Large-angle broadband electromagnetic wave absorber of metal-free floor structure based on reciprocity principle

Abstract

The invention discloses a reciprocity principle-based large-angle broadband electromagnetic wave absorber without a metal floor structure, and belongs to the technical field of electromagnetic stealth. The electromagnetic organelle comprises a plurality of vertically-placed wave-absorbing structure units which are arranged in a rectangular period and a plurality of horizontally-placed wave-absorbing suppression units which are arranged in a rectangular period. In the electromagnetic organelle, a wave-absorbing structure unit is vertically placed above a wave-absorbing suppression unit; the wave absorbing structure unit is used for absorbing large-angle incident electromagnetic waves; the wave-absorbing suppression unit plays a role in reducing the transmission coefficient as much as possible and achieving impedance matching; the structure can ensure the transmission of in-band low-frequency normal incident electromagnetic waves while absorbing out-of-band high-frequency large-angle incident electromagnetic waves.

Description

Technical Field

[0001] The invention belongs to the technical field of electromagnetic stealth, and in particular relates to a large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle. Background Art

[0002] In modern electronic warfare, with the rapid development of radar detection technology, the electromagnetic stealth problem of aircraft has also received widespread attention. In order to reduce the radar cross section (RCS) of aircraft, the method of loading electromagnetic absorbers on the surface of aircraft is usually adopted. In an increasingly complex application environment, when an aircraft is detected by an air-to-air radar, the detection electromagnetic wave of the radar is usually incident in a direction horizontal to the direction of the fuselage. At this time, the electromagnetic wave incident angle faced by the electromagnetic absorber loaded on the front and rear inner surfaces of the aircraft can be as high as 85° and the range of the incident angle is relatively small. The research direction of the existing technology is mostly focused on electromagnetic absorbers under some small angles of incidence. Existing research work shows that: under normal circumstances, when the electromagnetic wave is incident at ±30° obliquely, the absorption bandwidth of 90% absorption rate will be sharply narrowed, and the absorption efficiency in the absorption band will deteriorate, and as the oblique incidence angle of the electromagnetic wave increases, the above situation continues to intensify. When the oblique incidence angle of the electromagnetic wave reaches ±50°, the absorber has lost the ability to work normally and cannot reduce the RCS at slight incidence. Therefore, in order to ensure the stealth performance of the system, it is of great significance to study frequency selective surfaces with broadband absorbing performance when electromagnetic waves are incident at large angles.

[0003] The Chinese patent application with application number 202210553692.7 and invention name “A broadband dual-polarization electromagnetic absorber at extreme incident angles and its design method” discloses a broadband dual-polarization electromagnetic absorber at extreme incident angles and its design method. The structure can show efficient absorption performance when TE and TM dual-polarization electromagnetic waves are incident at an incident angle of 75° and the working frequency band is within 5.34-10.24GHz. However, it is loaded with a metal floor, which cannot allow the normal incident electromagnetic waves in the band to pass through, and the relative bandwidth of absorption is only 62.9%. The Chinese patent application with publication number CN108718005A and invention name “Dual-resonance microwave absorber” discloses a dual-resonance microwave absorber. When the incident angle is greater than 50°, the absorption efficiency deteriorates sharply, and the stability problem of large-angle incidence above 50° still needs to be solved. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a large-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle, which can achieve broadband electromagnetic absorption in the frequency band range of 1.65GHz to 16.56GHz when the incident angle is 85°, and the absorption rate is above 70%.

[0005] The technical problem proposed by the present invention is solved in this way:

[0006] A wide-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle, comprising N vertically placed absorbing structural units arranged in a rectangular periodic pattern and N horizontally placed absorbing suppression units arranged in a rectangular pattern, wherein N is a positive integer ≥ 2;

[0007] The absorbing structure unit comprises 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 comprises a metal resonance structure and a metal floor; the metal resonance structure comprises 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 symmetry axis 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 and the center position of the lower edge of the first dielectric layer; the metal floor is a grounding patch with a concave structure, which is distributed on both sides of the metal resonance structure, and a gap is left between the lower circular patch and the two rectangular metal strips; two resistors are respectively embedded between the first rectangular metal strip and the metal floor, between the lower circular patch and the metal floor, and between the second rectangular metal strip and the metal floor;

[0008] The wave absorbing suppression unit comprises 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 gaps at four positions closest to the edge of the second dielectric layer, and lumped resistance elements are embedded in the gaps;

[0009] Adjacent wave absorbing structural units are closely fitted and arranged, and adjacent wave absorbing suppression units are closely fitted and arranged, and the lower edges of the wave absorbing structural units are connected to the sides of the wave absorbing suppression units.

[0010] Furthermore, the short side W of the wave absorbing structure unit is 8.33 mm, and the long side L is 13.33 mm; the periodic side length W of the wave 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 floor is 0.5 mm, the spacing g2 between the lower circular patch and the metal floor is 0.24 mm, and the spacing g3 between the second rectangular metal strip and the metal floor is 0.07 mm; the width w of the second rectangular metal strip is g It is 0.33mm.

[0012] Furthermore, the left edge, right edge and bottom edge of the metal floor reach the left edge, right edge and bottom edge of the first dielectric layer, and the height range covers the second rectangular metal strip, the circular patch below 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 of the resistor in the absorbing structure unit H 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 of the embedded lumped resistor element R d 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 of the embedded lumped resistor element R x 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 the present invention, the wave absorbing structural unit is vertically placed above the wave absorbing suppression unit; the wave absorbing structural unit is used to absorb electromagnetic waves incident at a large angle; the wave absorbing suppression unit plays a role in minimizing the transmission coefficient and impedance matching; this structure can absorb high-frequency and large-angle incident electromagnetic waves outside the band while ensuring the transmission of low-frequency and normally incident electromagnetic waves within the band.

[0018] In the present invention, the design method of the absorbing structure unit is based on the analogy analysis of the antenna reciprocity principle in the design of electromagnetic absorbers. First, an antenna array with good radiation characteristics at a large angle is designed, and then the feeding port of the antenna is loaded with an appropriate matching load, so that an electromagnetic absorber array with absorbing performance equivalent to the radiation performance of the original antenna can be designed. In order to achieve better impedance matching, it is hoped that within a wide bandwidth, the real part of the input impedance of the antenna unit fluctuates within a smaller range, and the imaginary part is close to zero, so that a constant lumped resistance element can be used to achieve better impedance matching. However, since the antenna designed in the present invention has a wide bandwidth, it is difficult to achieve impedance matching through a resistor. For the symmetry of the structure and to achieve a better matching effect, the present invention adds six resistors on the basis of the designed antenna to improve the absorption rate of the absorbing structure. The design idea of ​​the absorbing suppression unit is to further optimize the impedance matching between the above-mentioned absorbing structure unit and the free space, and to achieve a wide-angle matching effect.

[0019] The beneficial effects of the present invention are:

[0020] 1. The electromagnetic organelle of the present invention can achieve broadband wave absorption at 85° incidence under specific polarization and specific azimuth, and the relative wave absorption bandwidth is 163.76%. It can ensure that the aircraft is not detected by out-of-band large-angle radar within a wider frequency, and has strong application requirements;

[0021] 2. The electromagnetic cell organelle of the present invention has no metal floor on the horizontal plane. While absorbing the electromagnetic waves incident at large angles outside the band in a wide frequency band, it does not affect the transmission of the electromagnetic waves incident normally within the band, which greatly improves the limitation that the absorber with a floor can only absorb waves outside the band but cannot transmit waves within the band.

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

[0023] 4. The design method of the electromagnetic organelle described in the present invention can be extended to design electromagnetic absorbers with specific incident angles and azimuth planes, and the design of the electromagnetic absorber can be converted into the design of the antenna. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the front structure of the large-angle broadband electromagnetic absorber of the present invention;

[0025] Figure 2 It is a schematic diagram of the side structure of the large-angle broadband electromagnetic absorber of the present invention;

[0026] Figure 3 is a schematic structural diagram of the wave absorbing structural unit of the present invention;

[0027] Figure 4 is a front structural schematic diagram of the wave absorbing and suppressing unit of the present invention;

[0028] Figure 5 is a schematic diagram of the back structure of the wave absorbing and suppressing unit of the present invention;

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

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

[0031] Figure 8 is the electric field polarization pattern of the coplanar waveguide broadband antenna described in the embodiment, wherein (a) 7 GHz, (b) 9 GHz, (c) 11 GHz;

[0032] Fig. 9It is a schematic diagram of the Port and polarization mode of the wide-angle broadband electromagnetic absorber described in the embodiment;

[0033] Fig.10 : is a transmission coefficient curve of the wide-angle broadband electromagnetic absorber described in the embodiment at 85° incidence at different ports and different polarization modes;

[0034] Fig.11 It is the y-polarized absorption rate of the wide-angle broadband electromagnetic absorber described in the embodiment under 85° incidence at the MIN port. DETAILED DESCRIPTION

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

[0036] This embodiment provides a large-angle broadband electromagnetic absorber without a metal floor structure based on the reciprocity principle, and its front view is as follows: Figure 1 As shown, the side view is Figure 2 As shown, it includes three vertically placed wave absorbing structural units arranged in a rectangular periodic pattern and three horizontally placed wave absorbing suppression units arranged in a rectangular pattern;

[0037] The structural schematic diagram of the wave absorbing structural unit is as follows Figure 3 As shown, it 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 resonance structure and a metal floor; the metal resonance 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 symmetry axis 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 and the center position of the lower edge of the first dielectric layer; the metal floor is a grounding patch with a concave structure, which is distributed on both sides of the metal resonance structure, and a gap is left between the lower circular patch and the two rectangular metal strips; the left edge, right edge and lower edge of the metal floor reach the left edge, right edge and lower edge of the first dielectric layer, and the 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 floor, between the lower circular patch and the metal floor, and between the second rectangular metal strip and the metal floor respectively;

[0038] The front structural diagram and the back structural diagram of the wave absorbing suppression unit are respectively as shown in 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 are provided with gaps at four positions closest to the edge of the second dielectric layer, and lumped resistance elements are embedded in the gaps;

[0039] Adjacent wave absorbing structural units are closely fitted and arranged, and adjacent wave absorbing suppression units are closely fitted and arranged, and the lower edges of the wave absorbing structural units are connected to the sides of the wave absorbing suppression units.

[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 floor is 0.5 mm, the spacing g2 between the lower circular patch and the metal floor is 0.24 mm, and the spacing g3 between the second rectangular metal strip and the metal floor is 0.07 mm; the width w of the second rectangular metal strip is g The height L1 of the metal floor is 6.97 mm. The resistance value R of the resistor in the wave absorbing structure unit is H 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 of the embedded lumped resistor element R d 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 of the embedded lumped resistor element R x 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 with a set azimuth plane and a set incident angle, comprising the following steps:

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

[0043] S2. Design an antenna structure with good working performance and a directional pattern and bandwidth that meet the set indicators; since it is hoped that the absorber has no metal floor, the designed antenna structure should also have no metal floor structure and use a coplanar waveguide feeding antenna, such as Figure 6 As shown;

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

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

[0046] Figure 7 : 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 frequency band range of 7.76-14.11 GHz.

[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 angle range of 0°±90° on the yoz plane.

[0048] Fig. 9 Schematic diagram of the Port port and polarization mode corresponding to this embodiment. The electromagnetic wave can be incident from the port MAX or port MIN as shown in the figure, the electromagnetic wave propagation direction is k, and the incident angle is θ. When the electromagnetic wave is incident vertically, the electromagnetic field can be divided into x-polarization and y-polarization according to the direction of the electric field along the x-axis and the y-axis. When the electromagnetic wave is incident at a certain angle θ, the electromagnetic field can be decomposed into a vertical polarization (TM polarization) component and a horizontal polarization (TE polarization) component according to the direction of the electric field perpendicular to the ground and the direction of the electric field parallel to the ground.

[0049] Fig.10The transmission curves of the wide-angle broadband electromagnetic absorber set in this embodiment are in four cases: x-polarization from MAX to MIN port, y-polarization from MAX port to MIN port, x-polarization from MIN port to MAX port, and y-polarization from MIN port to MAX port. It can be seen that when the electromagnetic wave is incident on the surface of the structure at a certain angle, because the structure is not a rotationally symmetric structure in the xoy plane, the port MAX and the port MIN are not reciprocal. Since the absorbing structural units are arranged only along the y direction, the electric field incident along the y polarization direction will be affected by it, and part of the electric field energy incident in the y polarization direction will be induced and absorbed 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 incident electric field in the x-polarization direction. In the range of 0-1.65GHz, the absorber has basically no effect on the transmission performance of the x-polarized electromagnetic wave, and introduces an insertion loss of about 7dB when the y-polarized electromagnetic wave is transmitted. It has a good inhibitory effect on electromagnetic waves incident in the high-frequency range of 1.65-16.56GHz.

[0050] Fig.11 is the absorption rate of the wide-angle broadband electromagnetic absorber of this embodiment under the y-polarization incident at 85° at the MIN port. As we know, the absorption rate can be defined as A=1-|S 11 | 2 -|S 21 | 2 Therefore, the absorber can achieve a high absorption efficiency of more than 70% at an incident angle of 85° within 1.65-16.56 GHz.

[0051] The present invention designs a wave absorber for broadband absorption of electromagnetic waves incident at large angles, which can be applied to the fields of wireless communication of antenna covers and stealth of aircraft.

[0052] In summary, the present invention discloses a design of a wide-angle broadband electromagnetic absorber without a metal floor structure. The absorber can achieve more than 70% efficient absorption at an incident angle of 85° within 1.65-16.56GHz (relative bandwidth 163.76%) without a metal floor in the incident direction, while maintaining the transmission of waves within the band with reasonable insertion loss.

Claims

1. A wide-angle broadband electromagnetic absorber with a metal-free floor structure based on the reciprocity principle, characterized in that: It comprises N vertically placed wave absorbing structural units arranged in a rectangular periodic pattern and N horizontally placed wave absorbing suppression units arranged in a rectangular pattern, wherein N is a positive integer ≥ 2; The absorbing structure unit comprises 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 comprises a metal resonance structure and a metal floor; the metal resonance structure comprises 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 symmetry axis 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 and the center position of the lower edge of the first dielectric layer; the metal floor is a grounding patch with a concave structure, which is distributed on both sides of the metal resonance structure, and a gap is left between the lower circular patch and the two rectangular metal strips; two resistors are respectively embedded between the first rectangular metal strip and the metal floor, between the lower circular patch and the metal floor, and between the second rectangular metal strip and the metal floor; The wave absorbing suppression unit comprises 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 gaps at four positions closest to the edge of the second dielectric layer, and lumped resistance elements are embedded in the gaps; Adjacent wave absorbing structural units are closely fitted and arranged, adjacent wave absorbing suppression units are closely fitted and arranged, and the lower edges of the wave absorbing structural units are connected to the sides of the wave absorbing suppression units.

2. The large-angle broadband electromagnetic absorber without metal floor structure based on the reciprocity principle according to claim 1 is characterized in that: The short side W of the wave absorbing structural unit is 8.33 mm, and the long side L is 13.33 mm; the periodic side length W of the wave absorbing suppression unit is 8.33 mm.

3. The large-angle broadband electromagnetic absorber without metal floor structure based on the reciprocity principle according to claim 1 is characterized in that: 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 floor is 0.5 mm, the spacing g2 between the lower circular patch and the metal floor is 0.24 mm, and the spacing g3 between the second rectangular metal strip and the metal floor is 0.07 mm; the width w of the second rectangular metal strip is g It is 0.33mm.

4. The large-angle broadband electromagnetic absorber without metal floor structure based on the reciprocity principle according to claim 1 is characterized in that: The left edge, right edge and bottom edge of the metal floor reach the left edge, right edge and bottom edge of the first dielectric layer, and the height range covers the second rectangular metal strip, the circular patch below 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 without metal floor structure based on the reciprocity principle according to claim 1 is characterized in that: The resistance value R of the resistor in the absorbing structure unit H is 70 ohms.

6. The large-angle broadband electromagnetic absorber without metal floor structure based on the reciprocity principle according to claim 1 is 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 of the embedded lumped resistor element R d is 1 ohm.

7. The large-angle broadband electromagnetic absorber without metal floor structure based on the reciprocity principle according to claim 1 is 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 of the embedded lumped resistor element R x is 100 ohms.

8. The large-angle broadband electromagnetic absorber without metal floor structure based on the reciprocity principle according to claim 1 is characterized in that: 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.

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