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The most prominent number in the specification book of RFID absorbing materials is usually the reflection loss: it reaches -15 dB at 915 MHz and -20 dB at 2.45 GHz, and the effective bandwidth covers how many MHz. The engineer selected the data with a straightforward logic: the stronger the absorption, the more advantageous it is for improving the read distance in metal environments.
The results of sample testing sometimes do not match this logic. Attach a waveplate with a reflection loss of -18 dB, and the label reading distance is actually shorter than attaching a waveplate with a reflection loss of -12 dB. Retesting the reflection loss of the absorbing wave plate, the data is correct. The problem lies in another parameter: the dielectric constant of the absorbing wave plate.
Wave absorbing plate is not a "transparent absorption layer", it is the medium in the near-field of the antenna
The RFID tag antenna is a resonant structure, and its resonant frequency is determined by the geometric dimensions of the antenna and the surrounding dielectric environment. The absorbing plate is attached near the antenna and participates in resonance through two parameters:
Magnetic permeability μ '
Changing the magnetic flux distribution in the near-field of the antenna affects the equivalent inductance. For HF frequency band labels mainly based on magnetic field coupling, this parameter is dominant; For UHF band tags, the influence of magnetic permeability is relatively small.
Dielectric constant ε′
Changing the electric field distribution around the antenna is equivalent to adding a component to the antenna capacitance. UHF tag antennas mainly emit electric field radiation, and the loading effect of ε 'is more significant than that of μ'.
Wave absorbing plates are usually composed of a composite of magnetic fillers and polymer matrices, with the polymer matrix having an ε 'in the range of 3-10 and the magnetic fillers possibly having a higher ε'. The effective ε 'after composite depends on the filler ratio and interface polarization, and may be significantly higher than the linear mixing value of the two components. This ε 'is not a part of' absorption ', it is a change in the resonant conditions of the antenna caused by the absorption plate acting as a dielectric layer.
The causal chain between resonance shift and reading distance decrease when ε 'is high
UHF tag antennas are tuned to the target frequency (such as 915 MHz) in free space or on specific substrates. After attaching the absorbing wave plate, its ε 'increases the equivalent capacitance around the antenna, causing the resonant frequency to shift towards lower frequencies.
Three variables that determine the offset
The offset depends on three variables: the ε 'of the absorbing plate, the thickness of the absorbing plate, and the distance between the absorbing plate and the antenna radiator. The higher the ε ', the greater the offset; The thicker the thickness, the greater the offset; The closer the distance, the greater the offset. For the typical scenario where a thin absorber plate (0.1-0.3 mm) is attached to the back of a tag antenna, the resonant frequency shift may reach tens of MHz as the ε 'increases from 5 to 15.
If the tag antenna has been tuned to 915 MHz before placement, the offset resonant frequency may fall to the edge or even beyond the 860-960 MHz frequency band. The input impedance of the antenna in the target frequency band deviates from the conjugate matching point of the chip, resulting in a decrease in backscatter efficiency and a shortened reading distance. At this point, the reflection loss of the absorbing plate may still be good - it does absorb electromagnetic waves, but it also "deflects" the tag antenna itself.
Why is the specification sheet not labeled with ε′: industry habits and testing separation
The specification sheet of the absorbing wave plate usually indicates the reflection loss, thickness, operating frequency band, and flame retardant level, and ε 'rarely appears. There are two reasons.
Firstly, the industry tends to evaluate absorbing plates as "shielding/absorbing materials"
Reflection loss describes the ability of a material to attenuate incident electromagnetic waves, which is a key indicator in shielding and darkroom scenarios. But in the RFID scenario, the function of the absorbing plate is not to shield, but to change the field distribution near the antenna - its dielectric loading effect is equally important as the absorption effect, but it is not within the traditional evaluation system.
Secondly, the testing of ε 'is separate from the testing of reflection loss
Reflection loss is measured using bow or waveguide methods, and ε 'is inverted from S-parameters using coaxial or waveguide methods. Both can be completed on the same vector network analyzer, but the report usually only presents the reflection loss curve, and the ε 'data does not enter the specification sheet.
This means that if engineers only look at the selection of reflection loss, their ability to predict what the antenna will look like after the absorption plate is attached is incomplete.
Strong absorption and long reading distance are not the same thing
Reflection loss describes the attenuation ability of an absorbing wave plate to external incident electromagnetic waves. In the RFID scenario, the absorbing plate faces two types of electromagnetic waves: the incident wave from the reader and the wave radiated/scattered by the tag antenna itself. The former needs to be absorbed to reduce interference from metal reflections, while the latter needs to be 'let go' to maintain the radiation efficiency of the label.
The absorption mechanism of a waveplate does not distinguish between these two types of waves. A strong absorbing wave plate may absorb external interference while also absorbing the energy radiated by the tag antenna itself, reducing the radiation efficiency of the tag. Meanwhile, its ε 'will alter the resonance conditions of the antenna. The direction of the change in read distance may not be determined by the combination of two effects, but rather by the net effect between absorption gain and loading loss. That's why 'the reading distance of the absorption plate with a reflection loss of -18 dB is not as good as that of the absorption plate with a reflection loss of -12 dB' is not an anomaly, but rather the result of dielectric loading and radiation efficiency losses exceeding the absorption gain under specific conditions.
Selection and judgment of RFID absorbing materials based on "combination resonance"
Firstly, the supplier is required to provide the ε 'data of the absorbing plate in the target frequency band, rather than just providing the reflection loss
ε 'is a necessary parameter for predicting dielectric loading effects. If the supplier can only provide the reflection loss curve, it is impossible to predict the direction and amplitude of the antenna resonance offset after the absorption plate is mounted.
Secondly, during the sample stage, use a combination of "tag antenna absorbing wave plates" to measure the resonant frequency and reading distance, rather than just measuring the reflection loss of the absorbing wave plates
Testing the absorption plate alone cannot reflect its changes in antenna resonance conditions. When selecting, the absorber plate should be attached to the actual tag antenna, and the combined resonant frequency, input impedance, and read distance should be measured to confirm whether the offset is within the tolerance range of the tag chip.
Thirdly, make a joint trade-off between the thickness of the absorbing plate and the value of ε ', rather than pursuing "thin" or "low ε'" separately
The loading effect of thin wave absorbing plates is relatively small, but the absorption depth may be insufficient; The resonance offset of the low ε 'absorbing wave plate is small, but the absorption bandwidth may be narrow. When selecting, the acceptable resonance offset should be determined based on the matching tolerance of the tag antenna, and then the optimal absorption performance scheme should be selected within this constraint.
Fourthly, in the tag antenna design stage, the dielectric loading of the absorbing wave plate should be included in the matching calculation, rather than adjusting the antenna first and then mounting it
If the project allows, the matching network of the tag antenna should be tuned in the combination state of the "antenna absorbing plate", so that the resonant frequency still falls within the target frequency band after being mounted. In this way, the ε 'of the absorbing wave plate changes from an "interference variable" to a "design input", resulting in better stability of the reading distance.
Advanced Institute (Shenzhen) Technology Co., LtdThe RFID absorbing material product line covers the UHF and 2.4 GHz frequency bands, providing reflection loss curves and ε′ data at the target frequency band, as well as resonance frequency and reading range evaluation data for the combination of "tag antenna absorbing plate".
The product supports joint matching of ε 'and thickness based on the matching tolerance of the tag antenna, helping engineers find an acceptable balance between absorption performance and dielectric loading. For more technical information, please visithttp://www.avanzado.cn/.