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In the product specifications of solid-state sheet absorbing materials, the peak reflection loss is often marked as -30 dB or even lower. But after engineers receive the physical object and install it on the actual equipment, the measured results often only range from -10 dB to -15 dB. This difference does not necessarily mean that there is a problem with the material itself - more likely, there is a systematic difference between the testing conditions in the specifications and the actual application scenarios.
Understanding the sources of these differences is more valuable in engineering than simply pursuing higher nominal values.
The testing method determines the meaning of the data: bow method, waveguide method, and arch method cannot be mixed
There are at least three commonly used methods in the industry for measuring the reflectivity of solid-state sheet absorbing materials: bow method, waveguide method, and arch method. They are each applicable to different frequency ranges and sample shapes, and the physical meanings of the measured data are not equivalent.
Arch MethodAccording to GJB 2038A-2011 standard, it is suitable for measuring the reflectivity of flat absorbing materials, with a frequency range covering 1-40 GHz. The testing requirement is that the surface flatness of the tested sample should not exceed 0.1 mm. This means that if the sheet has warping, uneven thickness, or rough surface, the test results themselves have a high degree of uncertainty. The advantage of the bow method is that it can be tested under free space conditions, which is closer to the electromagnetic wave incidence state in practical applications; The limitation lies in the large testing space and sample size required in the low frequency band, which limits the measurement accuracy below 1 GHz.
Waveguide methodPlace the sample in a standard waveguide segment, measure its complex dielectric constant and complex magnetic permeability, and then calculate the reflectivity through transmission line theory. The waveguide method has high controllability of testing conditions and good data repeatability, but it tests the electromagnetic parameters of materials in specific waveguide modes, rather than the reflection characteristics in free space. When the material is mounted on actual equipment, the polarization state, incident angle, and boundary conditions of the incident wave are different from the waveguide testing environment. Directly applying the reflectivity data measured by the waveguide method may lead to misjudgment.
Arch Reflection MethodSimilar to the bow method, but with stricter control over testing distance and angle, it is usually used in scenarios where higher reflectivity accuracy is required. If there are differences in the calibration standards, background noise treatment methods and metal backplane conditions adopted by different laboratories, the test results of the same material in different laboratories may have a deviation of several dB.
The following table summarizes the key differences among three commonly used testing methods:
GB/T 32596-2016 "General Specification for Electromagnetic Shielding Absorbing Plates" specifies the terminology, classification, technical requirements, and testing methods for solid-state sheet-like absorbing materials in the frequency range of 10 MHz to 40 GHz. Although this standard provides a unified testing framework for the industry, it does not mandate that all suppliers must use the same testing method or sample preparation conditions. This means that reflection loss data from different suppliers, even if the frequency range is the same, may not necessarily be directly compared horizontally.
Invisible variables in sample preparation: thickness tolerance and surface condition
Even with the same testing method, differences in sample preparation can still cause significant data bias.
Thickness is one of the most sensitive parameters that affect the peak frequency and intensity of reflection loss. According to the quarter wavelength matching principle, the matching thickness is inversely proportional to the target frequency. If there is a deviation of ± 10% between the actual thickness of the sheet and the nominal thickness, the matching frequency may shift by several hundred MHz, and the reflection loss value measured at a fixed frequency point may differ by several dB. For sheet absorbing materials with a thickness of 1-2 mm, a thickness tolerance of ± 0.1 mm is sufficient to cause the peak reflection loss to deviate from the target frequency band.
Surface flatness is another easily overlooked factor. The bow method requires the surface flatness of the sample to be no more than 0.1 mm. Solid sheet-like absorbing materials may have slight warping or uneven thickness on the surface after molding or vulcanization. If the sample is not flattened during testing, the reflection phase of the incident wave on the surface of the sample will be inconsistent, and the measured reflectivity will be lower - that is, it may appear to "absorb better", but the actual reason is surface scattering rather than material absorption.
The condition of the metal backplate is equally critical. Reflectance testing usually uses a metal backplate as a total reflection reference. If the adhesion between the backplate and the sample is not tight and there is an air gap, electromagnetic waves will generate additional reflection and interference in the gap, resulting in distortion of the test data. Even if the thickness of the air gap is only a few tens of micrometers, it will significantly change the reflection phase in the GHz frequency band.
Data validation strategy in engineering selection
Engineers can verify and judge the reflection loss data provided by suppliers from the following three dimensions.
Firstly, confirm the testing method and sample conditions in the test report
Require suppliers to provide complete test reports, rather than just one reflection loss curve. The report should include: test standard number, test frequency range, sample thickness and tolerance, sample surface flatness data, metal backplate type and bonding method. If the supplier is unable to provide this information, their nominal reflection loss value can only be used as a qualitative reference and cannot be used as a quantitative selection basis.
Secondly, prioritize suppliers who provide data on high temperatures or actual working conditions
The reflection loss data in most specifications are measured at room temperature (23 ° C) and in a dry environment. For scenarios such as in car electronics and base station amplifiers where the actual operating temperature may reach 85 ° C or above, the electromagnetic parameters of materials at high temperatures may differ significantly from those at room temperature. After high-temperature treatment at 450 ° C/1h, the electromagnetic parameters of the high-temperature resistant absorbent/polyimide composite material with oriented sheet-like structure showed little change when the doping ratio was 50% -60%. However, when the doping ratio increased to 70%, the real part of the dielectric constant decreased from 36.91-52.74 to 18.29-31.52, and the change amplitude significantly increased. This set of data indicates that the high-temperature stability of materials is closely related to the doping ratio of fillers, and the room temperature data cannot be directly extrapolated to high-temperature scenarios.
Thirdly, pay attention to whether the supplier provides consistent data across multiple batches
The consistency of the performance of solid-state sheet-like absorbing materials is affected by the batch stability of the preparation process. In processes such as directional freeze-drying, magnetic field orientation, or rolling forming, small fluctuations in temperature gradient, magnetic field uniformity, and rolling pressure can all cause changes in the orientation of sheet-like particles, thereby affecting reflection loss. If the supplier can provide continuous reflection loss data for multiple batches, and the peak deviation between batches is controlled within a reasonable range (such as ± 2 dB), it indicates that their process control capability is relatively reliable.
Advanced Institute (Shenzhen) Technology Co., LtdA standardized process has been established for the preparation and testing of solid-state sheet-like absorbing materials, covering the 1-18 GHz frequency band and supporting customized design of sheet-like particle orientation angle and filling ratio.
The product testing report includes complete information such as testing methods, sample thickness, and surface condition, and can provide electromagnetic parameter data under normal and high temperature conditions. The matrix system can be made of high-temperature resistant polyimide or epoxy resin, and the temperature resistance range can be adjusted within the range of -40 ° C to 200 ° C according to the matrix selection. For more technical information, please visithttp://www.avanzado.cn/.