1 Introduction
Radio Frequency Identification (RFID) is a non-contact automatic identification technology that uses radio frequency technology. It boasts advantages such as high transmission speed, impact resistance, large-volume reading, and reading during movement. Therefore, RFID technology has significant application potential in various fields, including logistics and supply chain management, production management and control, anti-counterfeiting and security control, and traffic management and control. Currently, RFID technology operates in low-frequency, high-frequency, ultra-high-frequency, and microwave bands, with high-frequency and ultra-high-frequency applications being the most widespread.
2. RFID Technology Principles
An RFID system mainly consists of a reader (target), a transponder (RFID tag), and a backend computer. The reader, which reads, writes, and stores data from the tag, comprises a control unit, a high-frequency communication module, and an antenna. The tag itself mainly consists of an integrated circuit chip and an external antenna. The circuit chip typically includes a radio frequency front-end, logic control, and memory circuits. Tags can be categorized by their power supply principle into active, semi-active, and passive tags. Passive tags are favored due to their low cost and small size.
The basic working principle of an RFID system is as follows: After the tag enters the reader's radio frequency field, the induced current obtained by the antenna is boosted by a voltage boosting circuit to power the chip. At the same time, the induced current carrying information is converted into a digital signal by the radio frequency front-end circuit and sent to the logic control circuit for processing. The information that needs to be replied is sent from the tag memory, sent back to the radio frequency front-end circuit by the logic control circuit, and finally sent back to the reader through the antenna.
3. Antennas in RFID Systems
From the perspective of RFID technology principles, the key to RFID tag performance lies in the characteristics and performance of the RFID tag antenna. The antenna plays a crucial role in the data communication process between the tag and the reader. On one hand, the tag's chip startup circuit needs to obtain sufficient energy from the electromagnetic field generated by the reader through the antenna to begin operation; on the other hand, the antenna determines the communication channel and communication method between the tag and the reader. Therefore, research on antennas, especially the internal antenna of the tag, has become a key focus.
3.1 Types of RFID System Antennas
Based on the power supply method of the RFID tag chip, RFID tag antennas can be divided into two categories: active antennas and passive antennas. Active antennas have lower performance requirements than passive antennas, but their performance is greatly affected by battery life. Passive antennas overcome the limitations of active antennas due to battery constraints, but they have very high performance requirements. Currently, the research focus of RFID antennas is on passive antennas. From the perspective of the operating frequency band of the RFID system, RFID systems operating in the LF and HF bands (e.g., 6.78MHz, 13.56MHz) transmit electromagnetic energy in the inductive field region (quasi-steady field), also known as inductive coupling systems. Systems operating in the UHF band (e.g., 915MHz, 2400MHz) transmit electromagnetic energy in the far-field region (radiation field), also known as microwave radiation systems. Because the energy generation and transmission methods of the two systems differ, the corresponding RFID tag antennas and front-end components have their own unique characteristics. Therefore, tag antennas are divided into near-field inductive coil antennas and far-field radiating antennas. Inductive coupling systems use near-field induction coil antennas, composed of multi-turn inductors. The inductors and their parallel capacitors form a parallel resonant circuit to couple maximum radio frequency energy. Microwave radiation systems primarily use far-field radiating antennas, mainly dipole antennas and slot antennas. Far-field radiating antennas are typically resonant, generally using half a wavelength. The shape and size of the antenna determine its frequency range and other performance characteristics; higher frequencies result in more sensitive antennas with smaller footprints. Higher operating frequencies allow for smaller tag sizes, and far-field radiating antennas offer higher radiation efficiency compared to near-field induction antennas.
3.2 Design Requirements for RFID Tag Antennas
The design requirements for RFID tag antennas mainly include: sufficiently small physical size to meet the miniaturization needs of tags; omnidirectional or hemispherical directional coverage; high gain to provide maximum signal strength to the tag's chip; good impedance matching so that the tag antenna polarization matches the reader's signal regardless of the tag's orientation; and robustness and low cost. When selecting an antenna, the main considerations are: antenna type, antenna impedance, RF performance on the applied object, and RF performance when other objects surround the tagged item.
4. Types and Research Status of RFID Tag Antennas
Tag antennas are mainly divided into three categories: coil type, dipole type, and slot (including microstrip patch) type. Coil type antennas are made by coiling metal wire into a plane or winding metal wire around a magnetic core; dipole antennas are composed of two straight wires of the same thickness and length arranged in a straight line, with the signal fed in from the two middle ends, and the length of the antenna determines the frequency range; slot type antennas are made by grooves cut into a metal surface, and microstrip patch antennas are composed of a circuit board with a rectangle at one end, the length and width of the rectangle determining the frequency range.
For RFID antennas used in low-to-medium frequency short-range applications with an identification distance of less than 1m, coil antennas are generally used due to their simple manufacturing process and low cost. For long-range applications in high-frequency or microwave bands above 1I1, dipole and slot antennas are required.
4.1 Coil Antenna
When the tag's coil antenna enters the alternating magnetic field generated by the reader, the interaction between the tag's antenna and the reader's antenna is similar to that of a transformer. The coils of both are equivalent to the primary and secondary coils of a transformer.
The carrier frequency used for bidirectional communication between tags and readers is crucial when the tag antenna coil requires a small size (small area) and a certain working distance. In such cases, the mutual inductance between the antenna coils of the RFID tag and reader is clearly insufficient to meet practical needs. To address this, a ferrite material with high magnetic permeability can be inserted inside the tag antenna coil to increase the mutual inductance, thus compensating for the small coil cross-section. Currently, coil antenna technology is mature and widely used in RFID systems such as identity recognition and cargo tagging. However, for RFID applications with high frequencies, large information volumes, and uncertain working distances and directions, coil antennas struggle to achieve the required performance specifications.
4.2 Dipole Antenna
Dipole antennas have the advantages of good radiation capability, simple structure and high efficiency. They can be designed into RFID systems suitable for omnidirectional communication and are widely used in the design of RFID tag antennas, especially in long-range RFID systems.
The biggest problem with traditional half-wave dipole antennas is their impact on tag size, such as a 915MHz half-wave dipole. Research shows that terminated, tilted, and folded dipole antennas, by selecting appropriate geometric parameters, can achieve the desired input impedance, offering advantages such as high gain, wide frequency coverage, and low noise. Their performance is excellent, and their size is much smaller than traditional half-wave dipole antennas. With the addition of brazed electrical terminals and an unbalanced transformer, gain, impedance matching, and bandwidth can be maximized. It is known that increasing the number of bends in the antenna helps reduce its size without sacrificing efficiency. Therefore, how can "bending" be performed within a limited space? How do the specific "bending" parameters affect the tag antenna's resonant frequency and input impedance? What kind of "bending" achieves the highest RF efficiency?
We know that objects with fractal structures generally possess proportional self-similarity and space-filling properties. When applied to antenna design, this can enable multi-band antenna performance and size reduction. Extensive research has been conducted both domestically and internationally on antennas with fractal structures, confirming their excellent size reduction characteristics and the ability to significantly improve antenna efficiency within limited space.
Applying Hilbert fractal transformation to different positions and dimensions of a half-wave dipole and simulating the Hilbert tag antenna using the method of moments yields simulation results of the tag antenna's resonant frequency and input impedance varying with the fractal dimension and order. Analyzing the antenna gain and efficiency in the results helps determine which dimension and order of the tag antenna best meets the design requirements of a practical tag antenna. Further, a physical antenna is fabricated, and the RF identification distance is tested. This is a commonly used research method.
4.3 Slotted (including microstrip patch) antennas
Slot antennas are characterized by low profile, light weight, simple processing, easy conformal design to objects, mass production, diverse electrical performance, and wide bandwidth integration with active devices and circuits into a unified component. They are suitable for large-scale production, which can simplify the manufacturing and debugging of the whole device and thus greatly reduce costs.
Microstrip patch antennas consist of radiating patch conductors attached to a dielectric substrate with a metal base plate. Depending on the antenna's radiation characteristics, the patch conductors can be designed in various shapes. They are commonly used in low-profile structures with frequencies above 100MHz. Typically, they consist of a rectangular or square metal patch placed on the surface of a thin dielectric layer (called the substrate) on a ground plane. The patch can be manufactured using photolithography, making it low-cost and easy to mass-produce.
As mentioned earlier, bent antennas are beneficial for reducing the physical size of tag antennas, meeting the design requirements of tag miniaturization. The same concept of bending can be utilized for slot antennas. In fact, bent slot antennas are suitable for RFID tags in the high-frequency microwave band, effectively reducing antenna size and offering superior performance. They have broad market prospects. The research method is similar to that of bent dipole antennas, using the method of moments to study the influence of the number, height, position, and width of the slot bends, as well as the size of the slot antenna flat plate, on the resonant characteristics of the rectangular antenna.
A folded slot antenna has a flat panel size of L x W, a slot bending width and height of s and h, and a slot distance from the center of the feed point. The following discussion focuses on the effects of variations in these parameters on the resonant characteristics, reflection coefficient, and antenna efficiency of the slot antenna.
Based on the impact of bending parameters on the performance of slot antennas, slot antennas for UHF RFID tags can be designed and physical antennas fabricated according to actual needs. It is anticipated that bent slot antennas will be a promising development direction in the field of UHF tag antenna design.
5. Hot Issues with RFID Tag Antennas
In the design of RFID tag antennas, in addition to the ever-important issue of reducing physical size, further improving the bandwidth and gain characteristics of miniaturized antennas to expand their practical application range, and analyzing the cross-polarization characteristics of miniaturized antennas to clarify their polarization purity are also important research directions. In addition, composite antenna design covering various frequencies, multi-tag antenna optimized distribution technology, reader smart beam scanning antenna array technology, design simulation software and platforms, tag antenna and attachment medium matching technology, consistency anti-interference and security and reliability technology are all topics worthy of further research.
On-chip antenna technology has recently become a hot research topic. The expanding application areas of RFID technology have led to increasing demands for miniaturization, lightweight design, multifunctionality, low power consumption, and low cost in RFID tags. However, current RFID tags still use external, independent antennas. The advantages of this approach are a high Q-factor, ease of manufacturing, and moderate cost. The disadvantages are its large size, fragility, and inability to perform tasks such as anti-counterfeiting or implantation as biometric tags in animals. If the antenna could be integrated onto the tag chip, it would operate without any external components, resulting in a smaller and more convenient tag. This has sparked research into on-chip antenna technology.
Integrating the antenna onto the chip not only simplifies the original tag manufacturing process and reduces costs, but also improves reliability. The on-chip antenna, acting as both an energy receiver and a signal sensor, determines the performance of the entire system. Its fundamental principle is based on Faraday's law of electromagnetic induction. It converts the energy of changing external magnetic fields into on-chip power supply voltage, serving as the operating power for the entire chip. Simultaneously, it uses changes in on-chip current or voltage caused by changes in the electromagnetic field to identify the received signal. The signal is transmitted to the receiving end by altering the change in the external magnetic field due to its own output impedance. To date, on-chip antennas implemented on standard CMOS processes still primarily use silicon-based integrated spiral inductors as their main structure.
In addition to the internal design of RFID tags, research in areas such as RFID smarttable antennas is also receiving increasing attention.
