Relationship between the Res1, resonant frequency and bandwidth.
Radio frequency identification (RFID) technology is a very important part of the Internet of Things. The antenna used in RFID system reader is one of its most important equipment, has become a research hotspot. Compared to far-field applications, RFID antennas typically use high-gain circularly polarized antennas or antenna arrays to increase their read distance. Near-field RFID reader antenna requires strong magnetic field, wide band and low gain characteristics, while the resulting magnetic field should be evenly distributed to avoid leakage read phenomenon. The main contents of this chapter are as follows. The RFID reader near field antenna based on the principle of magnetic field coupling has developed rapidly in recent years. In this paper, a double-layer open-circuit antenna are proposed. The near-field antennas have wideband and strong magnetic field characteristics, and are compact and simple to process. The open-loop antenna of the double-layer terminal is located at the lower level and the upper layer is the radiation ring. The antenna size is smaller, and the feeder part and the radiation part of the isolation, a good deal to avoid interference between each other.
- near field
- electrically large
1. Concept of near-field antenna
Radio frequency identification (RFID) technologies, which were developed during the World War II, provide wireless identification and tracking capability [1, 2]. The reader antenna is an important unit of RFID systems. Many new RFID antennas are developed for different applications [3–7]. Reader antennas can be classified into two types based on the working scope for different application purposes: near-field antenna and far-field antenna. Currently, ultra-high frequency (UHF) near-field RFID technology are fast developed in item-level identifications such as sensitive products tracking, biological products and medical products (blood, medicines, vaccines), biosensing applications and so on [8–12].
The basic consideration of UHF near-field RFID is to make it work in a short distance stable, just like what LF/HF near-field RFID does . Inductive coupling systems are selected in most applications in near-field UHF RFID, because most of the reactive energy is stored in magnetic field. Inductive coupling is more stable than the capacitive coupling and hardly affected by liquid or metal .
In a near-field RFID system, the reader and the tag antennas are coupled mainly through magnetic field. If the tag antenna is electrically small, the magnetic field of reader antenna is perturbed by tag rarely, and coupling coefficient C could be shown by the equation [14, 15].
One of the challenges in UHF near-field RFID applications is to design a reader antenna with wide bandwidth and strong near magnetic field simultaneously. Strong near magnetic field is important for extending the reading range. Low gain could reduce interferences. Wide bandwidth antenna could be applied at 840–960 MHz, which covers both ETSI of Europe and FCC of North America. In some special application scenarios, smaller size antennas are required because of the limited system space.
Some near-field antennas have been reported to generate strong and even magnetic field. Many travelling wave antenna is proposed to extend the bandwidth . A conventional travelling wave antenna called Mini-Guardrail from a famous company Impinj has wide bandwidth of more than 200 MHz at UHF band as well as low gain . But because, loads of traveling wave antenna consume too much power, current and near magnetic field are not strong enough. The magnetic field should be lower than −13 dBA/m at any direction if the distance above antenna is larger than 2 cm. Conventional standing wave antenna could present a strong magnetic field, but its gain is too high and bandwidth is too narrow to cover a wide UHF RFID bandwidth. Typical standing wave antenna with strong magnetic field like eye shape is proposed by Li et al. , but −10 dB bandwidth is only 30 MHz which could only cover FCC band. Narrow bandwidth antenna is easily detuned by environmental changes.
2. Typical design of electrically large near-field RFID antenna
2.1. Structure and character of a near-field RFID antenna
The current distribution of the near-field antenna is in-phase. That is, the current on the loops has the same orientation. For example, a two-layer and open circuit shape near-field antenna made by PCB board is shown in Figure 1 . The current character is explained by this example.
Figure 1(a) shows the structure of the antenna, which is composed of two PCB layers. The top layer is mainly composed of two quasi-half loops which are connected with two folded straight terminals. The bottom layer is feed network with ground and lead are printed onto top and bottom surface, respectively. Fed at edge of bottom layer, 50 ohm microstrip line is connected with two 100-ohm lines, which are connected to two metal columns in another end. The other end of each column is connected to metal quasi-half loop strip. There are four loads on two quasi-half loops which are marked by 1, 2, 3 and 4 in Figure 1(a) . The two PCB boards are connected by two metal columns and fixed by two nylon columns.
Figure 1(b) shows the top view. Angle and radius of loop are marked by
2.2. Performance of the antenna
The proposed antenna can be printed onto any substrate and optimized at specific operating frequency by properly selecting the geometrical parameters. The antenna prototype is printed onto a FR4 substrate (
2.2.1. S11 performance
The impedance matching measurement of the antennas was carried out using the Agilent N5230A vector network analyzer. Figure 3 shows the simulated and measured return loss. The proposed antenna exhibits broadband impedance bandwidth, the frequency range for −15 dB return loss is from 826 to 950 MHz or 124 MHz bandwidth. The measured result agrees well with the simulation.
2.2.2. Far-field gain and directivity
The far-field gain and directivity are shown in Figure 4 . It is clear that the gain is lower than −10 dB at any direction and about 15 dB lower than directivity.
2.2.3. Current distribution
Figure 5 shows the current distribution of different phases. Different from conventional traveling wave antenna, current on the loop assumes standing wave distribution. One important factor of current is in-phase. Because magnetic field produced by the currents on the adjacent sides of the antenna cancel out each other and is thus very weak in the central portion of interrogation zone if the current distribution is out-phase. In Figure 5 , current of this antenna is not out-phase because of small electrical length of the quasi-half loop. At the phases of 45, 90 and 135 degree, the currents are all strong. Actually, the magnetic field at these phases are also strong. The result could be verified by the comparison between Figures 5 and 6 .
2.2.4. Magnetic field distribution
Figure 6 shows that magnetic field is concentrated and uniform around the center region of antenna at different phases (0°, 45°, 90°, 135°). Moreover, as a result of folded terminal, average current can be enhanced on the outer loop so that strong magnetic field intensity is obtained.
Figure 7(a) shows the magnetic field distribution at different
Compared to other antenna, it could be found that the
2.3. Reading range
To further verify the performance of the proposed two-layer and two quasi-half loops antenna, the prototype was used as the reader antenna in a UHF near-field RFID system to detect UHF near-field tags. Test system is shown in Figure 8(a) , and the proposed antenna was connected to the reader operating at both 865–868 MHz of ETSI and 902–928 MHz of FCC with 30 dBm output to detect tag. Tag is positioned on a foam board, which has a size of 70 mm×70 mm, could be shown as Figure 8(b) . Grid is marked on the top of the foam board with the size of 1 cm×1 cm. The data of detected tag on each intersection were recorded.
This chapter adopts one annular tag which could be activated when magnetic field intensity is stronger than −13 dBA/m. The system choose ETSI band as the operating frequency because the operating mechanism of FCC is hopping frequency (HF).
The measurement results of reading range are exhibited in Figure 9 ; it is clear that reading scope is reduced if distance increased. Compared between Figures 9 and 7(a) , it could be found that at each
3. Optimization of near magnetic field
3.1. Parameter studies
The antenna’s structure is a distorted symmetrical dipole. So, the length of metal loops and the input impedance could be estimated based on dipole theory. If resonant frequency is determined, the electric length of a metal column, quasi-half loop and folded terminal should match the resonant frequency.
After extensive simulations, it is found that loads (
3.1.1. Loads on quasi-half loops,
Figure 10(a) and (b) shows the magnetic field distribution of antennas at different values of
|S11 at resonant frequency||−15 dB bandwidth||−10 dB bandwidth|
|10 ohm||−11.95 dB at 884 MHz||0 MHz||61 MHz (859–920 MHz)|
|30 ohm||−33.5 dB at 884 MHz||123 MHz (828–951 MHz)||190 MHz (808–998 MHz)|
|60 ohm||−13.81 dB at 884 MHz||0 MHz||179 MHz (818–986 MHz)|
|100 ohm||-8.35 dB at 893 MHz||0 MHz||0 MHz|
|S11 at resonant frequency||−15 dB bandwidth||−10 dB bandwidth|
|10 ohm||−16.7 dB at 877 MHz||42 MHz (857–899 MHz)||124 MHz (817–941 MHz)|
|30 ohm||−21.7 dB at 879 MHz||72 MHz (844–916 MHz)||145 MHz (810–955 MHz)|
|50 ohm||−33.5 dB at 884 MHz||123 MHz (828–951 MHz)||190 MHz (808–998 MHz)|
|90 ohm||−25.4d B at 894 MHz||96 MHz (850–946 MHz)||184 MHz (811–996 MHz)|
Both magnetic field and return loss are more sensitive to
3.1.2. Effect of the space between two layers,
Figure 11(a) and (b) exhibits the magnetic field distribution of the antennas with varying heights of
3.1.3. Radius of quasi-half loop
Radius of quasi-half loop is also an important influencing factor of magnetic field. In the experiments, other parameters are unchanged except
3.2. Structure improvement
Figure 13(a) shows the structure of the antenna. The top layer contains two quasi-half loops with two inductance-like terminals. The bottom layer is the feed network with the ground and lead. Fed at the edge, a 50 ohm microstrip line is connected to two 100 ohm microstrip lines which connect metal columns. The other end of each column is connected with a metal loop strip. On loops, there are four printed loads which are marked by 1, 2, 3 and 4 in Figure 13(a) . The two PCB boards are supported by two nylon columns.
Figure 13(b) shows the top view of antenna. The angle, width and radius of the quasi-half loop are marked by
4. Conclusion of near-field antenna of RFID system
It is challenging to design UHF near-field RFID antennas with strong magnetic fields and broad bandwidth. The UHF RFID antenna is demonstrated to be able to achieve a broad bandwidth and a low gain. The proposed antenna has also been proven to have strong magnetic fields with concentrated field distribution in the near-field region of antenna, which is very suitable for UHF near-field RFID reader applications.
Moreover, the investigation has shown that the UHF RFID antenna has produced the stronger magnetic field distribution. The most impact factor for near-field RFID antenna is magnetic field distribution. In recent years, near field radio frequency identification system has developed, many antennas are developed, but there are still have some problems. The combination of magnetic field construction and band widening technology is not perfect, especially the low-loss large-scale standing wave near-field antenna band widening technology is rarely reported.
The near-field theory is the research foundation of the near-field antenna. The breakthrough of the near-field electromagnetism theory and its innovation not only play an extremely important role in the development of the electromagnetism itself but also promotes the radio frequency identification directly and also a series of modern electronic technology development, so the analysis of the near-field problem is very basic in the electromagnetism, valuable and challenging research work.