Radio frequency identification (RFID) System RFID tag antennas (1)

【 Summary 】 using the traditional bar code identification for goods will bring a range of inconveniences: no longer distances recognition, requiring human intervention, many items cannot be identified, and so on.

On the contrary, radio frequency identification (RFID) systems using an electromagnetic wave with a penetrating, so you can be identified for longer distances, without human intervention, you can identify a wide variety of items.

1 Introduction

Modern society is becoming more and more rich, product data management needs are increasing, people need to be varied in the production, sale and distribution of goods in the process to identify, manage, and positioning.

Using the traditional bar code identification for goods will bring a range of inconveniences: no longer distances recognition, requiring human intervention, many items cannot be identified, and so on. On the contrary, radio frequency identification (RFID) systems using an electromagnetic wave with a penetrating, so you can be identified for longer distances, without human intervention, you can identify a wide variety of items.

Radio frequency identification technology is a non-contact automatic recognition technology.

It consists of an electronic tag (Tag/Transponder), reader (Reader/Interrogator) and middleware (Middle-Ware) three parts of a short-range wireless communication system. Radio frequency identification tags is radio frequency identification tags and tag antenna combination. Labelling according to its working mode and is divided into passive and active tags. Active tags itself carry battery to provide the reader communication required energy; passive tags using inductive coupling or inverse scattering mode, i.e. through the tag antenna from a reader in electromagnetic fields or electromagnetic energy activation chip, and adjust the RFID chip and label matches your antenna, will be stored in the tag chip information back to the reader. Therefore, RFID antenna impedance must label chip input impedance matching to allow conjugation tag chip can maximize RFID reader of electromagnetic energy. In addition, the tag antenna design must also be considered, and the electronic label, such as application in where metal objects on the surface of the tag antenna and used in normal surface tag antenna in antenna structure and material there are great differences. Suitable for a variety of chip, low cost, versatile tag antenna is radio frequency identification in our widely popular as one of the key technologies.

2 radio frequency identification system and the classification of the antenna

For passive tags in the RFID system, according to the working frequency of the different has two working mode.

One is the inductive coupling (Inductive Coupling) mode, which is also called near field work mode, it is mainly used for low-frequency and high frequency RFID system; the other is the inverse scattering (Back scattering) mode, which is also known as far-field operation, mainly applicable to UHF and microwave RFID system.

Induction coupling model mainly refers to the reader antenna and tag antenna using coil forms.

When the reader is reading the label, issued without modulation signals, is in the reader antenna near-field of the tag antenna receives this signal and activate the tag chip after tag chip according to the internal storage of global unique identification number (ID) of the control tag antenna current size. The current size further enhance or reduce the reader antenna's magnetic field. At this point, the reader of near-field component, show the characteristics of being modulated, reader internal circuit detects this as a label for the amount of modulation and demodulation and tag information.

In inverse scattering work mode, the reader and tag between electromagnetic wave for information transmission.

When a reader reads the tag recognition, first issued without modulation of electromagnetic waves, which is located in the far field RFID tag antennas receive electromagnetic signals and antenna on the induction voltage, electronic tag internal circuit to the induction voltage rectifier and magnification used to activate the tag chip. When tag chip activated, with its own global unique identification number on the label to change chip impedance, when the impedance of the RFID chip and tag chip impedance matching between good, basic non-reflective signal, impedance matching is not good when it will be almost entirely reflected signal, this reflection amplitude signal occurs, this situation is similar to the reflective signal amplitude modulation. The reader through the received signal is modulated reflection to determine the electronic label identification number and be recognized. This type of antenna including microstrip antenna, planar dipole antenna and loop antenna. Figure 2 is developed to work in multiple recognition environment of UHF RFID tag antennas.

3 electronic tag antenna design and test

As mentioned earlier, because of the work to low-frequency and high frequency RFID system uses inductively coupled model for communication, so it works in both band of readers and RFID tag is used in coil form of antenna.

Work in these two frequency bands for radio frequency identification systems are subject to the scope of the near field effects, leading to the recognition of a shorter distance. Under the present circumstances, the use of near field communication of radio frequency identification system maximum recognition distance less than 1 m.

Because of the low frequency and high frequency band for radio frequency identification system is the electromagnetic field coupled model, so the system's antennas using coil forms.

Use this form of the main reasons are as follows: 1. Electromagnetic coupling between relatively tight coil; 2. The antenna uses coil forms to further reduce the size of the antenna and thereby reduce the size of the label; 3. Tag chip characteristics required tag antenna has a reactor.

In the UHF and microwave-band, transponder and reader communication using backscatter works.

Then, connect the electronic label and a bridge between the reader and is no longer a near magnetic field instead of electromagnetic waves. At this point, passive RFID tag at the reader of the electromagnetic far field. According to the frequency band of wavelengths and antenna portDiameter can compute the band RFID system for far-field and the reader. Generally, passive tags in the UHF range working distance of up to 10 meters, according to available information, the work on microwave bands (primarily means 2.45GHz) passive tags working distance of only 1 meters. So currently uses the inverse scattering mode of radio frequency identification system is mainly used in 860 ~ 960 MHz of UHF band.

In the passive tag antenna composed of radio frequency identification system, and the labels from the readers of electromagnetic fields generated by or obtained the activation energy of electromagnetic wave tag chip, RFID part circuits specifically designed to detect tag antenna on induced electromotive force or induced voltage, and through a rectifier diode circuit and other circuit voltage amplification, etc.

These circuits are integrated in the tag chip. When the chips are encapsulated usually also will introduce part of distributed capacitance. However, the antenna design itself does not need to know the specific circuit chip but only need to master the chip and encapsulated after chip impedance, and take advantage of maximum energy transfer rule design antenna input impedance.

Because RFID tag chip output impedance with the reactor components, in order to achieve maximum transfer of energy, you need an antenna input impedance design for tag chip impedance of conjugate [1].

In General, the RFID tag chip input impedance is Z = R-jX forms, in order to obtain the impedance of the conjugated forms, electronic tag antenna impedance should be as Z = R + jX forms.

As mentioned, the work in low frequency and high frequency RFID system uses passive tags antenna coil form, this can introduce a coil forms used to offset the gankang equivalent circuit capacitance for tag chip and antenna maximum energy transfer.

And for work on UHF and microwave band label antenna, in order to introduce a sense of resistance to offset chip of capacitance, antenna design joined the ring structure of Kansei feed [2], or to join the T-type match [3], and other structures.

In addition, in order to provide the equivalent isotropically radiated power (EIRP) for further reading distance, apart from the requirements of the electronic label also has a high gain antenna, also called RFID tag chip antenna and be able to have enough between good match [4].

In the tag antenna design and simulation and achieve an ideal result after processing the antenna and test to verify the correctness of design and simulation.

Because the label before those with complex impedance of the antenna characteristics, test methods and have real impedance antenna test method. In addition, in the same tag antenna tests, according to the required data in different ways in their tests.

Typically, the process of testing the antenna does not need to specifically test the input impedance of the antenna.

But the tag antenna impedance is negative impedance, and its imaginary and real part of a larger (usually X/R > 10), this impedance curve in smith circle diagrams close to short round, it is difficult to observe by smith circle graph antenna impedance bandwidth. In order to obtain the label input impedance of the antenna, you can test the device's output ports directly connected to the antenna input port. Because this method does not consider the tag antenna itself has a complex impedance of the characteristics of the antenna and test equipment to obtain conjugate between did not, at this time can only be matched antenna impedance parameters, such as the scattering matrix parameter and standing wave ratio, commonly used to measure the antenna circuit parameters cannot directly access.

In order to obtain the scattering parameters and VSWR circuit parameters so that the antenna impedance bandwidth characteristics for evaluation, measurement of impedance parameters into the relevant calculation formula or method using impedance matching in test equipment and antenna to join matching circuits.

Matching circuits available two ways, one is the work of the higher frequency components of the separation of powers, the second is the use of microwave circuit [1] [2]