RF antenna in RFID

- Jun 21, 2018-

In an RF device, the matching between the antenna and the tag chip becomes more severe when the operating frequency is increased to the microwave region. The goal of the antenna is to transmit maximum energy into and out of the tag chip. This requires careful design of the antenna and the free space and the matching of its attached tag chips. The frequency bands considered in this paper are 435 MHz, 2.45 GHz, and 5.8 GHz for use in retail merchandise.

In an RF device, the matching between the antenna and the tag chip becomes more severe when the operating frequency is increased to the microwave region. The goal of the antenna is to transmit maximum energy into and out of the tag chip. This requires careful design of the antenna and the free space and the matching of its attached tag chips. The frequency bands considered in this paper are 435 MHz, 2.45 GHz, and 5.8 GHz for use in retail merchandise.

Antenna must meet the conditions:

Small enough to fit on the desired item;

Has directionality of omnidirectional or hemispherical coverage;

Provide the largest possible signal to the tag's chip;

Regardless of the direction of the item, the polarization of the antenna can match the reader's interrogation signal;

Very cheap.

The main considerations when choosing an antenna are:

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 around labeled items.

Possible choices

There are two ways to use this:

a) The tagged items are placed in a warehouse, there is a portable device, may be handheld, ask for all the items, and they need to give information feedback;

b) Install readers at the entrance of the warehouse to ask for and record items. Another major choice is active tags or passive tags.

Optional antenna

There are several types of antennas available for use in RFID systems with 435 MHz, 2.45 GHz and 5.8 GHz frequencies. 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 gain size affects the antenna's range of action. The first three types of antennas in the table below are linearly polarized, but a microstrip antenna can make a circularly polarized, logarithmic spiral antenna only circularly polarized. Since the directionality of the RFID tag is not controllable, the card reader must be circularly polarized. A circularly polarized tag antenna can generate a strong signal of 3dB.

Impedance problem

For maximum power transmission, the input impedance of the chip after the antenna must match the output impedance of the antenna.

For decades, the antenna was 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 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 the printed patch antenna can provide a wide range of impedance (usually 40 to 100 ohms).

The type of antenna is selected so that its impedance can be matched with the input impedance of the tag chip. Another problem is that other objects 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 spacing between the dipole antenna and a ketchup did some actual measurements and showed some changes.

Other objects also have similar effects. In addition, the dielectric constant of the object, not 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 3.0dB insertion loss, 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 behavior of the antenna varies for different objects and different object distances. It is not feasible for omnidirectional antennas, so designing directional antennas is not affected by this problem.

Radiation pattern

The mode of the antenna was tested in a non-reflective environment, including various objects that need to be labeled, and the 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 back-to-back signal drops by more than 20dB. When the distance between the antenna and the center of the object is 100 to 150 mm, the back-to-back signal drops by about 10 to 12 dB. At a distance of 100 mm from the antenna, several bottles of water (plastic and glass) were measured and the back-return signal was reduced by more than 10 dB. Experiments on wax carton liquids and even apples have yielded similar results.

Influence of local structure

When using hand-held instruments, a large number of other nearby objects severely distort the radiation patterns of the reader antenna and the tag antenna. This can be calculated for an operating frequency of 2.45 GHz. Assuming a representative geometry, the displayed return signal is reduced by 10 dB compared to the free space, and when the dual antenna is used simultaneously, it drops more than the expected mode. In the warehouse environment, an item box has a problem with a label, and several labels are attached to a 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 detection of the access control device, so that the influence of the local structure becomes less important because the reader antenna of the access control device is fixed in and out of the warehouse and directly points to the labeled object.

Distance

The gain of the RFID antenna and whether or not to use an active tag chip will affect the system's usage distance. Optimistic considerations, when the radiant intensity of the electromagnetic field complies with relevant UK standards, in the passive case of 2.45 GHz, the full-wave rectification, the driving voltage is not greater than 3 volts, the optimized RFID antenna impedance environment (impedance 200 or 300 ohm), use distance About 1 meter [3]. If the WHO limit [4] is used, it is more suitable for global use, but the effect distance is reduced by half. These limit the electromagnetic field power of the reader to the tag. The effect distance decreases as the frequency increases. If you use an active chip, you can reach a distance of 5 to 10 meters.