Technical characteristics and application analysis of near-field and far-field RFID antennas

　　Technical characteristics and application analysis of near-field and far-field RFID antennas

　　RFID (Radio Frequency Identification) technology is one of the core technologies of the perception layer of the Internet of Things. Its antenna performance directly determines the identification distance, accuracy and environmental adaptability. Near-field and far-field RFID antennas are based on completely different electromagnetic coupling mechanisms, and show significant differences in communication distance, anti-interference ability and application scenarios. Starting from electromagnetic theory, this article systematically analyzes the working principles, technical parameters and design optimization methods of the two types of antennas to provide theoretical support for the engineering selection and performance improvement of RFID systems.

　　Near-field RFID antenna: inductive coupling mechanism and technical characteristics

　　Working principle and electromagnetic field distribution

　　Near-field RFID antennas work based on the principle of inductive coupling. Its core is to generate a time-varying magnetic field in the antenna coil through an alternating current to achieve energy transmission and data interaction with the tag. According to Maxwell's equations, an alternating magnetic field will excite an eddy electric field in the surrounding space. When the RFID tag enters the magnetic field region (usually defined as a near field region less than λ/2π from the antenna, where λ is the working wavelength), the tag coil generates an induced electromotive force due to electromagnetic induction, which powers the tag chip and modulates the load to feedback information.

　　In common near-field frequency bands such as 13.56MHz (ISO 15693 standard), the magnetic field energy is mainly concentrated near the antenna coil axis, showing a magnetic field-dominated characteristic (the electric field component can be ignored). Through finite element simulation (HFSS software), it can be seen that its magnetic field intensity (H-field) decays with the cube of the distance (H∝1/r³). This rapid decay characteristic creates the precise distance control capability of near-field communication (usually ≤30cm).

　　Key technical parameters and design optimization

　　Coil structure: The typical near-field antenna adopts a spiral coil design. The relationship between its inductance (L) and the number of coil turns (N) and diameter (D) can be expressed as L=μ₀N²D/(8D) (approximate formula). In the 13.56MHz frequency band, the coil inductance is usually designed to be 1-3μH, and the matching capacitor (C) forms an LC resonant circuit (f=1/(2π√(LC))) to achieve 50Ω impedance matching.

　　Q value control: The quality factor Q=ωL/R determines the concentration of the magnetic field. A high Q value (30-50) can enhance the magnetic field strength, but will reduce the bandwidth; in practical applications, the Q value needs to be adjusted through a series resistor to balance the read and write distance and data transmission rate.

　　Anti-metal design: The use of magnetic core (such as ferrite) shielding technology can reduce the eddy current loss of the metal surface to the magnetic field by more than 60%. Experimental data show that the attenuation of the identification distance in a metal environment is reduced from 40% to 15% for a near-field antenna equipped with a 0.5mm thick ferrite layer.

　　Far-field RFID antenna: electromagnetic wave propagation and backscattering mechanism

　　Working principle and wave field characteristics

　　Far-field RFID antennas rely on electromagnetic wave propagation and backscattering mechanisms and work in UHF (860-960MHz) and higher frequency bands (such as 2.45GHz). According to electromagnetic radiation theory, when the antenna size is comparable to the working wavelength, significant electromagnetic radiation will be generated, forming a plane wave (E/H=377Ω, free space wave impedance) in which the electric field (E) and the magnetic field (H) are perpendicular to each other and propagate in phase.

　　The far-field region is defined as a space greater than 2D²/λ (D is the maximum size of the antenna) from the antenna. At this time, the electromagnetic wave propagates in the transverse electromagnetic mode (TEM). After the tag receives the electromagnetic wave energy, it changes the reflection cross section (RCS) through impedance modulation and loads the information into the reflected wave. This process is called backscattering modulation. The receiving end extracts tag information through coherent detection to achieve long-distance communication (usually ≥1m, and active tags can reach more than 100m).

　　Key performance indicators and design methods

　　Gain and directivity: The gain (G) of the far-field antenna directly determines the communication distance, and its theoretical limit is G=4πA/λ² (A is the effective aperture area). The gain of the commonly used microstrip patch antenna in the UHF band is about 5-8dBi, and the directional beam width (3dB) is controlled at 60°-120°, which can effectively suppress sidelobe interference.

　　Polarization matching: The polarization mode (linear polarization/circular polarization) of the tag antenna and the reader antenna needs to match, otherwise polarization loss will occur. The circularly polarized antenna can tolerate an angle deviation of ±45°. In scenarios such as logistics pallet identification, the recognition success rate is increased by more than 30% compared with the linearly polarized antenna.

　　Anti-multipath design: Diversity reception technology (such as spatial diversity and frequency diversity) can reduce signal fading caused by multipath effects. Experiments show that the bit error rate (BER) of the dual-antenna diversity system in a warehouse environment is reduced from 10⁻³ to 10⁻⁵.

　　Technical comparison and application scenarios of near-field and far-field antennas

　　Performance parameter comparison

　　The coupling mechanism of the near-field antenna (13.56MHz) is inductive coupling (magnetic field dominated), the communication distance is ≤30cm, the data rate is 106-848kbps, the metal resistance is strong (magnetic core shielding is required), and the multi-tag recognition ability is low (≤10 / second).

　　The far-field antenna (UHF) is based on electromagnetic wave propagation (backscattering), with a communication distance of 1-15m (passive) and >100m (active), a data rate of 26.7-640kbps, weak metal resistance (special tag design required), and high multi-tag recognition capability (≥100/second).

　　Analysis of typical application scenarios

　　Near-field application: In payment terminals (such as NFC), 13.56MHz near-field antennas achieve secure transactions through precise coupling within 10cm, and their magnetic field boundaries are clearly identifiable to avoid misreading of adjacent cards; in the medical field, near-field antennas can penetrate liquid environments (such as blood sample tubes) and maintain a recognition rate of more than 99.5% for RFID tags.

　　Far-field application: UHF far-field antennas can realize batch identification of whole pallets of goods in smart warehousing. The directional antenna with 8dBi gain and 4W transmission power can read more than 50 tags at the same time within a distance of 8m; in the highway ETC system, the circularly polarized far-field antenna can tolerate the angle changes of vehicles during high-speed driving, ensuring a 99.9% recognition success rate.

　　Frontier technology and development trends

　　Adaptive switching technology

　　The new RFID reader has realized the adaptive switching of near-field/far-field mode, and automatically adjusts the working frequency band (13.56MHz/915MHz) and coupling mode by real-time monitoring of the received signal strength (RSSI). Experimental data shows that this technology can increase the recognition success rate by 20%-30% in complex environments.

　　Metamaterial antenna design

　　The introduction of metamaterials has brought breakthroughs to RFID antennas. Its negative dielectric constant characteristics can compress the magnetic field distribution in the near field area, making the volume of 13.56MHz antennas 40% smaller while maintaining a 30cm communication distance; in far-field applications, the metamaterial reflector can increase the gain of UHF antennas by more than 3dBi.

　　Anti-interference algorithm optimization

　　For electromagnetic interference in industrial environments, the signal processing algorithm based on machine learning can effectively distinguish between tag backscattered signals and noise. Tests show that the demodulator using LSTM neural network reduces the bit error rate from 10⁻² to 10⁻⁴ at a -10dB signal-to-noise ratio.

　　Conclusion

　　Near-field and far-field RFID antennas are based on different electromagnetic coupling mechanisms, forming complementary technical characteristics: near-field antennas use inductive coupling to achieve centimeter-level accurate identification, which is suitable for scenarios requiring high accuracy; far-field antennas use electromagnetic wave propagation and backscattering to achieve long-distance batch identification of more than meters. In the future, with the integration of new materials and intelligent algorithms, RFID antennas will develop in the direction of miniaturization, multi-function and adaptability, further expanding the application boundaries in intelligent manufacturing, smart logistics and other fields.