A large-angle broadband electromagnetic wave absorber based on the principle of reciprocity for metal-free floor structure

Abstract

This invention discloses a large-angle broadband electromagnetic absorber based on the reciprocity principle and a metal-free floor structure, belonging to the field of electromagnetic stealth technology. The electromagnetic absorber of this invention includes several vertically placed, rectangularly arranged absorbing structural units and several horizontally placed, rectangularly arranged absorbing suppression units. In this electromagnetic absorber, the absorbing structural units are vertically placed above the absorbing suppression units; the absorbing structural units are used to absorb electromagnetic waves incident at large angles; the absorbing suppression units serve to minimize the transmission coefficient and achieve impedance matching; this structure can absorb high-frequency, large-angle incident electromagnetic waves outside the band while ensuring the transmission of low-frequency, normally incident electromagnetic waves within the band.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic stealth technology, specifically relating to a large-angle broadband electromagnetic absorber based on a metal-free floor structure using the reciprocity principle. Background Technology

[0002] In modern electronic warfare, with the rapid development of radar detection technology, the electromagnetic stealth of aircraft has received widespread attention. To reduce the radar cross section (RCS) of an aircraft, electromagnetic absorbers are typically installed on its surface. In increasingly complex environments, when an aircraft is detected by air-to-air radar, the radar's electromagnetic waves are usually incident horizontally to the fuselage. In this case, the electromagnetic absorbers installed on the front and rear inner surfaces of the aircraft face electromagnetic wave incident angles as high as 85°, and the range of these angles is relatively small. Current research focuses primarily on electromagnetic absorbers for small incident angles. Existing research shows that, typically, when electromagnetic waves are incident at an angle of ±30°, the absorption bandwidth with a 90% absorption rate narrows sharply, and the absorption efficiency within the absorption band deteriorates. Moreover, this situation worsens with increasing incident angles. When the incident angle reaches ±50°, the absorber loses its ability to function properly and cannot reduce the RCS at slightly oblique angles. Therefore, in order to ensure the stealth performance of the system, it is of great significance to study frequency-selective surfaces with broadband absorption performance when electromagnetic waves are incident at large angles.

[0003] Chinese patent application No. 202210553692.7, entitled "A Broadband Dual-Polarized Electromagnetic Absorber and Its Design Method under Extreme Incident Angles," discloses a broadband dual-polarized electromagnetic absorber and its design method under extreme incident angles. This structure exhibits high absorption performance for TE and TM dual-polarized electromagnetic waves in the 5.34-10.24 GHz operating frequency band when the incident angle is 75°. However, it has a metal ground plane, which prevents the transmission of in-band normally incident electromagnetic waves, and the relative absorption bandwidth is only 62.9%. Chinese patent application No. CN108718005A, entitled "Dual-Resonant Microwave Absorber," discloses a dual-resonant microwave absorber. When the incident angle is greater than 50°, the absorption efficiency deteriorates sharply, and the stability problem at large incident angles above 50° still needs to be solved. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a wide-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle. It can achieve broadband electromagnetic absorption in the frequency band from 1.65 GHz to 16.56 GHz when the incident angle is 85°, with an absorption rate of over 70%.

[0005] The technical problem addressed by this invention is solved as follows:

[0006] A wide-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle includes N vertically placed, rectangularly arranged absorbing structural units and N horizontally placed, rectangularly arranged absorbing suppression units, where N is a positive integer ≥ 2.

[0007] The absorbing structure unit includes a first dielectric layer and a first metal layer located on the upper surface of the first dielectric layer; the first dielectric layer is rectangular with its short side horizontal; the first metal layer includes a metal resonant structure and a metal ground plane; the metal resonant structure includes two circular patches and two rectangular metal strips; the centers of the two circular patches and the two rectangular metal strips are located on the vertical axis of symmetry of the first dielectric layer; the first rectangular metal strip connects the two circular patches, and the second rectangular metal strip connects the lower circular patch to the center position of the lower edge of the first dielectric layer; the metal ground plane is a grounding patch with a recessed structure, distributed on both sides of the metal resonant structure, with a gap between it and the lower circular patch and the two rectangular metal strips; two resistors are embedded between the first rectangular metal strip and the metal ground plane, between the lower circular patch and the metal ground plane, and between the second rectangular metal strip and the metal ground plane, respectively.

[0008] The wave absorption suppression unit includes a second dielectric layer, a second metal layer located on the upper surface of the second dielectric, and a third metal layer located on the lower surface of the second dielectric; the second dielectric layer is square; the second metal layer and the third metal layer are open metal rings with different diameters; the open metal rings are provided with slits at the four positions closest to the edge of the second dielectric layer, and lumped resistor elements are embedded in the slits;

[0009] Adjacent absorbing structural units are arranged in close fit, and adjacent absorbing and suppressing units are arranged in close fit. The lower edge of the absorbing structural unit is connected to the edge of the absorbing and suppressing unit.

[0010] Furthermore, the short side W of the absorbing structure unit is 8.33 mm, and the long side L is 13.33 mm; the periodic side length W of the absorbing suppression unit is 8.33 mm.

[0011] Furthermore, the radius r1 of the upper circular patch of the metal resonant structure is 2.67 mm, and the radius r2 of the lower circular patch is 2.07 mm; the spacing g1 between the first rectangular metal strip and the metal ground plane is 0.5 mm, the spacing g2 between the lower circular patch and the metal ground plane is 0.24 mm, and the spacing g3 between the second rectangular metal strip and the metal ground plane is 0.07 mm; the width w of the second rectangular metal strip... g It is 0.33mm.

[0012] Furthermore, the left, right, and bottom edges of the metal floor extend to the left, right, and bottom edges of the first dielectric layer, respectively, and the height range covers the second rectangular metal strip, the lower circular patch, and part of the first rectangular metal strip; the height L1 of the metal floor is 6.97 mm.

[0013] Furthermore, the resistance value R in the absorbing structure unit H It is 70 ohms.

[0014] Furthermore, the inner diameter r3 of the open metal ring in the second metal layer is 2.78 mm, the outer diameter r4 is 3.13 mm, and the resistance value R of the embedded lumped resistor element is... d It is 1 ohm.

[0015] Furthermore, the inner diameter r5 of the open metal ring in the third metal layer is 2.08 mm, the outer diameter r6 is 2.78 mm, and the resistance value R of the embedded lumped resistor element is... x It is 100 ohms.

[0016] Furthermore, the dielectric constant of the first dielectric layer is 2.67, the loss tangent is 0.0375, and the dielectric thickness is 0.2 mm; the dielectric constant of the second dielectric layer is 2.2, the loss tangent is 0.0009, and the dielectric thickness is 0.34 mm.

[0017] In this invention, the absorbing structure unit is placed vertically above the absorbing suppression unit; the absorbing structure unit is used to absorb electromagnetic waves incident at large angles; the absorbing suppression unit serves to minimize the transmission coefficient and achieve impedance matching; this structure can absorb high-frequency electromagnetic waves incident at large angles outside the band while ensuring the transmission of low-frequency electromagnetic waves incident normally within the band.

[0018] In this invention, the design method of the absorbing structure unit is based on an analogous analysis of the antenna reciprocity principle in the design of electromagnetic absorbers. First, an antenna array with good radiation characteristics at large angles is designed. Then, an appropriate matching load is applied to the antenna's feed port, allowing the design of an electromagnetic absorber array with absorption performance comparable to the original antenna's radiation performance. To achieve better impedance matching, it is desirable that the real part of the antenna unit's input impedance fluctuates within a small range over a wide bandwidth, while the imaginary part approaches zero. This allows for good impedance matching using a constant lumped resistance element. However, due to the wide bandwidth of the antenna designed in this invention, impedance matching with a single resistor is difficult to achieve. For structural symmetry and to achieve better matching, six resistors are added to the designed antenna, improving the absorption rate of the absorbing structure. The design concept of the absorbing suppression unit is to further optimize the impedance matching between the aforementioned absorbing structure unit and free space, achieving a wide-angle matching effect.

[0019] The beneficial effects of this invention are:

[0020] 1. The electromagnetic absorber described in this invention can achieve broadband absorption when incident at 85° under specific polarization and azimuth plane, with a relative absorption bandwidth of 163.76%. It can ensure that the aircraft is not detected by out-of-band large-angle radar within a wide frequency range, and has strong application requirements.

[0021] 2. The electromagnetic absorber of the present invention has no metal floor on the horizontal plane. While absorbing electromagnetic waves incident at large angles outside the band within a wide frequency band, it does not affect the transmission of electromagnetic waves incident normally within the band. This greatly improves the limitation of previous absorbers with loaded floors that could only absorb waves outside the band and could not transmit waves inside the band.

[0022] 3. The electromagnetic absorber of the present invention has a profile height of only 0.075 relative to the lowest operating frequency, which is low and suitable for scenarios with strict requirements on absorber thickness;

[0023] 4. The design method of the electromagnetic absorber described in this invention can be extended to design electromagnetic absorbers with specific incident angles and azimuth planes, transforming the design of electromagnetic absorbers into the design of antennas. Attached Figure Description

[0024] Figure 1 This is a front structural schematic diagram of the large-angle broadband electromagnetic absorber described in this invention;

[0025] Figure 2 This is a side view of the large-angle broadband electromagnetic absorber described in this invention.

[0026] Figure 3 This is a schematic diagram of the structure of the wave-absorbing structural unit described in this invention;

[0027] Figure 4 This is a front structural schematic diagram of the wave-absorbing suppression unit described in this invention;

[0028] Figure 5 This is a schematic diagram of the back structure of the wave-absorbing suppression unit described in this invention;

[0029] Figure 6 This is a schematic diagram of the coplanar waveguide broadband antenna designed in the embodiment;

[0030] Figure 7 This is the reflection coefficient curve of the coplanar waveguide broadband antenna described in the embodiment;

[0031] Figure 8 These are the electric field polarization patterns of the coplanar waveguide broadband antenna described in the embodiments, where (a) 7 GHz, (b) 9 GHz, and (c) 11 GHz.

[0032] Figure 9This is a schematic diagram of the port and polarization mode of the large-angle broadband electromagnetic absorber described in the embodiment;

[0033] Figure 10 These are the transmission coefficient curves of the large-angle broadband electromagnetic absorber described in the embodiment under different ports and different polarization modes at 85° incident angle;

[0034] Figure 11 It is the absorption rate of the large-angle broadband electromagnetic absorber described in the embodiment under 85° incident at the MIN port with y-polarization. Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0036] This embodiment provides a large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle, as shown in the front view below. Figure 1 As shown, the side view is as follows Figure 2 As shown, it includes three vertically placed, rectangularly arranged, periodically absorbing structural units and three horizontally placed, rectangularly arranged, absorbing and suppressing units.

[0037] The structural diagram of the absorbing structure unit is shown below. Figure 3 As shown, the system includes a first dielectric layer and a first metal layer located on the upper surface of the first dielectric layer. The first dielectric layer is rectangular with its short side horizontal. The first metal layer includes a metal resonant structure and a metal ground plane. The metal resonant structure includes two circular patches and two rectangular metal strips. The centers of the two circular patches and the two rectangular metal strips are located on the vertical axis of symmetry of the first dielectric layer. The first rectangular metal strip connects the two circular patches, and the second rectangular metal strip connects the lower circular patch to the center of the lower edge of the first dielectric layer. The metal ground plane is a grounding patch with a recessed structure, distributed on both sides of the metal resonant structure, with a gap between it and the lower circular patch and the two rectangular metal strips. The left, right, and lower edges of the metal ground plane reach the left, right, and lower edges of the first dielectric layer, respectively, and its height range covers the second rectangular metal strip, the lower circular patch, and part of the first rectangular metal strip. Two resistors are embedded between the first rectangular metal strip and the metal ground plane, between the lower circular patch and the metal ground plane, and between the second rectangular metal strip and the metal ground plane.

[0038] The front and back structural diagrams of the wave-absorbing suppression unit are shown below. Figure 4 and Figure 5As shown, it includes a second dielectric layer, a second metal layer located on the upper surface of the second dielectric, and a third metal layer located on the lower surface of the second dielectric; the second dielectric layer is square; the second metal layer and the third metal layer are open metal rings with different diameters; the open metal rings have gaps at four positions closest to the edge of the second dielectric layer, and lumped resistor elements are embedded in the gaps;

[0039] Adjacent absorbing structural units are arranged in close fit, and adjacent absorbing and suppressing units are arranged in close fit. The lower edge of the absorbing structural unit is connected to the edge of the absorbing and suppressing unit.

[0040] In this embodiment, the short side W of the absorbing structure unit is 8.33 mm, and the long side L is 13.33 mm; the periodic side length W of the absorbing suppression unit is 8.33 mm. The radius r1 of the upper circular patch of the metal resonant structure is 2.67 mm, and the radius r2 of the lower circular patch is 2.07 mm; the spacing g1 between the first rectangular metal strip and the metal ground plane is 0.5 mm, the spacing g2 between the lower circular patch and the metal ground plane is 0.24 mm, and the spacing g3 between the second rectangular metal strip and the metal ground plane is 0.07 mm; the width w of the second rectangular metal strip... g The thickness is 0.33mm. The height L1 of the metal floor is 6.97mm. The resistance R in the absorbing structure unit is... H The resistance is 70 ohms. The inner diameter r3 of the open metal ring in the second metal layer is 2.78 mm, the outer diameter r4 is 3.13 mm, and the resistance R of the embedded lumped resistor element is... d The resistance is 1 ohm. The inner diameter r5 of the open metal ring in the third metal layer is 2.08 mm, the outer diameter r6 is 2.78 mm, and the resistance R of the embedded lumped resistor element is... x The dielectric constant is 100 ohms. The dielectric constant of the first dielectric layer is 2.67, the loss tangent is 0.0375, and the dielectric thickness is 0.2 mm; the dielectric constant of the second dielectric layer is 2.2, the loss tangent is 0.0009, and the dielectric thickness is 0.34 mm.

[0041] This embodiment also provides a design method for an electromagnetic absorber that sets the azimuth plane and the incident angle, including the following steps:

[0042] S1. Set the electrical performance parameters of the electromagnetic absorber, including polarization direction, incident angle, azimuth plane and absorption bandwidth;

[0043] S2. Design an antenna structure with a radiation pattern and bandwidth that meet the set specifications and exhibit good performance; since it is desirable for the absorber to have no metal ground plane, the designed antenna structure should also have no metal ground plane, employing a coplanar waveguide-fed antenna, such as... Figure 6 As shown;

[0044] S3. Replace the low-noise amplifier, mixer, and other components connected to the antenna end with impedance-matching resistors of appropriate values. The principle used here is as follows: the antenna reciprocity principle states that when an antenna is used as a transmitting antenna, its input impedance and impedance bandwidth are the same as when it is used as a receiving antenna. If the low-noise amplifier, mixer, and other components connected to the receiving antenna end are replaced with lossy components (such as resistors) of appropriate values ​​with impedance matching, the antenna can still receive electromagnetic waves in free space and achieve good absorption under specific angles of electromagnetic wave incidence. This method can be extended to design electromagnetic absorbers with defined incident angles and azimuth planes, transforming the design of electromagnetic absorbers into the design of antennas.

[0045] S4. Perform simulation optimization on the initial model to obtain a large-angle broadband electromagnetic absorber that meets the electrical performance indicators.

[0046] Figure 7 This is the reflection coefficient curve of the coplanar waveguide broadband antenna described in this embodiment. It can be seen that the antenna is well matched in the 7.76-14.11GHz frequency band.

[0047] Figure 8 (a) Figure 8 (b) and Figure 8 (c) are the electric field polarization patterns of the coplanar waveguide broadband antenna described in this embodiment at 7 GHz, 9 GHz and 11 GHz respectively. The electric field polarization direction is along the x direction, and the radiation pattern forms omnidirectional radiation within the 0°±90° angle range of the yoz plane.

[0048] Figure 9 This diagram illustrates the Port and polarization scheme corresponding to this embodiment. Electromagnetic waves can be incident from either Port MAX or Port MIN as shown in the diagram, with the propagation direction being k and the incident angle being θ. When the electromagnetic wave is incident perpendicularly, the electromagnetic field can be divided into x-polarization and y-polarization based on the electric field direction along the x-axis and y-axis. When the electromagnetic wave is incident at a certain angle θ, the electromagnetic field can be decomposed into vertical polarization (TM polarization) and horizontal polarization (TE polarization) components based on whether the electric field direction is perpendicular to the ground or parallel to the ground.

[0049] Figure 10The transmission curves of the large-angle broadband electromagnetic absorber in this embodiment are shown for four cases: x-polarization from the MAX to the MIN port, y-polarization from the MAX to the MIN port, x-polarization from the MIN to the MAX port, and y-polarization from the MIN to the MAX port. It can be seen that when an electromagnetic wave is incident on the surface of the structure at a certain angle, because the structure is not rotationally symmetric in the xoy plane, ports MAX and MIN are not reciprocal. Since the absorbing structure units are arranged only along the y-direction, the electric field incident along the y-polarization direction is affected, inducing and absorbing some of the electric field energy incident along the y-polarization direction at low frequencies, resulting in a decrease in the transmission coefficient of the y-polarized electromagnetic wave. However, the structure has no effect on the transmission coefficient of the electric field incident along the x-polarization direction. In the 0-1.65 GHz range, the absorber has virtually no effect on the transmission performance of x-polarized electromagnetic waves, but introduces an insertion loss of about 7 dB when transmitting y-polarized electromagnetic waves. It effectively suppresses electromagnetic waves incident in the high-frequency range of 1.65 – 16.56 GHz.

[0050] Figure 11 The absorption rate of the wide-angle broadband electromagnetic absorber described in this embodiment is the y-polarized absorption rate at an 85° incident angle at the MIN port. It is known that the absorption rate can be defined as... Therefore, this absorber can achieve a high absorption efficiency of over 70% at an 85° incident angle within the 1.65-16.56 GHz range.

[0051] This invention designs a microwave absorber for broadband absorption of electromagnetic waves incident at large angles, which can be applied to wireless communication of radomes and aircraft stealth.

[0052] In summary, this invention discloses a design for a large-angle broadband electromagnetic absorber with a metal-free floor structure. This absorber achieves over 70% efficient absorption within the 1.65-16.56 GHz band (163.76% relative bandwidth) at an 85° incident angle, while maintaining reasonable insertion loss and internal wave transmission within the band, all without the need for a metal floor in the incident direction.

Claims

1. A large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle, characterized in that, It includes N vertically placed rectangularly arranged absorbing structural units and N horizontally placed rectangularly arranged absorbing suppression units, where N is a positive integer ≥ 2; The absorbing structure unit includes a first dielectric layer and a first metal layer located on the upper surface of the first dielectric layer; the first dielectric layer is rectangular with its short side horizontal; the first metal layer includes a metal resonant structure and a metal ground plane; the metal resonant structure includes two circular patches and two rectangular metal strips; the centers of the two circular patches and the two rectangular metal strips are located on the vertical axis of symmetry of the first dielectric layer; the first rectangular metal strip connects the two circular patches, and the second rectangular metal strip connects the lower circular patch to the center of the lower edge of the first dielectric layer; the metal ground plane is a grounding patch with a recessed structure, distributed on both sides of the metal resonant structure, with a gap between it and the lower circular patch and the two rectangular metal strips; two resistors are embedded between the first rectangular metal strip and the metal ground plane, between the lower circular patch and the metal ground plane, and between the second rectangular metal strip and the metal ground plane, respectively. The wave absorption suppression unit includes a second dielectric layer, a second metal layer located on the upper surface of the second dielectric, and a third metal layer located on the lower surface of the second dielectric; the second dielectric layer is square; the second metal layer and the third metal layer are open metal rings with different diameters; the open metal rings are provided with slits at the four positions closest to the edge of the second dielectric layer, and lumped resistor elements are embedded in the slits; Adjacent absorbing structural units are arranged in close fit, and adjacent absorbing and suppressing units are arranged in close fit. The lower edge of the absorbing structural unit is connected to the edge of the absorbing and suppressing unit.

2. The large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle according to claim 1, characterized in that, The short side W of the absorbing structure unit is 8.33 mm, and the long side L is 13.33 mm; the periodic side length W of the absorbing suppression unit is 8.33 mm.

3. The large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle according to claim 1, characterized in that, The upper circular patch of the metal resonant structure has a radius r1 of 2.67 mm, and the lower circular patch has a radius r2 of 2.07 mm; the distance g1 between the first rectangular metal strip and the metal ground plane is 0.5 mm, the distance g2 between the lower circular patch and the metal ground plane is 0.24 mm, and the distance g3 between the second rectangular metal strip and the metal ground plane is 0.07 mm; the width w of the second rectangular metal strip... g It is 0.33mm.

4. The large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle according to claim 1, characterized in that, The left, right, and bottom edges of the metal floor extend to the left, right, and bottom edges of the first dielectric layer, respectively, and the height range covers the second rectangular metal strip, the lower circular patch, and part of the first rectangular metal strip; the height L1 of the metal floor is 6.97 mm.

5. The large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle according to claim 1, characterized in that, The resistance value R in the absorbing structure unit H It is 70 ohms.

6. The large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle according to claim 1, characterized in that, The inner diameter r3 of the open metal ring in the second metal layer is 2.78 mm, the outer diameter r4 is 3.13 mm, and the resistance value R of the embedded lumped resistor element is... d It is 1 ohm.

7. The large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle according to claim 1, characterized in that, The inner diameter r5 of the open metal ring in the third metal layer is 2.08 mm, the outer diameter r6 is 2.78 mm, and the resistance value R of the embedded lumped resistor element is... x It is 100 ohms.

8. The large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle according to claim 1, characterized in that, The first dielectric layer has a dielectric constant of 2.67, a loss tangent of 0.0375, and a dielectric thickness of 0.2 mm; the second dielectric layer has a dielectric constant of 2.2, a loss tangent of 0.0009, and a dielectric thickness of 0.34 mm.

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