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Introduction
From the research and development of electromagnetic wave absorbing materials, incoming material inspection to finished product acceptance, the testing of complex magnetic permeability and complex dielectric constant runs through the entire process. This article starts from three core testing methods, namely coaxial method, waveguide method, and bow method, to explain the engineering traps of the NRW algorithm, such as applicable boundaries, half wavelength divergence problems, and high-order mode interference. It analyzes the systematic effects of sample preparation tolerances and temperature environment on test results, and provides a verification and judgment framework for supplier electromagnetic parameter data.
The same batch of electromagnetic wave absorbing materials was sent to two laboratories for electromagnetic parameter testing, and the imaginary part of the complex magnetic permeability, μ ", may differ by more than 15% in the 2-6 GHz frequency band. This is not a problem with the material itself, but rather a result of differences in the selection of testing methods and sample preparation.
For engineers of absorbing materials, μ "and ε" are the core criteria for selection. The reflection loss curve can be calculated through electromagnetic parameters and transmission line theory, but the prerequisite is that the electromagnetic parameters themselves are reliable. If there is a systematic deviation in the test results of μ "and ε", all subsequent performance evaluations will be based on unreliable foundations.
Applicable boundaries of three types of testing methods
The mainstream methods for testing the electromagnetic parameters of electromagnetic wave absorbing materials in the industry are divided into three categories: coaxial method, waveguide method, and free space method. They each have boundaries in frequency coverage, sample morphology, and data processing.
Coaxial methodLoad the powder or solid sample into a coaxial fixture, measure the S parameter using a vector network analyzer (VNA), and then invert the complex dielectric constant and complex magnetic permeability using the NRW algorithm. Its frequency range usually covers 2-18 GHz, suitable for powder samples and small-sized solid samples that can be processed into rings. The core limitation of the coaxial method is that the sample must be tightly adhered to the inner wall of the fixture, and any air gap will introduce phase errors in the S parameters, resulting in distortion of the calculated values of ε "and μ".
Waveguide methodProcess the sample into thin sheets that match the cross-section of a standard rectangular waveguide, and measure the S ₁₁ and S ₂ coefficients in the waveguide section. This method has high testing accuracy and requires a small amount of samples, making it one of the most commonly used methods for measuring microwave electromagnetic parameters in laboratories. But the frequency range of the waveguide method is determined by the waveguide size, and each waveguide corresponds to a specific frequency band. In high-temperature environments, the testing fixture needs to be replaced with high-temperature alloys or carbon fiber materials, and the testing atmosphere needs to be switched to vacuum or argon to prevent oxidation, which increases the complexity of the testing system.
Arched method (free space method)In a microwave anechoic chamber, electromagnetic waves are emitted towards a flat sample through an antenna, and the reflectivity is measured at different incident angles. Its advantages lie in non-contact testing, support for multi angle measurement, and broadband coverage, with a frequency range of 1-40 GHz. However, the bow method requires a large darkroom space and a large area of samples, with strict requirements for background reflectivity - the background reflectivity needs to be below -40 dB to ensure testing accuracy.
The following table summarizes the differences in core parameters among the three methods:
GB/T 32596-2016 "General Specification for Electromagnetic Shielding Absorbing Wave Plates" and GJB 2038A-2011 provide a standardized framework for testing the reflectivity of absorbing materials, while SJ 20512-1995 specifies the testing methods for the complex permittivity and complex permeability of microwave high loss solid materials. However, these standards do not mandate the standardization of testing methods and sample preparation conditions for all suppliers, resulting in data from different laboratories not necessarily being directly comparable horizontally.
The engineering trap of NRW algorithm
Whether using coaxial or waveguide methods, the acquisition of electromagnetic parameters relies on the Nicolson Ross Weir (NRW) algorithm to invert ε and μ from S-parameters. There are two easily overlooked pitfalls in the engineering application of this algorithm.
Half wavelength divergence problem
The NRW algorithm exhibits numerical divergence near the half wavelength condition where the sample thickness is an integer multiple of the wavelength corresponding to the test frequency, resulting in significant fluctuations in the calculated values of ε″ and μ ″. The solution is to use samples with a quarter wavelength thickness, or to use the multi thickness averaging method in data processing to smooth out the divergent region.
Phase shift caused by air gap
The coaxial method requires no air gap between the sample and the inner wall of the fixture, but in practical operation, after the powder sample is mixed with paraffin and pressed into a ring, there may be a micrometer sized gap between the outer diameter of the ring and the inner wall of the fixture. This gap will introduce significant phase errors above 10 GHz, resulting in higher ε 'and lower μ'. The waveguide method also faces this problem: any gap between the sample sheet and the waveguide wall will change the phase relationship of the S parameter, affecting the accuracy of the inversion results.
The systematic impact of temperature on test results
The electromagnetic parameters of electromagnetic wave absorbing materials are temperature sensitive, but the temperature dependence of different parameters is not consistent. A study based on electromagnetic frequency dispersion coefficient shows that the complex dielectric constant and complex magnetic permeability of materials exhibit different temperature response trends in the range of 298 K to 473 K, but their dispersion coefficients remain relatively stable under temperature changes. This discovery suggests that if only focusing on the data of μ "and ε" at a single temperature point, it may not be possible to accurately evaluate the absorption performance of materials at actual operating temperatures.
For application scenarios with large temperature fluctuations such as in vehicle electronics and base station RF units, it is recommended that suppliers provide variable temperature electromagnetic parameter data covering at least -40 ° C to 125 ° C. If the peak frequency deviation of the material at high temperature exceeds one octave, the room temperature test data will not be able to be used for selection judgment under actual working conditions.
Data validation strategy in engineering selection
Engineers can verify the electromagnetic parameter data provided by suppliers from the following dimensions.
Confirm the testing method, sample thickness, and preparation conditions
Request the supplier to provide a complete test report, including the test standard number, method type, sample thickness and tolerance, and preparation process (powder/solid/paraffin mixture). If this information is missing from the report, the values of μ "and ε" can only be used as qualitative references. It is particularly important to confirm whether the sample thickness avoids the half wavelength divergence region and whether the fixture bonding method has been calibrated.
Pay attention to consistency across multiple batches
The batch consistency of electromagnetic wave absorbing materials is affected by the preparation process. For products of the same model, suppliers are required to provide electromagnetic parameter data for multiple consecutive batches to confirm the batch to batch deviation of peak frequency and intensity. If the peak frequency deviation between batches exceeds 0.3 GHz, it may be necessary to readjust the mounting scheme between different batches in practical applications.
Verify the coverage of high temperature or actual operating condition data
The electromagnetic parameters in most specifications are measured at room temperature (23 ° C) and in a dry environment. For high-temperature application scenarios, suppliers should be required to provide electromagnetic parameter data near the upper limit of the operating temperature, covering at least two temperature points, the midpoint and upper limit of the expected operating temperature range.
Advanced Institute (Shenzhen) Technology Co., LtdA standardized process has been established for electromagnetic parameter testing of absorbing materials. The test report includes complete information such as testing methods, sample thickness, and preparation conditions, and can provide electromagnetic parameter data under normal and high temperature conditions.
The Yanbo brand absorbing material series covers three main models: XJY-MH100, XJY-MH150, and XJY-MH200. The real part of the magnetic permeability covers ≥ 100 to ≥ 200, and the recommended frequency range covers 10 MHz to 18 GHz. The standard thickness can be as low as 0.2 mm, and the working temperature range is -40 ° C to 125 ° C. The flame retardant rating reaches UL94 V-0. For more technical information, please visithttp://www.avanzado.cn/.