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A common on-site issue with RFID systems is that when the tag is placed on the reader antenna, the reading distance is actually worse than when pulled apart by a few centimeters. The first reaction of engineers is usually 'the tag chip is saturated' - this judgment does hold in UHF passive tags, where the rectifier circuit of the chip undergoes nonlinear changes in impedance characteristics when the input power exceeds a certain threshold.
But the problem is not always on the chip. When absorbing materials are introduced between tags and metals, or arranged around readers to suppress multipath reflections, the absorbing plate itself also has a "power capacity" boundary. Beyond this boundary, the absorbing wave plate is no longer a passive, parameter fixed magnetic flux guiding layer, but will become an unpredictable detuning source that varies with field strength.
Saturation of absorption plate: Magnetic permeability is not a constant independent of field strength
The core function of a waveplate relies on the magnetic permeability of soft magnetic fillers such as ferrite, carbonyl iron powder, FeSiAl, etc. Magnetic permeability describes the ability of a material to respond to an external magnetic field, but this ability has an upper limit: when the external magnetic field strength increases to a certain extent, the magnetization of the material approaches saturation magnetization, and the magnetic permeability begins to decrease. This phenomenon is called 'partial saturation'.
The difference between small signal calibration and high-power actual working conditions
In conventional RFID range testing, the magnetic field strength at the tag is usually in the range of a few A/m to tens of A/m, far below the saturation field of most soft magnetic materials. The absorbing wave plate operates in the linear region, with stable magnetic permeability and parameters consistent with the specifications. But when the tag is placed close to the reader antenna or when the reader operates at high power, the magnetic field strength at the tag may increase by one to two orders of magnitude and enter the nonlinear region of the absorbing plate. In the application notes of the 13.56 MHz RFID antenna matching by Omicron Lab, it is clearly pointed out that this effect occurs: ferrite materials "exhibit nonlinear effects under high magnetic field strength, especially when partially saturated", and the matching conditions of the resonant system will shift. The core issue of this note is whether the antenna matching calibrated with a small signal (0 dBm) still holds true under high-power signals. The answer is - not necessarily for systems containing ferrites.
Two failure paths after saturation: absorption degradation and antenna detuning
After the absorption plate enters partial saturation, performance degradation occurs along two paths simultaneously.
The first issue is the deterioration of absorption performance
The decrease in the real part of the magnetic permeability μ 'means that the ability of the absorbing plate to guide the magnetic flux is weakened, and the magnetic field lines that were originally confined in the low magnetic resistance path begin to "overflow" towards the metal back plate or surrounding environment. The variation of the imaginary part of magnetic permeability is more complex: in some material systems, partial saturation may cause the loss tangent to rise first and then fall, and the position and depth of the absorption peak to shift simultaneously. The reflection loss curve marked on the specification sheet of the absorption plate is measured under small signal conditions, and the actual absorption performance under high field strength may significantly deviate from this curve.
The second issue is antenna mismatch
The absorbing plate is attached near the tag antenna or in the near-field region with the reader/writer antenna, and its magnetic permeability directly participates in the equivalent inductance of the antenna. The decrease in μ 'means that the equivalent inductance of the antenna decreases and the resonant frequency shifts towards higher frequencies. If the antenna is tuned to the target frequency (such as 13.56 MHz or 915 MHz) under small signals, the frequency offset under high field strength may cause the antenna to deviate from the conjugate matching point of the chip, resulting in decreased backscatter efficiency and shortened read distance.
The superposition effect of two paths is the phenomenon observed on site, where 'the closer you get, the harder it is to read'. When the tag is close to the reader/writer, the field strength increases, the absorbing plate saturates, the μ 'decreases, the antenna becomes detuned, and the power obtained by the chip is actually lower than when the distance is extended. IEEE's research on coupled UHF tag nonlinearity observed a similar phenomenon: when the reader power increases or the tag is pulled closer, some tags "suddenly stop responding". This study attributes the reason to the combined effect of coupling between tag antennas and chip nonlinearity, but the saturation of the absorbing plate provides an additional and often overlooked explanatory path for this phenomenon.
Why can't the regular specification sheet expose this issue
The specifications of the waveplate are almost entirely measured under small signal conditions. Reflection loss is measured using a vector network analyzer at low power, magnetic permeability is extracted using an impedance analyzer at small signals, and antenna matching is calibrated using small signal S11. These data describe the behavior of the 'linear region'.
But the power of RFID readers can be very high under near-field conditions. The output power of a fixed UHF reader can reach 30 dBm (1W) or even higher, and the antenna gain further amplifies the field strength. The magnetic field strength in the near-field region may be several orders of magnitude higher than under small signal testing conditions. Within this field strength range, the nonlinear behavior of the absorbing wave plate begins to manifest, and the specification does not provide parameter curves at high field strengths. That's why absorption plates that meet the specifications may perform poorly in actual high-power near-field scenarios. The problem is not that the material itself is unqualified, but that the evaluation conditions do not match the service conditions.
Selection and judgment of RFID absorbing materials based on "power capacity x working field strength"
Firstly, confirm the actual field strength range in the application scenario, rather than assuming that "small signal conditions represent everything"
If the tag or absorbing plate is placed in the near-field region of the reader/writer antenna (distance less than λ/2 π), or if the reader/writer operates at near maximum power, the field strength may enter the nonlinear region of the absorbing plate. When selecting, the magnetic field strength at the target location should be estimated, and the magnetic permeability retention rate of the absorbing plate at that field strength should be confirmed with the supplier.
Secondly, suppliers are required to provide magnetic permeability or reflection loss data under high field strength, rather than just providing small signal parameters
The reference testing method is to measure the antenna matching or reflection loss with absorbing wave plates at different excitation power levels (such as 0 dBm and 20 dBm), and observe whether the parameters undergo significant shifts. The application notes of Omicron Lab demonstrate the feasibility of this method: by comparing the S11 curves under small and large signals, the power related nonlinearity of ferrite can be quantified.
Thirdly, in system design, avoid operating the absorbing wave plate at the position with the highest field strength
If the function of the absorption plate is to suppress multipath reflections around the reader or eddy currents on the metal backplate, it should be placed at a certain distance from the antenna radiator to reduce the field strength that the absorption plate can withstand. The power capacity boundary of the absorbing wave plate can be "bypassed" through layout optimization, rather than forcibly using thicker materials to harden it.
Fourthly, during the sample stage, verify the reading distance using actual reader power and actual label distance, rather than extrapolating data from a small signal network analyzer
The reflection loss and magnetic permeability on the specification sheet are described in the linear region, and the actual performance under high-power near-field needs to be confirmed through system level testing. When testing, multiple distance points from far to near should be covered to observe whether there is a non monotonic decrease in the reading distance at a certain distance - this is a typical signal of wave plate saturation.
Advanced Institute (Shenzhen) Technology Co., LtdThe RFID absorbing material product line covers the UHF and HF frequency bands, providing magnetic permeability and reflection loss data under small signal and high field strength conditions, and supporting power capacity matching evaluation based on customer's actual reader power and tag antenna distance.
The product is suitable for RFID scenarios that require stable absorption and antenna matching in high-power near-field environments. For more technical information, please visithttp://www.avanzado.cn/.