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At the EMC review meeting for mobile phones, engineers often present a frequency band list: 2.4 GHz Wi-Fi、5 GHz Wi-Fi、6 GHz Wi-Fi 6E、n78（3.5 GHz）、n79（4.9 GHz）、24.25–27.5 GHz、37–43.5 GHz Millimeter wave, UWB at 6.5/8 GHz, NFC at 13.56 MHz, wireless charging at 110-205 kHz.
Then the question arises: Is there a type of absorbing material that can cover all of these frequency bands?
The answer is no. It's not that the supplier lacks capability, but rather that the material physics itself does not allow it.
The frequency band span determines that a single material cannot cover it
The loss mechanism of absorbing materials has frequency band selectivity. Magnetic loss materials rely on the natural resonance and eddy current loss of soft magnetic fillers, and their peak values appear near specific frequencies. The natural resonance frequency of ferrite is usually in the range of several hundred MHz to several GHz, while the resonance frequency of nanocrystalline materials is related to grain size and composition. The loss peak of carbonyl iron powder is in the GHz range. Once the target frequency deviates from the peak value by more than one octave, it is difficult to achieve effective absorption even with an increase in thickness.
The situation is similar for dielectric loss materials. The polarization relaxation time of carbon based materials and conductive polymers determines their effective loss frequency band, while the impedance gradient of porous structures is related to pore size and thickness. Fine pore foam is effective in the millimeter wave frequency range of 24-43.5 GHz, but if it is placed at 2.4 GHz, a thickness of 0.3-5 mm cannot form sufficient absorption depth at low frequencies.
By stacking the frequency band list and material loss spectrum together, the conclusion is clear: 2.4-6 GHz requires magnetic loss type thin films, 24-43.5 GHz requires porous or millimeter wave specific absorbing layers, and 13.56 MHz and 110-205 kHz require magnetic separators rather than absorbing plates. These types of materials have different operating frequency bands, loss mechanisms, and thickness requirements, and no material can simultaneously meet these conditions within the full thickness budget of a mobile phone.
Multi band "is not about stacking materials, but segmenting them according to the noise path
Since a single material cannot cover it, the engineering approach shifts to "segmented matching". But segmented matching does not mean stacking several absorbing materials together - stacking introduces new problems: interlayer impedance mismatch, total thickness exceeding the stacking budget, and low-frequency materials increasing reflection in the high-frequency range.
A more reasonable organizational approach is to segment by noise path rather than by frequency list.
The propagation of noise inside mobile phones can be roughly classified into three types of paths:
The first type is near-field coupling near the chip and FPC
The harmonics of high-speed digital interfaces and the switching noise of DCDC converters are coupled to adjacent RF antennas or sensitive circuits through near-field magnetic fields. The main frequency of this type of noise is usually in the range of several hundred MHz to 6 GHz, mainly composed of magnetic field components. It is suitable for magnetic loss type thin films with a thickness of 0.1-0.5 mm, which are mounted between the noise source and the sensitive circuit.
The second type is cavity resonance inside the shielding cover
The metal shielding covers of millimeter wave modules and Wi Fi modules form standing waves at specific frequencies, and the resonance peaks are coupled to external antennas through gaps. The main frequency of this type of problem is above 24 GHz, and it is suitable for fine pore absorbing foam or millimeter wave dedicated absorbing layer with a thickness of 1-3 mm, which is mounted on the inner wall of the cavity.
The third type is surface wave coupling on metal structures (frames, middle frames, camera decorations)
Millimeter waves form surface waves when propagating on metal surfaces, transmitting noise from one end to the other. This type of problem requires thin absorbers with specific dielectric/magnetic loss spectra, typically ranging in thickness from 0.05 to 0.3 mm.
Three types of paths correspond to three material requirements. A mobile phone may have these three types of problems simultaneously, but their spatial positions, noise sources, and frequencies are different. The core of segmented matching is to place each material on its corresponding noise path, rather than attempting to cover all paths with one material.
The following table summarizes the matching relationship between three types of noise paths and material systems:
The transition zone between frequency bands is the most easily overlooked blind spot
Segmented matching may sound clear, but in practical engineering, the most problematic area is the transition zone between frequency bands.
Taking 5-8 GHz as an example. 5 GHz Wi Fi (5.15-5.85 GHz), 6 GHz Wi Fi 6E (5.925-7.125 GHz), and UWB (6.5/8 GHz) are densely distributed in this range. This frequency band is located in the "middle zone" between magnetic loss type thin films and millimeter wave absorbing layers: the μ "of magnetic loss materials above 6 GHz usually begins to decrease, and the design focus of millimeter wave absorbing layers is above 24 GHz, which may not have sufficient absorption depth for 6-8 GHz.
If engineers use the classification of "thin sheets for low frequencies and foam for high frequencies", the 5-8 GHz range may have insufficient coverage on both sides, manifested as fluctuations in Wi Fi 6E throughput under specific cavity conditions, or a decrease in UWB ranging accuracy near metal structures. The solution to this blind spot is not to forcefully find a "universal material", but to separately configure materials for the transition frequency band in the segmented scheme. For example, selecting a magnetic loss sheet with a tuned peak at around 6 GHz, or a medium thickness foam validated in the effective frequency range of 5-8 GHz. When selecting, suppliers should be required to provide reflectivity or attenuation data in the 5-8 GHz range, rather than just looking at the performance of the two endpoints at 2.4 GHz and 24 GHz.
Allocation of thickness budget: who accounts for 0.1 mm, who accounts for 3 mm
Another constraint of segmented matching is the allocation of thickness budget. The stacking space inside the phone varies greatly in different positions: the space above the chip is usually only 0.1-0.3 mm, the space inside the shield may be 1-3 mm, and the inner side of the metal frame is between the two.
This means that material selection should not only focus on "which frequency band has stronger absorption", but also on "how thick this position can fit". In a space of 0.1 mm, millimeter wave foam cannot function at all; Covering the millimeter wave cavity resonance with ultra-thin magnetic loss plates in a 3mm space, the absorption depth is not sufficient. The engineering judgment sequence should be: first determine the available space for each noise path, and then select the material with the best absorption capacity for the target frequency band under the constraints of that space, rather than selecting the material first and then finding a place to paste it.
Segmented absorption selection judgment based on noise path
Firstly, draw a three-dimensional table of "noise path frequency band space" instead of a simple frequency list
Label each noise problem with the dominant frequency, propagation path (near-field/cavity/surface wave), and available space, which together determine the material type and thickness. The frequency list can only tell you "which frequency bands are available", not "what materials should be used or where to place them".
Secondly, confirm material data separately for the transition frequency band (5-8 GHz)
Require suppliers to provide reflectance or attenuation curves for this range, rather than just peak data for 2.4 GHz and 24 GHz. The performance gap in transition frequency bands is often not exposed in single frequency point testing, but is reflected in fluctuations in overall throughput or ranging accuracy.
Thirdly, confirm in the segmentation scheme that interlayer boundaries will not introduce new reflections
If absorbing materials of different frequency bands are adjacent in space, sudden impedance changes at the boundary may trigger new reflection paths. When selecting, it should be confirmed whether the supplier provides multi-layer composite solutions and impedance matching data of adjacent materials in the transition zone.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of mobile phone absorbing materials covers magnetic loss thin sheets, fine pore absorbing foam, and thin layer millimeter wave absorbers.
Magnetic loss type thin film is designed for near-field coupling in the frequency range of several hundred MHz to 6 GHz, with a thickness of 0.1-0.5 mm. The pore size of the fine hole absorbing foam can be customized to 80-300 μ m, with a thickness of 0.3-5.0 mm. The effective frequency range covers 1-40 GHz, with a focus on cavity resonance in the frequency range of 24-43.5 GHz; Thin layer absorption surface coupled with metal structure surface waves, with a thickness that can be reduced to the order of 0.05 mm.
The product supports material matching and multi-layer composite design for segmented schemes, and can provide selection support based on specific noise paths, frequency bands, and spatial constraints. For more technical information, please visithttp://www.avanzado.cn/.