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One of the most familiar issues for hardware engineers is the sharp drop in reading distance when NFC tags are placed close to the metal backplate. The common coping action is to find a magnetic separator with higher magnetic permeability and stick it on. The reading distance has changed from 50 to 150 and from 150 to 300, but it has not recovered proportionally and sometimes even becomes shorter.
The problem is not that the magnetic permeability is not high enough, but that a key parameter equivalent to μ 'was missed during the selection process: the imaginary part of the magnetic permeability, μ'.
At 13.56 MHz, μ 'is responsible for' leading 'and μ' is responsible for 'consuming'
NFC operates in the near-field magnetic field coupling region at 13.56 MHz. The core function of the magnetic separator is to insert a low magnetic resistance path between the antenna and the metal backplate, pulling the magnetic field lines away from the metal surface to avoid the reverse magnetic field generated by metal induced eddy currents canceling out the antenna magnetic flux.
In this pathway, the roles played by μ 'and μ' are completely opposite. μ 'represents the energy storage and magnetic conductivity of the material, with higher values indicating more magnetic flux that can be guided; μ "represents magnetic loss, which is the part of the magnetic field energy entering the material that is converted into heat. The research of ShanghaiTech University has a clear statement on this: NFC magnetic sheets need to have a large μ 'for magnetic conductivity at 13.56 MHz and below, and at this time, μ' 'is basically zero' to avoid dissipating useful magnetic fields in the form of eddy current losses.
The magnetic field energy of NFC antenna itself is very limited. When the magnetic flux emitted by the reader attenuates through space and reaches the mobile phone end, the energy available to excite antenna resonance is already very small. If the μ "of the magnetic separator is too high, this energy will be directly consumed by the material during the" guided "process, reducing the energy obtained by the antenna and naturally shortening the reading distance. At this point, even if the μ 'is very high, the actual effect may not be as good as a piece of material with medium μ' but extremely low μ ''.
Magnetic isolation and absorption are two different things in the NFC frequency band
In engineering practice, the terms "magnetic isolation plate" and "absorption plate" are often used interchangeably, but in the NFC frequency band, their loss mechanism requirements are opposite. The magnetic separator pursues high μ ', low μ' ', and low tan δ, with the goal of' magnetic conductivity without energy consumption '; Wave absorbing plates may pursue higher μ "or dielectric losses for converting electromagnetic energy into heat.
Placing a piece of absorbing material designed for absorption in the GHz frequency band between the NFC antenna and metal may still result in a significant loss of μ "around 13.56 MHz. This loss will be directly loaded onto the NFC antenna, resulting in an increase in equivalent series resistance and a decrease in Q value. Conversely, placing a magnetic isolation sheet optimized for NFC low loss magnetic conductivity into a millimeter wave cavity may not provide sufficient broadband absorption, and high μ 'may also alter the antenna's matching conditions. Therefore, the first step in selecting NFC anti metal materials is not to compare "whose magnetic permeability is higher", but to confirm whether the material's loss level at 13.56 MHz is low enough. The data of μ "and tan δ can better predict whether the read distance can be restored than the absolute value of μ '.
Low loss engineering boundary: from tan δ ≤ 0.05 to Q ≈ 28
Industry data provides an actionable quantitative boundary for 'low loss'.
The NFC shielding scheme disclosed in the patent literature shows that at a working frequency of 13.56 MHz, the μ 'range of Ni Zn ferrite sheet is 100-250, and tan δ (=μ "/μ') is required0.05 or lessIn the same scheme, the μ 'of the metal polymer sheet is 30-70, and tan δ is also required to be ≤ 0.05. This means that in the NFC frequency band, tan δ 0.05 is an engineering validated boundary line, beyond which material loss begins to erode the NFC signal budget and cannot be ignored.
Product level reference for TDK IFQ06
At 13.56 MHz, the corresponding Q value (μ '/μ') is approximately 28, with μ '≈ 55 and μ' ≈ 2. This Q-value level is already able to provide an effective bypass for magnetic flux without significantly consuming NFC magnetic field energy. The μ 'of IFQ06 is not high - only 55, far lower than the hundreds or even thousands of nominal values of many wireless charging separators. But this set of data indicates that,NFC magnetic separators do not require extremely high μ ', they require sufficiently low μ' 'The value of 55 μ 'is sufficient to provide effective magnetic flux guidance, but if μ' 'increases from 2 to 20, the Q value will drop to less than 3, and the antenna will still not function properly.
Academic research also supports this boundary. The study of Co replacing NiCuZn ferrite shows that at 13.56 MHz, the μ 'can reach over 150, while the μ' remains below 2. The project data from the Cloud High Tech Expo shows that the μ 'of the ferrite substrate material used for NFC antennas is greater than 175@13.56MHz ， μ "is less than 3.5@13.56MHz The batch production level can reach a level of μ ′>150 and μ ″<2.5. The common point of these data is that the low loss goal of NFC magnetic separators is to suppress μ 'at the level of single digits or even lower while maintaining the availability of μ'.
Two material pathways: "intrinsic low loss" of ferrite and "process controlled loss" of nanocrystals
There are mainly two technical paths to achieve this low loss boundary.
Ferrite pathway: natural low loss
The resistivity of NiZn ferrite is extremely high, and eddy current losses can be ignored at 13.56 MHz. The main sources of μ "are hysteresis losses and residual losses, which can be reduced to lower levels through formulation and sintering processes. The Ni Zn ferrite sheets with μ'100-250 and tan δ ≤ 0.05 in the aforementioned patents, as well as the μ "≈ 2 in IFQ06, are all manifestations of this pathway. The limitation of ferrites is that the upper limit of μ 'is constrained by the intrinsic properties of the material, typically not exceeding 250-300 at 13.56 MHz. The adjustable magnetic flux guiding ability is limited, and their hard and brittle characteristics limit their use in curved surface mounting scenarios.
Nano crystal pathway: process loss control
The μ 'of nanocrystalline strip can be much higher than that of ferrite, and the Bs is also higher, but its resistivity is extremely low. Untreated continuous strip has significant eddy current loss at 13.56 MHz, and the μ' is high. Directly using it for NFC will cause Q value collapse. Therefore, the prerequisite for nanocrystals to enter the NFC scene is not that "μ 'is high enough", but that "the eddy current is first pressed down". The engineering method is magnetic fragmentation: dividing the continuous strip into small, mutually insulating fragments, cutting off the closed path of eddy currents over a large area. The effective μ 'of the finished product after magnetic fragmentation usually falls within the range of 150-800, and the control level of μ' depends on the fragment size, gap filling method, and layer design. The method for preparing nanocrystalline magnetic separators disclosed in the patent involves introducing transverse cracks through transverse stripe crushing rollers and forming scale like fragments through grid patterned crushing rollers, with the aim of reducing eddy current losses while minimizing magnetic field line losses.
The choice between the two paths depends on the constraints of the scene. If the space allows for flat mounting and the demand for μ 'is within 250, the intrinsic low loss and mature cost structure of ferrite are more reliable starting points. If it is necessary to mount on curved surfaces or narrow borders, or to require a higher μ 'to cope with thicker metal backboards, the flexibility advantage and μ' space of nanocrystals become the decisive factors - but the prerequisite is that the supplier's fragmentation process can compress the μ 'at 13.56 MHz to a Q-value usable level.
Selection judgment based on μ "and Q value
Firstly, use μ "and tan δ at 13.56 MHz as the first screening criteria, rather than μ '
Require suppliers to provide data on the target frequency points of μ ', μ', and tan δ, rather than just providing magnetic permeability at 100 kHz or 1 MHz. The engineering boundary that can be referred to is tan δ ≤ 0.05, and μ ″ should be controlled at the single digit level as much as possible. A scheme with μ '=200 and μ'=4 may be superior to the scheme with μ '=600 and μ'=40 in NFC range recovery. If the supplier cannot provide loss data at 13.56 MHz, its applicability to NFC scenarios cannot be determined.
Secondly, distinguish between "tape μ '" and "effective μ' and Q values after the finished product is attached to the antenna"
The initial μ 'of nanocrystalline magnetic separators may be as high as tens of thousands, but the effective μ' of the finished product after fragmentation and lamination is usually only 150-800. The inductance design of NFC antennas relies on the actual loading effect after the finished product is attached to the back of the antenna. When selecting, the supplier should be required to provide the finished product status, effective μ 'and Q value change data after fitting the NFC antenna structure, rather than the initial values on the tape specification sheet.
Third, according to the conditions of the metal backplane and the mounting space, make a system level trade-off between ferrite and nanocrystals
When the metal backplate is thin, the antenna area is sufficient, and the space allows for flat mounting, the natural low μ "of ferrite is a more robust starting point. When the metal backplate is thick, the antenna area is limited, or curved surface mounting is required, the μ 'space and flexibility advantages of nanocrystals become necessary conditions - but it must be confirmed that their magnetic fragmentation process compresses μ "to a Q-value usable level at 13.56 MHz. When selecting, one should inquire about the process correspondence between fragment size, gap filling method, and finished product μ ", rather than just looking at the final nominal value.
Advanced Institute (Shenzhen) Technology Co., LtdThe NFC anti metal magnetic isolation sheet covers two routes: ferrite composite sheet and nanocrystalline composite sheet. Ferrite composite sheets can provide frequency band data of μ 'and μ "at 13.56 MHz, suitable for flat mounting scenarios; The nanocrystalline composite sheet controls eddy current loss through magnetic fragmentation and insulation treatment, and the effective μ 'of the finished product can be adjusted within the range of 150-800, supporting 0.03-0.2 mm thickness and curved surface mounting.
The product can provide Q value change and reading distance recovery data after fitting the NFC antenna structure, and match the material system according to the metal backplane conditions and space constraints. For more technical information, please visithttp://www.avanzado.cn/.