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Introduction
Starting from the evolution law of high-frequency electromagnetic parameters with frequency, analyze the mechanism transition phenomenon of dielectric loss attenuation above 10 GHz and rapid increase of magnetic loss. This paper explains how the material system adapts to the characteristics of this frequency band, compares the applicable boundaries of metamaterial structure, graphene aerogel and ferrite based materials in the high frequency band, and provides a judgment framework for the selection of high-frequency absorbing materials in 5G millimeter wave and vehicle borne radar scenarios.
Engineers often encounter a counterintuitive phenomenon in the selection of high-frequency absorbing materials: a magnetic loss type absorbing plate that performs well in the 2-6 GHz frequency band, when switched to 24 GHz or 77 GHz scenarios, experiences a significant decrease in absorption performance, which cannot be compensated for even with an increase in thickness.
The problem lies in the frequency band dependence of the loss mechanism. The weight of the loss mechanism of the same material in different frequency bands will fundamentally change, and this change is not always visually apparent.
Transition of loss mechanism near 10 GHz
In the 2-10 GHz frequency band, dielectric loss is usually the dominant attenuation mechanism of absorbing materials. The interface polarization and dipole polarization in carbon based materials, conductive polymers, and composite systems contribute mainly to electromagnetic energy dissipation in alternating electric fields.
But after the frequency exceeds 10 GHz, this pattern reverses. A study based on starch based carbon aerogel/FeCoNi composite microwave absorbing material showed that in the range of 2 – 10 GHz, the dielectric loss angle tangent (tan δ ε) and magnetic loss angle tangent (tan δ µ) both showed an upward trend, and the dielectric loss was significantly higher than the magnetic loss, indicating that the frequency band was dominated by dielectric loss. Above 10 GHz, dielectric loss begins to decrease, while magnetic loss rapidly increases and becomes the main loss mechanism.
The physical reason for this transformation is that the contribution of dielectric loss mainly comes from dipole polarization and interface polarization. These polarization mechanisms have a decreasing ability to follow alternating electric fields at high frequencies, and the polarization relaxation time no longer matches the high-frequency period, resulting in a decrease in dielectric loss with increasing frequency. At the same time, natural resonance and exchange resonance of magnetic materials begin to dominate at high frequencies. The analysis of the magnetic loss mechanism of FeCoNi/SCA composite materials shows that exchange resonance controls the magnetic loss behavior in the 14-18 GHz frequency band.
For engineers of absorbing materials, this mechanism shift means that the design of high-frequency absorbing materials cannot simply "translate" the formula of low-frequency magnetic loss materials to the high-frequency band, but needs to readjust the material composition and structural design based on the loss weight of the target frequency band.
Three technical routes for high-frequency absorbing materials
There are currently three main technical routes in engineering for the absorption requirements of millimeter wave frequency bands, each suitable for different scenarios.
The first type is metamaterial structural absorbers
Metamaterial absorbers achieve near perfect absorption of electromagnetic waves through periodic resonant elements, and their absorption frequency is determined by the geometric dimensions of the elements rather than the intrinsic electromagnetic parameters of the material. A dual frequency metamaterial absorber for 5G millimeter wave communication achieved an absorption rate of 99.71% and a shielding effectiveness of 51.33 dB at 24 GHz, and an absorption rate of 99.43% and a shielding effectiveness of 46.7 dB at 26 GHz, with performance not affected by polarization angle. The advantage of metamaterial absorbers lies in their extremely thin thickness and strong absorption peak, making them suitable for chip level or module level EMI suppression scenarios with limited space. But its absorption bandwidth is usually narrow, requiring precise design of unit size for the target frequency.
The second type is lightweight carbon based absorbers
Graphene aerogels provide a variety of inherent attenuation paths by virtue of three-dimensional porous networks and rich oxygen-containing functional groups. However, the effective absorption bandwidth of pure graphene aerogels is limited, only 6.46 GHz (11.54 – 18.00 GHz) at 1.95 mm thickness, and lack of magnetic loss components, which limits their absorption capacity at low frequencies. By coupling graphene aerogel with structure induced resonance, a graphene aerogel based metamaterial absorber with a stacking cone configuration has achieved an effective absorption bandwidth of 40.7 GHz (9.1 – 49.8 GHz), covering an ultra wide frequency range from X-band to V-band, and maintains stable absorption at ± 70 ° incidence angle. The engineering value of this type of material lies in balancing lightweight and ultra wideband coverage, making it suitable for weight sensitive scenarios such as drones and aerospace.
The third type is ferrite based materials
M-type hexagonal ferrite enhances absorption through ferromagnetic resonance in the millimeter wave frequency band and can also integrate beam control function. A review paper systematically analyzed the multifunctional metasurface applications of M-type ferrites in millimeter wave absorption and beam control, with particular emphasis on the role of ferrites in ferromagnetic resonance and surface wave capture mechanisms. This type of material is suitable for complex scenarios that require simultaneous absorption and electromagnetic wave modulation, such as ground stations and satellite communication systems in non terrestrial networks (NTNs).
The following table summarizes the applicable characteristics of three technical routes in the millimeter wave frequency band:
Engineering verification of vehicle mounted millimeter wave radar
The 77 GHz vehicle mounted millimeter wave radar is one of the most typical engineering application scenarios for high-frequency absorbing materials. The size of radar modules is usually around 5 × 5 centimeters, with extremely limited internal space. However, with a working frequency of up to 77 GHz, self-excited signals are easily generated inside the cavity, which affects the normal operation of the receiving and transmitting channels.
For this scenario, high loss flexible absorbing materials can provide attenuation values of 70 dB/cm or more at a frequency of 77 GHz, effectively absorbing self-excited signals generated by millimeter wave radar. Another type of solution adopts co injection molding of absorbing PBT particles and radar shell, which exhibits good electromagnetic wave absorption characteristics in the 77 GHz frequency band, can effectively absorb clutter and antenna sidelobe signals, reduce electromagnetic interference, and improve radar recognition sensitivity.
These engineering cases illustrate that the selection of high-frequency absorbing materials should not only focus on the laboratory reflection loss curve, but also evaluate the material's attenuation ability under actual module size and assembly conditions.
Three engineering judgments in selection
Firstly, confirm the weight of the material's loss mechanism in the target frequency band
Require suppliers to provide dielectric loss tangent and magnetic loss tangent data for the target frequency band, rather than just providing reflection loss curves. If the target frequency exceeds 10 GHz and the material is still dominated by dielectric loss, the absorption performance may not meet expectations. The ideal high-frequency absorbing material should achieve the synergy of two loss mechanisms in the target frequency band, rather than extreme bias towards one.
Secondly, evaluate the actual attenuation value of the material in the millimeter wave band
The peak of the reflection loss curve may occur at specific frequencies and under specific testing conditions, while the incident angle and polarization state of electromagnetic waves in engineering scenarios often differ from laboratory tests. For high-frequency applications such as 77 GHz, suppliers should be required to provide attenuation values (dB/cm or dB/mm) for that frequency band, rather than just providing reflection loss data. The attenuation value more directly reflects the material's ability to absorb electromagnetic waves per unit thickness, and has a stronger correlation with actual engineering effects.
Thirdly, pay attention to the polarization stability and angular stability of metamaterial structures
If a metamaterial structured absorber is selected, its performance retention at different polarization angles and incident angles should be confirmed. Some metamaterial absorbers exhibit excellent performance under vertical incidence, but their absorption rates sharply decrease under oblique incidence or cross polarization conditions. The wide angle stable metamaterial design (such as maintaining stable absorption within ± 70 ° range) is more reliable in scenarios such as vehicle radar and base station antennas.
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