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Introduction
The RFID system covers a frequency range from 125 kHz low frequency to 860-960 MHz ultra-high frequency, and the magnetic separator does not face the problem of "adjusting different parameters with the same material" in this frequency range, but rather a systematic reversal of evaluation weights. In the low frequency range, high magnetic permeability and high saturation flux are emphasized to guide magnetic flux and suppress eddy currents; High frequency and ultra-high frequency bands prioritize low loss and low dielectric constant, while high permeability materials may become a burden due to eddy current loss and dielectric loading. Starting from the frequency square dependence of eddy current losses, this article analyzes the differential requirements of LF, HF/NFC, and UHF frequency bands for μ ', μ', ε ', and resistivity. Combined with engineering boundaries such as tan δ ≤ 0.1, ε'<20, and volume resistivity ≥ 10 ⁶Ω· cm, a framework for selecting RFID magnetic separators with "working frequency band" as the first sieve is provided.
The selection and discussion of RFID magnetic separators often treat "RFID" as a unified scenario: find a high permeability, low loss magnetic sheet, stick it between the tag antenna and metal, and restore the reading distance to complete it.
But the frequency range covered by RFID extends from 125 kHz to 960 MHz, spanning nearly four orders of magnitude. On this span, the failure mechanism and material evaluation weight of the magnetic separator have undergone fundamental changes. Materials that are effective in the low-frequency range may fail in the ultra-high frequency range due to excessive eddy current losses; Low dielectric materials available in the ultra-high frequency range may not provide sufficient magnetic flux guidance in the low frequency range due to insufficient magnetic permeability.
Eddy current loss is the starting point for understanding this watershed
The core mechanism of RFID tag failure caused by metal environment is eddy current. The alternating magnetic field induces eddy currents on the metal surface, and the reverse magnetic field generated by the eddy currents cancels out the magnetic flux emitted by the reader/writer. The effective magnetic field obtained by the tag antenna is weakened, and the reading distance decreases or even returns to zero.
The function of the magnetic separator is to insert a low magnetic resistance path between the antenna and the metal, guiding the magnetic flux away from the metal surface. But the magnetic separator itself is also a conductor or semiconductor, which not only guides the magnetic flux, but also generates eddy currents inside. The approximate relationship of eddy current loss is:
Pe∝ (f² · B² · t²) / ρ
f: Frequency B: Magnetic induction intensity t: Thickness ρ: Electrical resistivity
This relationship directly explains the source of the frequency band watershed: eddy current losses are proportional to the square of the frequency. The frequency increased from 125 kHz to 13.56 MHz, and the f ² increased by about ten thousand times; From 13.56 MHz to 900 MHz, f ² has increased by thousands of times. Eddy current losses, which can be ignored in the low frequency range, become the dominant loss mechanism in the high frequency range.
Therefore, the higher the frequency band, the more materials need to be thinned and have high resistivity to suppress eddy currents. In the low frequency range, a thicker magnetic layer can be used to exchange for magnetic flux capacity, while in the high frequency range, thickness and resistivity must be the first screening criteria.
LF frequency band: prioritizing magnetic flux capacity, eddy current has not yet become the main contradiction
Low frequency RFID with frequencies of 125 kHz and 134.2 kHz, typical applications include animal ear tags, access cards, and industrial code carriers. In this frequency band, f ² is very small, and the proportion of eddy current loss in the total loss is limited. The core task of a magnetic separator is to provide sufficient magnetic flux guidance capability to confine the low-frequency magnetic field lines emitted by the reader/writer near the tag antenna, avoiding being "short circuited" by the metal substrate.
LF recommendation window
The LF recommendation window provided by the selection data of Advanced Institute Technology is: nanocrystalline or amorphous materials, initial magnetic permeability of u'3000-8000 @ 1 MHz, thickness of 0.1-0.3 mm, and key control parameters of high saturation magnetic flux Bs ≥ 1.0 T. High μ 'is effective here because eddy currents have not yet become a bottleneck at low frequencies, and the magnetic flux capacity directly determines the upper limit of read distance recovery.
HF/NFC frequency band: low loss becomes the first constraint
The 13.56 MHz HF frequency band is the main frequency band for NFC payments, electronic tags, and access control systems. The f ² has increased by about ten thousand times compared to LF, and the weight of eddy current loss has significantly increased. Meanwhile, the magnetic field energy of NFC antenna itself is limited, and the magnetic loss of the magnetic separator will directly consume the signal budget.
HF Recommended Window and Engineering Criteria
The recommended material system for the HF frequency band is ferrite or nanocrystalline composite, with an initial frequency range of 150 to 800 MHz and a thickness of 0.05 to 0.2 mm. The key control parameter is low magnetic loss tan δ ≤ 0.1. The patent literature provides a stricter criterion for this: at 13.56 MHz, μ ′ ≥ 50, tan Δμ ≤ 0.1, and a volume resistivity ≥ 10 Ω· cm to suppress eddy currents.
TDK's IFL04 series provides a product level reference: at 13.56 MHz, the magnetic layer thickness is 0.05 mm with a μ'45 and μ'1.3. The μ'is not high, but the μ'is pressed to an extremely low level - in the HF frequency band, "sufficiently low loss" is more predictive of NFC performance than "sufficiently high magnetic permeability".
UHF frequency band: dielectric constant and resistivity replace magnetic permeability as the primary variables
860-960 MHz ultra-high frequency RFID, used for logistics tags, retail inventory, and asset management. The f ² is already very large in this frequency band, and eddy current loss has become the core constraint in the design of magnetic separators. More importantly, the working wavelength of UHF tag antennas is in the centimeter range, and the dielectric constant of the magnetic separator directly participates in the near-field distribution and impedance matching of the antenna.
UHF recommendation window
The recommended UHF window for Advanced Institute Technology is nickel zinc ferrite or low loss composite, with an initial μ'50-200 @ 1 MHz, a thickness of 0.1-0.5 mm, and a key control parameter of dielectric constant ε′<20. The target range of μ 'is significantly lower than the HF frequency band, because at this frequency, excessively high magnetic permeability is often accompanied by higher eddy current losses and dielectric loading, which can actually damage the matching conditions of the antenna. The design goal of UHF magnetic separators has shifted from "guiding magnetic flux" to "providing a stable near-field environment for antennas without introducing excessive losses and dielectric disturbances".
Comparison of parameter weights for three frequency bands
The following table summarizes the weight differences of key parameters of magnetic separators in the LF, HF, and UHF frequency bands:
Selection and judgment of RFID magnetic separators based on "frequency band"
Firstly, confirm the operating frequency band before discussing material parameters
LF, HF, and UHF have completely different weights for μ ', μ ", ε′, and ρ. Using the high permeability logic of LF to select UHF materials, or ignoring tan δ in HF scenarios, can lead to selection deviating from actual constraints. The first step in selection is not to compare the μ 'values, but to confirm which range the target RFID system's operating frequency falls within.
Secondly, in the HF frequency band, use tan δ as the first sieve
Tan δ ≤ 0.1 is a referenceable engineering boundary. The material with μ'45 and μ ″ only 1.3 may be superior to the scheme with higher μ 'but tan δ close to the 0.1 boundary in NFC scenarios. If the supplier only provides μ 'and does not provide μ "or tan δ at 13.56 MHz, the applicability of HF cannot be determined.
Thirdly, in the UHF frequency band, the dielectric constant and volume resistivity are placed before the magnetic permeability
ε '<20 is the key boundary to avoid antenna detuning. Meanwhile, a volume resistivity of ≥ 10 Ω· cm is the fundamental condition for suppressing eddy currents. UHF magnetic separators do not need to pursue high μ ', they require a composite balance of' sufficiently low loss, sufficiently low dielectric, and sufficiently high resistance '.
Fourthly, in cross band compatibility scenarios, priority should be given to layered or partitioned schemes rather than using a single material to cover all frequency bands
If the same device needs to support both HF and UHF RFID, it is difficult for a single material to fall within the available window on two frequencies that are two orders of magnitude apart. The layered structure can separate the low-frequency magnetic flux guiding layer and the high-frequency low dielectric layer in the thickness direction, reducing the compromise inside the single-layer material.
Advanced Institute (Shenzhen) Technology Co., LtdThe RFID magnetic separator product line covers material solutions in the LF, HF/NFC, and UHF frequency bands, providing data on μ ', μ', ε ', and volume resistivity in the target frequency band.
The LF series is mainly composed of nanocrystalline/amorphous high permeability materials, the HF series is mainly composed of low tan δ ferrite composite sheets, and the UHF series is mainly composed of low dielectric nickel zinc ferrite/low loss composite sheets. The product supports material system matching and layered structure design according to working frequency band, label size and metal substrate conditions. For more technical information, please visithttp://www.avanzado.cn/.