Impedance measurement method for UHF band RFID near-field antenna

introduction

Ultra-high frequency (UHF) band radio frequency identification (RFID) near-field reader antenna (NFRA) is attracting much attention due to its potential for application in single-item identification [1], insensitivity to the environment, and higher read/write speed than HF antennas. UHF band NFRA is usually implemented using an electrically large loop structure with balanced ports.

For NFRA, a good matching network is crucial [2,3]. Usually, NFRA antennas in the UHF band are designed to be installed in a metal cavity to reduce the impact of the environment on the antenna performance, as shown in Figure 1. However, due to the existence of the metal cavity, the impedance of the antenna will change dramatically with the frequency, which will result in the impedance value obtained in the simulation software being inaccurate. It is difficult to design a matching network with good performance based on this inaccurate impedance. Usually, we divide the design of NFRA into 3 steps:

1. First, the design and processing of the loop antenna;

2. The second step is to measure the impedance of the loop antenna;

3. The third step is the design of the matching network and the joint simulation of the matching network and the loop antenna. In this article, we designed a method to obtain the precise impedance of the antenna by combining the coaxial line and de-embedding technology for step 2. Based on the impedance obtained by this method, the matching network and the NFRA antenna are designed and manufactured.

Figure 1 Structure of UHF RFID near-field reader antenna

1 Measurement method

Generally, antennas with balanced ports, especially small antennas like the one shown in Figure 2, require a balun [4]. The function of the balun is to complete the conversion from a balanced port to an unbalanced port. Usually, a 1:1 balun is used between the coaxial line and the antenna structure to suppress the influence of the common-mode current on the coaxial line and complete the conversion.

Figure 2 Impedance measurement of an electrically small antenna with a balanced port

However, for an electrically large balanced port antenna, the common-mode current on the coaxial line can be ignored and the coaxial line can be directly connected to the antenna for measurement, as shown in Figure 3.

Figure 3 Impedance measurement of an electrically large antenna with a balanced port

In the UHF band, the wavelength in the air is about 33cm, which is smaller than the size of a general NFRA. We take a European band standard (865MHz-868MHz) NFRA as an example to illustrate the impedance measurement method. Figure 4 shows a simplified model of this antenna. It can be seen that the antenna is an elliptical ring structure with a circumference of 42cm, which is much longer than the wavelength at 866MHz. When measuring, we can directly connect the port to the coaxial line without going through the balun.

Figure 4: Simplified NFRA model of European frequency band standards

Figure 5 is a photo of the impedance measurement of the antenna. It can be seen that the antenna is directly connected to a coaxial line with a length of l and connected to the vector network analyzer. Table I gives the main dimensions of the antenna when measuring.

2 De-embedding Technology

By using the method in the first section, the NFRA return loss parameters with coaxial line parameters can be obtained. De-embedding technology is a technology used to eliminate the influence of coaxial line parameters and obtain the real impedance of NFRA [5,6]. Figure 6 shows the equivalent circuit model measured using the De-embedding technology, in which the coaxial line is equivalent to a transmission line with a length of l.

3 Measurement results

Figure 7 shows the comparison between the measured values ​​and simulation results of the S parameters when no matching network is added. It can be seen that the measured results are basically consistent with the simulation results obtained using HFSS software.

865MHz-868MHz is very small, which will lead to inaccurate impedance values ​​in simulation.

The simulated return loss is 0.88dB and the measured return loss is 1.3dB.

Figure 7 Comparison of simulated and measured S parameters without adding matching network

In Figure 8, we compare the simulated and measured impedance values. From the small-scale impedance comparison, we can see that the impedance of the antenna changes dramatically with frequency, which means that the bandwidth of the antenna after matching is very narrow. At 866MHz, the simulated impedance value is 366.9+j467.03(Ohm), while the impedance value measured after de-embedding is 460.8+j309(Ohm), and the Q value of the two differs by about 0.6. For narrowband matching, any slight difference in Q value will lead to matching failure, so accurate impedance measurement is crucial for the design of matching network. This is also the reason why we need to perform de-embedding technology on antenna measurement.

Figure 8 Comparison of simulated and measured impedance (a) Comparison of resistance values ​​(b) Comparison of reactance values

Based on the impedance value measured at 866MHz, we can design a matching network. Figure 9 shows the comparison between the simulated and measured S parameters of NFRA after adding the designed matching network. It can be seen that the bandwidth obtained by simulation is

Figure 9 Comparison of simulated and measured S parameters of NFRA after adding matching network

4 Conclusion

Taking a designed NFRA as an example, a low-loss impedance measurement method is described. Through joint measurement and de-embedding technology, the accurate value of the antenna impedance is obtained. Based on the measured impedance, a matching network with good performance is designed. The S-parameter simulation value and measured value of the matched NFRA are in good agreement, which proves the effectiveness and accuracy of this method.