruments need to use them to search for items in the warehouse, and the personnel costs are also expensive. In the access control system, each item box only needs 2 instead of 4 or 6 RFID tags.