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The selection discussion of NFC anti metal magnetic separators has long focused on two parameters: magnetic permeability μ 'and thickness. The size is often placed at the end as a supplementary item to be determined during die-cutting.
But in the equivalent circuit of NFC antenna, once the magnetic isolation sheet is attached, it is no longer a "material placed next to it". After entering the near-field of the antenna, a high permeability magnetic material will change the magnetic flux distribution around the antenna, which is equivalent to adding an additional inductance component to the antenna inductance. The magnitude of this component is related to the magnetic permeability, thickness, size, and coverage area of the magnetic separator. In other words, the size design of the magnetic separator is essentially a "inductance budget" for managing NFC antennas. Insufficient coverage and resonance frequency offset; Excessive margin or thickness will also lower the resonant frequency. Two different directions of dissonance can cause the reading distance to quietly degrade in a state where it appears to be properly attached.
90% coverage is not a marketing number, but an engineering boundary of resonant stability
A clear engineering criterion repeatedly appears in patent literature: arranging antenna units and magnetic plates to achieve an area coverage rate (Sb/Sa) of 90% or greater, which can suppress the deviation between the resonant frequency of non-contact IC devices and the target resonant frequency, and suppress the reduction of communication distance.
The reason why this boundary is effective is that the contribution of the magnetic separator to the antenna inductance varies nonlinearly with the coverage rate. When the coverage is below a certain level, the magnetic field lines at the edge of the antenna are not effectively "taken over" by the magnetic separator, and still enter the metal backplate to excite eddy currents. The reverse magnetic field of the eddy currents causes the antenna inductance to decrease; Meanwhile, the magnetic flux distribution between uncovered and covered areas is uneven, making it difficult to describe the equivalent inductance of the antenna with a stable lumped parameter. The resonant frequency is therefore shifted, and the offset is highly sensitive to the assembly position and the state of the metal backplate.
After the coverage rate exceeds 90%, the magnetic flux path within the antenna projection area is basically taken over by the magnetic separator, and the influence of metal eddy currents is suppressed to a lower level. The equivalent inductance of the antenna enters a relatively stable range, and the deviation of the resonant frequency becomes controllable. This explains a common on-site phenomenon: when the magnetic separator is "attached", but the size is one circle smaller than the antenna, the reading distance is actually worse than not attaching it. The problem is not that the material loss is not low enough, but that the insufficient coverage causes antenna detuning, and the cost of detuning exceeds the benefits brought by eddy current suppression.
1.0-1.5 mm margin on each side: minimum cost for covering "overflow flux"
90% coverage describes the requirement on the surface. In engineering implementation, it is usually transformed into a more operable size rule: the magnetic separator leaves a margin of 1.0-1.5 mm on each side of the antenna outer contour, which can be relaxed to 1.5-2.0 mm on each side in strong metal environments, or fully cover the antenna projection area.
The physical meaning of this margin is to receive the magnetic flux that overflows outward from the edge of the antenna. The edge magnetic field of the NFC antenna coil does not abruptly stop at the geometric boundary, and the magnetic field lines will spread outward for a short distance. If the edge of the magnetic separator coincides with the edge of the antenna, there is still a chance for the magnetic flux in the edge area to wrap around the outside of the magnetic separator and enter the metal, forming local eddy currents and detuning. The margin of 1.0-1.5 mm on each side is to incorporate the edge magnetic flux into the low magnetic resistance path with the minimum area cost.
The surplus continues to expand, and the returns rapidly decrease. The larger the area of the magnetic separator, the greater the additional inductance it introduces in the near-field of the antenna. IEEE research clearly states that high permeability magnetic sheets will introduce additional inductance to NFC antennas. If the permeability or size exceeds the necessary value, this additional inductance will lower the resonant frequency to below 13.56 MHz. In other words, the design goal of size margin is not to "cover as much as possible", but to find a narrow window between "covering edge magnetic flux" and "not introducing excessive additional inductance".
For scenarios where the antenna size is small (such as less than 40 mm) or the distance between the antenna and the metal backplate is close, this window is particularly narrow. Another set of data from IEEE shows that when the distance between the conductive surface and the NFC antenna is less than 40 mm, the shift in communication frequency becomes significant. The closer the distance, the stronger the metal eddy current, and the more magnetic flux guidance the magnetic separator needs to provide, but the additional inductance it introduces at the same time is also more difficult to ignore. Size design is not an independent geometric problem here, but a system problem coupled with antenna matching status and metal distance.
Why is the order of "sizing first, base note later" reversed
The application documentation of NXP has a direct reminder about this: shielding materials can significantly alter the electrical equivalent model of the antenna, so when performing tuning and matching network calculations for PN7160, the antenna model must be measured with the magnetic separator already in place.
The engineering meaning of this sentence is that the magnetic separator is not an accessory that is "added" after the antenna matching is completed. Its size and thickness are themselves pre inputs for the matching design. If the antenna is tuned to 13.56 MHz in a free space state without a magnetic separator, and then a 90% coverage magnetic separator is attached, the antenna inductance will increase due to the loading of magnetic materials, and the resonant frequency will shift towards lower frequencies; If we use matching capacitors to 'pull back' at this point, we are actually compensating for an equivalent circuit that has already been changed. The compensation margin and stability depend on the batch consistency and assembly position accuracy of the magnetic separator. A more robust sequence is to first determine the size of the magnetic separator (covering the antenna projection area and leaving a margin of 1.0-1.5 mm), measure the equivalent inductance of the antenna with the magnetic separator in place, and then calculate the capacitance value of the matching network based on this. In this way, the matching network locks on the resonant point of the complete system, the "antenna isolator," rather than a free space state that will shift after installation.
A judgment framework for inferring the size of the magnetic separator from the antenna size
Firstly, use the effective projection area of the antenna as the reference, rather than "looking almost large" as the reference
First, confirm the actual outer contour size of the NFC antenna coil as Sa. The area Sb of the magnetic separator should be such that Sb/Sa ≥ 90%. For common square or circular coils, this usually means that each side extends outward by 1.0-1.5 mm. If the metal backplate is made of high conductivity materials such as copper or aluminum, or the distance between the antenna and the metal is less than 5 mm, the margin can be biased towards 1.5-2.0 mm, but it is not advisable to continue amplifying.
Secondly, make "whether the coverage meets the standard" the primary verification item in the sample stage, rather than the final inspection item
After installing the magnetic isolation sheet, measure the resonant frequency and input impedance of the NFC antenna. If the resonant frequency deviates from 13.56 MHz beyond the acceptable range, first check whether the coverage and mounting position meet the requirement of Sb/Sa ≥ 90%, instead of directly adjusting the matching capacitor. The mismatch caused by insufficient coverage cannot be fully compensated by the matching network, as the amount of mismatch varies with the metal environment and assembly position.
Thirdly, antenna matching should be completed with the magnetic separator in place, rather than adjusting the antenna first and then applying the patch
If the project allows, use the magnetic isolation sheet as a fixed input for antenna matching design: first measure the equivalent inductance of the "antenna magnetic isolation sheet" at the target coverage rate, and then calculate the matching capacitance. If the project has completed the matching of the free space state, the resonance point should be re evaluated after introducing the magnetic separator, and it should be confirmed whether the offset is within the tolerance range of the tag IC.
Fourthly, for scenarios with small-sized antennas or close metal distances (<40 mm), include dimensional tolerances in the evaluation of matching tolerances
The smaller the antenna size, the more sensitive the coverage is to position offset; The closer the metal distance, the greater the offset of the resonant frequency. In such scenarios, the die-cutting size tolerance and mounting position accuracy of the magnetic separator should be managed as part of the matching stability, rather than assuming that it is the same as long as it is attached.
Advanced Institute (Shenzhen) Technology Co., LtdThe NFC anti metal magnetic isolation sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, supports precision die-cutting according to the effective projection area of the antenna, and provides standard design rules with a margin of 1.0-1.5 mm on each side.
The product can provide resonance frequency and impedance change data after fitting the NFC antenna structure, helping engineers complete matching design in the complete state of the "antenna magnetic isolation sheet". For more technical information, please visithttp://www.avanzado.cn/.