A frequency selective surface with low frequency transmission and high frequency absorption characteristics

Abstract

This invention discloses a transmittance-absorbing integrated frequency selective surface with low-frequency transmission and high-frequency absorption characteristics, exhibiting wave transmission in the 0-2 GHz band and wave absorption in the 8-12 GHz band. First, a low-pass frequency selective surface is designed, achieving low-pass filtering with an insertion loss of less than 1.4 dB in the 0-2 GHz band when TE and TM polarized waves are incident normally. To absorb X-band incident waves, a resistive absorber is placed above the low-pass frequency selective surface with a 4.5 mm air gap, forming a transmittance-absorbing integrated frequency selective surface with both low-frequency transmission and high-frequency absorption characteristics. When the incident angle of TE and TM polarized waves increases from 0° to 40°, this structure has a wideband transmission window greater than -1.8 dB in the 0-2 GHz band and a wideband absorption window less than -10 dB in the 8-12 GHz band. When TE and TM polarized waves are incident normally, the monostatic RCS of the penetration-integrated frequency selective surface decreases by more than 10 dB in the 7.85–12 GHz band compared to a metal plate of the same size.

Description

Technical Field

[0001] This invention belongs to the field of antenna engineering technology, and specifically relates to a frequency selective surface that integrates low-frequency transmission and high-frequency absorption characteristics. Background Technology

[0002] Frequency selective surfaces (FSS) have been widely used in antenna systems, electromagnetic stealth, and wireless communication systems due to their unique spatial filtering characteristics. Traditional FSSs are classified into four basic configurations: patch-type low-pass FSSs, grid-type high-pass FSSs, metal ring-type band-stop FSSs, and slotted ring-type band-pass FSSs. With beyond-visual-range warfare becoming the primary form of modern warfare and the continuous advancement of radar detection technology, improving the electromagnetic stealth performance of our combat equipment faces significant challenges, and the antenna is one of the main factors affecting radar cross-section. Traditional FSS radomes can achieve lossless transmission of electromagnetic waves within a specific frequency range while reflecting electromagnetic waves of other frequency bands in different directions, thereby enhancing the stealth effect of monostation radar. With the rapid development of multistation radar systems, traditional FSSs, due to their strong out-of-band reflections, result in a large radar cross-section, making them difficult to apply in modern radar systems. Therefore, there is an urgent need for a new composite material that combines wave transmission and absorption. A frequency selective radiator (FSR) absorbs electromagnetic waves of a specific frequency by covering the radome surface with a layer of artificial dielectric material. It not only allows wave transmission within a specific frequency band but also possesses broadband absorption capabilities. Based on their different absorption characteristics, they are categorized into low-frequency absorption / high-frequency transmission type, low-frequency transmission / high-frequency absorption type, mid-frequency transmission / two-sided frequency absorption type, and mid-frequency absorption / two-sided frequency transmission type.

[0003] In 2019, the paper "Miniaturized Frequency-Selective Rasorber With a Wide Transmission Band Using Circular Spiral Resonator" proposed an AT-type (low absorption, high transmission) FSR, which consists of a resistive element and a bandpass frequency selective surface (FSS). The hexagonal physical spacing is 12 mm, and the overall height of the FSR is 15.35 mm. The 0.5 dB transmission bandwidth of the resistive element and the bandpass FSS are 8.68 ~ 11.34 GHz and 8.2 ~ 11.33 GHz, respectively. After integrating the resistive element and the bandpass frequency selective surface, the 1 dB transmission bandwidth of the FSR is 8.3 ~ 11.07 GHz, and the 10 dB absorption bandwidth is 2.4 ~ 7.1 GHz. The transmission performance is independent of the polarization direction and the incident angle. This FSR unit uses an air layer design with a height of 12 mm, resulting in a profile height of 15.35 mm and a scanning angle of only 30°, which presents the problem of excessive profile height and small scanning angle.

[0004] In 2020, the paper "Broadband Dual-Polarized Band-Absorptive Frequency-Selective Rasorber Using Absorptive Transmission / Reflection Surface" proposed a TA-type (low transmittance, high absorption) FSR, employing a structure of two layers of resistive patch panels, a 10.5mm air layer, and two layers of 0.508mm thick Rogers RO5880 dielectric substrate. It uses 270-ohm and 100-ohm resistors. A dual-resonator and multi-resonator band-stop FSS are used to broaden the absorption bandwidth, with a double-loop design chosen as the dual-resonator band-stop FSS. The overall height of the FSR is 11mm. The FSR has a low-pass filtering bandwidth of 0~860MHz and an absorption bandwidth of 3.84~10.83GHz. The 10.5mm high air layer design of this FSR unit results in a profile height of 11mm, leading to issues of excessive profile height and a narrow low-frequency passband.

[0005] In 2021, the paper "Hybrid Frequency-Selective Rasorber With Low-Frequency Diffusion and High-Frequency Absorption" proposed an ATA-type (middle-pass, two-side-absorb) bandpass FSR. The FSR consists of three layers: a broadband second-order bandpass FSS at the bottom layer, and circuit-simulated absorbers and encoder surfaces at the middle and top layers, respectively. These utilize absorption and diffusion to reduce the RCS in both high and low frequency bands. The overall height of the FSR is 11.81 mm. In the 7.3–12.6 GHz band, the transmission amplitude is greater than -1.5 dB. Compared to a metal flat panel of the same size, the designed FSR achieves a 10 dB single-station RCS reduction in the 3.3 GHz–20 GHz range. The bandpass FSS of this FSR unit consists of three metal layers printed on two RO4003C substrates, each 1.524 mm thick, which presents issues such as excessive air layers and an overly thick bottom bandpass FSS substrate.

[0006] In patent CN121440194A, the inventors proposed a broadband transmittance integrated frequency selective surface with high selectivity, comprising a first loss layer, a second loss layer, and a frequency selective layer. Both the first and second loss layers utilize double-order resonance to achieve a transmission band with both high selectivity and broadband characteristics. The second loss layer and the frequency selective layer are cascaded to achieve integrated transmittance. It achieves transmittance in the 7.72 ~ 11.89 GHz frequency band and absorbance in the 1.96 ~ 7.12 GHz and 12.95 ~ 17.33 GHz frequency bands. However, the transmission and reflection characteristics at different scanning angles are not provided, and the surface exhibits problems such as overly complex unit structure and high manufacturing difficulty.

[0007] In patent CN121282627A, the inventors proposed a broadband low-frequency transparent and high-frequency absorbing integrated frequency selective surface, comprising three absorbing layers and two transmitting layers stacked from top to bottom. The three absorbing layers are, from top to bottom, an upper absorbing layer, a middle absorbing layer, and a lower absorbing layer; the two transmitting layers are, from top to bottom, an upper transmitting layer and a lower transmitting layer. This achieves insertion loss <1dB in the 0~3GHz frequency band, and under both TE and TM polarization, satisfies |S 11 | < -10dB. However, the transmission and reflection characteristics at different scanning angles are not given, and the number of unit layers reaches 5, with a total profile height of 34mm.

[0008] To address the aforementioned issues, this invention discloses a frequency selective surface that integrates low-frequency transmission and high-frequency absorption characteristics, aiming to miniaturize and reduce the profile of FSR units, as well as to achieve stable transmission and absorption characteristics of FSR units when plane waves are incident at large angles. Summary of the Invention

[0009] The purpose of this invention is to address the limitations of traditional FSS radomes, which can achieve lossless transmission of electromagnetic waves within a specific frequency range while reflecting electromagnetic waves of other frequency bands in different directions to enhance the stealth effect of monostatic radar. With the rapid development of multistatic radar systems, traditional frequency selective surfaces (FSS) suffer from strong out-of-band reflections, resulting in a large bistatic radar cross-section, making them unsuitable for modern radar systems. Therefore, a novel integrated transmittance and absorption composite FSS radome is urgently needed. Existing integrated transmittance and absorption frequency selective surfaces are mostly band-stop and band-pass configurations, which cannot meet the requirements of the 0.2~2GHz passband and X-band absorption band of ultra-wideband tightly coupled antenna arrays. Furthermore, their high profile and numerous layers prevent the achievement of a low profile.

[0010] First, a low-pass frequency selective surface was designed. A Jerusalem cross, a square ring with lumped elements, and a centrally connected square ring were printed from bottom to top on two layers of "Rogers RO5880 (tm)" dielectric substrates, with a total thickness of 0.254 mm. The low-pass frequency selective surface achieved good low-pass performance in the 0–2 GHz band and completely reflected incident waves in the 6–12 GHz band. Next, a resistive absorber was designed. A bent Jerusalem cross with a lumped resistor at its center was printed on the top and bottom sides of a 0.203 mm thick "Rogers RO4003 (tm)" dielectric substrate, placed 4.5 mm above the low-pass frequency selective surface, absorbing incident waves in the 8–12 GHz band.

[0011] To achieve the aforementioned objectives, the present invention employs the following technical solution: a frequency selective surface integrating low-frequency transmission and high-frequency absorption characteristics. This integrated frequency selective surface comprises a low-pass frequency selective surface, an air layer, and a resistive absorber. The low-pass frequency selective surface uses a Jerusalem cross 101, a square ring 102 with a 25-ohm lumped resistor 104, and a centrally connected square ring 103, printed from bottom to top on two substrates 105. The substrate used for the low-pass frequency selective surface is "Rogers RO5880 (tm)", with a relative permittivity of 2.2 and a thickness of 0.127 mm. When TE and TM polarized waves are incident perpendicularly, the low-pass frequency selective surface exhibits an insertion loss (IL) of less than 1.4 dB in the 0–2 GHz frequency band, achieving excellent low-pass performance. In the 6–12 GHz frequency band, it acts as a metal floor, completely reflecting the incident wave and, in conjunction with the subsequent absorption layer, achieving high-frequency absorption. The resistive absorber consists of two metal layers printed on a thin dielectric substrate 203, positioned above a 4.5mm air gap from the low-pass frequency selective surface, ensuring the total cross-sectional height of the integrated frequency selective surface is less than 5mm. The metal pattern of the resistive absorber uses a Jerusalem cross 201 with zigzag lines, with a 160-ohm lumped resistor 202 placed at the center of the cross as the absorbing element. To facilitate resistor soldering and avoid resistor crossover, the two intersecting strips of the Jerusalem cross are placed on the top and bottom layers of the substrate, respectively. The dielectric substrate of the resistive absorber is made of Rogers RO4003 (tm) with a relative permittivity of 3.55 and a thickness of 0.203mm. The entire integrated frequency selective surface unit is rotationally symmetric, thus insensitive to the polarization of the incident wave. The use of angularly stable structures such as square rings and the Jerusalem cross ensures that the integrated frequency selective surface maintains good wave transmission and absorption characteristics even when the incident angle of TE and TM polarized waves increases from 0° to 40°.

[0012] In summary, this integrated penetrating frequency selective surface has the following innovations: (1) It achieves both low-frequency transmission and high-frequency absorption characteristics, ensuring normal radiation from the back-end antenna while avoiding the problem of large radar cross-section caused by strong out-of-band reflection in traditional frequency selective surfaces, achieving a 10dB RCS reduction in the X-band; (2) It achieves a miniaturized design of the integrated penetrating frequency selective surface, where the lateral dimension of a traditional integrated penetrating frequency selective surface unit is greater than 0.2. The transverse dimension of the integrated frequency selective surface unit of this invention is only 0.14 mm. (3) A low-profile design of the integrated frequency selective surface for penetration absorption is achieved. The total profile height of the integrated frequency selective surface for penetration absorption of the present invention is only 0.033 mm. (4) When TE and TM polarized waves are obliquely incident, the frequency selection table of the absorption and transmission integration has angular stability and still has good wave transmission and absorption characteristics when the incident angle increases from 0° to 40°. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a complete structure of a frequency selective surface with integrated penetration and high-frequency absorption characteristics. It consists of a Jerusalem cross 101, a square ring 102 with a 25-ohm lumped resistor 104, a centrally connected square ring 103, two 0.127mm thick "Rogers RO5880 (tm)" substrates 105, a Jerusalem cross 201 with a zigzag line, a 160-ohm lumped resistor 202 placed at its center, a 0.203mm thick "Rogers RO4003 (tm)" dielectric substrate 203, and a 4.5mm air layer.

[0014] Figure 2 A schematic diagram of the surface structure selected for low-pass frequency is shown. The Jerusalem cross 101, the square ring 102 with a 25-ohm lumped resistor 104 and the center connecting square ring 103 are printed from bottom to top on two 0.127mm thick "RogersRO5880 (tm)" substrates 105.

[0015] Figure 3 A schematic diagram of the Jerusalem Cross 101 structure on the surface selected for low-pass frequency selection.

[0016] Figure 4 A schematic diagram of a square ring 102 with a 25-ohm lumped resistor 104 is shown on the surface for selecting low-pass frequencies. The lumped resistor is placed at the center of the four sides of the square ring to widen the stopband.

[0017] Figure 5 A schematic diagram of the central connecting square ring 103 structure in the surface selected for low-pass frequency selection.

[0018] Figure 6 This is a schematic diagram of the resistive absorber. The Jerusalem cross 201 with zigzag lines is printed on a 0.203mm thick "Rogers RO4003 (tm)" substrate. A 160-ohm lumped resistor 202 is placed in the center of the cross as the absorption element. Each FSR unit consists of four Jerusalem cross metal patterns.

[0019] Figure 7 The diagram shows the structure of the Jerusalem Cross 201 with its zigzag lines. To facilitate the soldering of the 160-ohm lumped resistor 202 and to avoid their intersection, the two intersecting strips of the Jerusalem Cross are placed on the top and bottom layers of the substrate, respectively.

[0020] Figure 8 Simulation results of transmission and reflection coefficients when selecting a surface for perpendicular incidence of TE and TM polarized waves at low-pass frequencies.

[0021] Figure 9 The simulation results show the reflection coefficients when TE-polarized waves are incident on FSR units at different angles.

[0022] Figure 10 The simulation results show the transmission coefficients when TE-polarized waves are incident on FSR cells at different angles.

[0023] Figure 11 The simulation results show the reflection coefficients when TM polarized waves are incident on FSR elements at different angles.

[0024] Figure 12 The simulation results show the transmission coefficients when TM polarized waves are incident on FSR cells at different angles.

[0025] Figure 13 The results of monostatic RCS simulations of TE and TM polarized waves perpendicularly incident on FSR cells are compared with those of a metal floor of the same size. Detailed Implementation

[0026] like Figure 1 As shown, the Jerusalem cross 101, the square ring 102 with a 25-ohm lumped resistor 104, and the central connecting square ring 103 are printed from bottom to top on two 0.127mm thick "Rogers RO5880 (tm)" substrates 105 to form a low-pass frequency selective surface. The Jerusalem cross 201 with zigzag lines and the 160-ohm lumped resistor 202 placed at its center are printed on a 0.203mm thick "Rogers RO4003 (tm)" dielectric substrate 203 to form a resistive absorber, which is placed above the low-pass frequency selective surface with an air gap of 4.5mm.

[0027] Figure 2 The substrate used for the low-pass frequency selective surface shown is "Rogers RO5880 (tm)", with a relative permittivity of 2.2 and a thickness of 0.127 mm. When TE and TM polarized waves are incident perpendicularly, the low-pass frequency selective surface has an insertion loss (IL) of less than 1.4 dB in the 0 ~ 2 GHz band, achieving a low-pass effect. In the 6 ~ 12 GHz band, it acts as a metal ground plane, completely reflecting the incident wave, and together with the absorption layer, it achieves the function of high-frequency absorption.

[0028] Figure 9 and Figure 10Simulation results for the reflection coefficient and transmission coefficient of TE-polarized waves incident on the FSR unit at different angles are shown. It can be seen that the FSR unit achieves the desired reflection coefficient and transmission coefficient in the 0~2 GHz frequency band when the TE-polarized wave is incident perpendicularly. 21 |> -1.41 dB, in the 8 ~ 12 GHz band, satisfies |S 11 |< -10 dB; When the incident angle of the TE polarized wave increases from 0° to 40°, in the 0~2 GHz frequency band, |S 21 |> -1.8 dB, in the 8 ~ 12 GHz band, satisfying |S 11 |< -10 dB.

[0029] Figure 11 and Figure 12 Simulation results for the reflection coefficient and transmission coefficient of TM polarized waves incident on the FSR unit at different angles are shown. It can be seen that the FSR unit achieves the desired reflection coefficient and transmission coefficient in the 0 ~ 2 GHz frequency band when the TM polarized wave is incident perpendicularly. 21 |> -1.4dB, in the 8 ~ 12 GHz band, satisfies |S 11 |< -10 dB; When the incident angle of the TM polarized wave increases from 0° to 40°, in the 0~2GHz frequency band, |S 21 |> -1.4 dB, in the 8 ~ 12 GHz band, basically meets |S 11 |< -10 dB.

[0030] Figure 13 The simulation results show the monostatic RCS of the FSR cell when TE and TM polarized waves are incident perpendicularly. It can be seen that when TE and TM polarized waves are incident normally, compared with a metal plate of the same size, the monostatic RCS of the FSR cell is reduced by more than 10 dB in the 7.85 ~ 12 GHz frequency band.

[0031] The embodiments described above are merely illustrative of specific implementations of the present invention. Their descriptions are detailed and specific, and should be understood as being presented only as examples and not as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A frequency selective surface with low frequency transmission and high frequency absorption characteristics, characterized in that The Jerusalem cross (101), the square ring (102) with a 25-ohm lumped resistor (104) and the central connecting square ring (103) are printed from bottom to top on two 0.127mm thick "Rogers RO5880 (tm)" substrates (105) as the bottom low-pass frequency selection surface. The Jerusalem cross (201) with a 160-ohm lumped resistor (202) in the center is printed on the top and bottom sides of the 0.203mm thick "Rogers RO4003 (tm)" substrate (203) as the top resistive absorber. There is a 4.5mm thick air layer between the bottom low-pass frequency selection surface and the top resistive absorber.

2. The transmissive and absorptive frequency selective surface with low frequency transmission and high frequency absorption characteristics according to claim 1, characterized in that The bottom layer consists of three metal layers and two "Rogers RO5880 (tm)" substrates, forming a low-pass frequency selective surface with a multi-layer coupling structure, with a cross-sectional height of only 0.

001. The reflection window exhibits a second-order frequency response in the 0-2 GHz band and a passband in the 6-12 GHz band. The resistive absorber consists of a Jerusalem cross and a 160-ohm lumped resistor placed at its center as the absorption element. To facilitate the soldering of the lumped resistor and avoid its crossing, the two intersecting strips of the Jerusalem cross are placed on the top and bottom layers of the "Rogers RO4003 (tm)" substrate, respectively. Each FSR unit consists of four Jerusalem cross metal patterns to achieve absorption of X-band incident waves.

3. The transmissive and absorptive frequency selective surface with low frequency transmission and high frequency absorption characteristics according to claim 1, characterized in that The low-pass frequency selective surface uses a centrally connected square ring with a bend in the line, while the resistive absorber uses a Jerusalem cross with a bend in the line to increase the current path, resulting in a transverse dimension of only 0.14 for the integrated frequency selective surface unit. The total height of the cross-section is only 0.

033. The entire integrated frequency selective surface unit is a rotationally symmetric structure, so it is not sensitive to the polarization of the incident wave. It also adopts structures with angular stability, such as square rings and Jerusalem crosses, so that when the incident angle of TE and TM polarized waves increases from 0° to 40°, the integrated frequency selective surface unit still has good wave transmission and absorption characteristics.

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