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Introduction
Starting from the coexistence of passband signals and out of band interference in antennas, this article analyzes how frequency selective absorbers can achieve absorption of out of band electromagnetic waves rather than reflection through the cascading design of loss layers and bandpass frequency selective layers, while maintaining low insertion loss in the passband. The article explains the working principle of the integrated structure of absorption and transmission, compares the applicable boundaries of different types of frequency selective absorbers, and provides an engineering selection and judgment framework for antenna covers and broadband antenna systems.
In the electromagnetic compatibility design of electronic systems, there is a long-standing design contradiction: antennas need to efficiently transmit and receive signals within their operating frequency band, but environmental electromagnetic noise outside the operating frequency band needs to be suppressed. Traditional metal shielding can block out of band interference, but it also shields the passband signal; Traditional absorbing materials can absorb out of band interference, but the loss medium also introduces insertion loss within the passband, reducing antenna efficiency.
This contradiction is particularly prominent in radar antenna covers and broadband communication antennas. Traditional metal frequency selective surface (FSS) antenna covers rely on geometric shapes to reflect out of band signals away from incoming waves. However, with the development of multi base radar and anti stealth technology, reflective frequency selective surfaces have increased the risk of detection. A better solution is to absorb out of band signals instead of reflecting them. Frequency Selective Rasorber (FSR) is the technological path developed to address this issue. Its design goal can be summarized in one sentence: to allow low loss transmission of electromagnetic waves within a specified frequency band, and to absorb electromagnetic waves outside the frequency band instead of reflecting them.
The working principle of the integrated structure of suction and transmission
The core structure of a frequency selective absorber consists of two functional layers: the upper loss layer and the lower bandpass frequency selective layer.
The loss layer is usually composed of a resistive film or conductive pattern, which exhibits absorption characteristics for out of band electromagnetic waves. The bandpass frequency selective layer is located below the loss layer, presenting a transmission window for electromagnetic waves with passband frequencies, while exhibiting reflection characteristics for electromagnetic waves with out of band frequencies. The 'reflection' here is not to reflect electromagnetic waves back into free space, but to reflect the residual electromagnetic waves that have penetrated the loss layer back, so that they can be absorbed again through the loss layer. This multiple process of absorption reflection reabsorption significantly improves the attenuation efficiency of out of band electromagnetic waves.
The double-layer metamaterial composite structure proposed by Professor Kong Jie's team at Northwestern Polytechnical University is a typical representative of this design concept. The research team combined a three-dimensional metamaterial absorber with a frequency selective surface to construct a composite system with low-frequency transmission and high-frequency absorption functions. The total thickness of this structure is only 6 millimeters, achieving efficient signal transmission in the 3.5-7 GHz frequency band and efficient electromagnetic wave absorption in the 12-18 GHz frequency band. The comprehensive bandwidth and thickness are superior to existing frequency selective absorbers. Its absorption mechanism can significantly reduce the radar cross section, which is of great significance for dealing with dual/multi base radar systems.
Energy Selective Surface: Balancing High Power Protection and Out of Band Absorption
Another technical approach for frequency selective absorbers is to introduce energy selection functionality. Energy Selective Surface (ESS) can adaptively switch operating modes based on the power of incident electromagnetic waves - maintaining transmission mode when low-power signals are incident and automatically enhancing shielding effectiveness when high-power microwaves are incident.
A representative ESS design integrates adaptive protection and out of band absorption by introducing a dual-mode resonant absorption layer in the upper layer. In the passband of 1.88-2.47 GHz, the insertion loss is less than 1.5 dB; in the out of band frequency range of 7.8-23.18 GHz, the absorption rate exceeds 90%. When exposed to high-power microwave radiation, the shielding effectiveness within the passband is enhanced to over 30 dB, while maintaining effective absorption performance in the range of 7.5-22.9 GHz. This power adaptive feature makes it uniquely valuable in scenarios where both conventional electromagnetic interference and high-power microwave threats need to be addressed simultaneously.
Another approach combines energy selective surfaces with loss layers, utilizing the nonlinear response of p-i-n diodes to achieve switchable passband stopband characteristics. Under low-power incidence, a low insertion loss of 0.9 dB is achieved at 3.58 GHz, and ultra wideband absorption is achieved in the range of 5.87-17.88 GHz. When the incident field strength exceeds 740 V/m, the passband transmission power rapidly decreases, and the maximum shielding effectiveness reaches 18 dB. This characteristic of "transparency during normal times and shielding during threats" provides a feasible engineering solution for balancing high-power microwave protection and electromagnetic stealth.
Applicable boundaries for different types of FSR
Frequency selective absorbers can be classified into several different configurations based on the relative position of the passband and absorption band:
Types of passband at high frequencies and absorption band at low frequenciesSuitable for scenarios where absorption is required in the P/L band and transmission is required in the C/X/Ku band. A miniaturized FSR design achieved over 90% absorption (relative bandwidth 127.3%) in the range of 500 MHz – 2.25 GHz, while achieving a transmission bandwidth of -1 dB in the range of 4.6-13.8 GHz. The unit size of this structure is only 0.026 λ L × 0.026 λ L × 0.10 λ L, which is suitable for low-frequency RCS reduction and EMI suppression in ultra wideband antenna systems.
Types of passband at low frequencies and absorption band at high frequenciesIt is suitable for scenarios that require low-frequency communication and high-frequency stealth. The double-layer metamaterial structure of Xigong University mentioned above belongs to this category, with a low-frequency passband covering 3.5-7 GHz and a high-frequency absorption band covering 12-18 GHz.
The following table summarizes the functional characteristics of different types of frequency selective absorbers:
Three judgments in engineering selection
For scenarios that require the integration of absorption and transmission functions in antenna systems, it is recommended to prioritize the following three points when selecting.
Firstly, the balance between passband insertion loss and antenna gain
The loss layer of the frequency selective absorber inevitably introduces a certain insertion loss within the passband. When selecting, the insertion loss data of the material in the passband should be confirmed, usually requiring less than 1-1.5 dB. If the gain margin of the antenna itself is limited, the design scheme with lower passband insertion loss should be prioritized. The square resistance of the resistive film in the loss layer directly affects the balance between passband insertion loss and out of band absorption. A low square resistance will increase passband loss, while a high square resistance will weaken out of band absorption capability.
Secondly, matching the out of band absorption bandwidth with the target threat frequency band
There are significant differences in the out of band absorption bandwidth between FSRs of different designs. When selecting, it should be confirmed whether the out of band absorption frequency band of the material fully covers the interference frequency band that needs to be suppressed. If the interference source frequency band spans multiple discontinuous intervals, a single FSR may be difficult to cover simultaneously, and in this case, multi-layer composite structures or segmented design schemes need to be considered.
Thirdly, the incident angle and polarization stability
The incident angle of electromagnetic waves in antenna covers and antenna systems is often not perpendicular, and the TE and TM polarization states also vary with the incident angle. The absorption performance of FSR is sensitive to the incident angle and polarization state. When selecting, it is necessary to confirm the material's absorption performance within the expected incident angle range (such as 0 ° -30 ° or 0 ° -45 °), as well as the consistency of performance under TE/TM polarization. Some designs can maintain stable absorption and transmission performance even at a 30 ° incident angle.
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