A broadband microwave wave absorber for 75°~85° extreme oblique incidence

Abstract

A kind of wideband microwave wave absorber for 75°~85° extreme oblique incidence belongs to microwave wave absorber design and manufacturing field.Wideband microwave wave absorber is formed by not less than 3*3 identical mixed units arranged uniformly according to fixed interval.Each mixed unit includes two absorption structures based on asymmetric filter network, respectively for wideband impedance matching of space wave under 75° and 80° oblique incidence angle.Two absorption structures differ in geometric dimension of slot line, microstrip line and resistance value of resistive load, so that the characteristic impedance of transmission line is different, impedance matching under different incidence angles is realized.The present application combines the two wideband absorption structures, the mixed unit can not only absorb electromagnetic wave with high efficiency under 75°, 80° incidence angle, but also maintain excellent electromagnetic absorption performance under 85° incidence angle, finally realize wideband absorption characteristics in 75°~85° extreme oblique incidence angle domain;It also has the advantages of single-layer PCB, easy to process and the like.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorber design and manufacturing, and relates to a broadband microwave absorber for 75°~85° extreme oblique incidence. Background Technology

[0002] The continuous development of radar detection technology has placed higher demands on the radar absorbers equipped on stealth targets: they must possess stable absorption characteristics in both wide frequency ranges and specific angular domains. Specifically, radar absorbers should achieve wideband or even ultra-wideband absorption to counter multi-band radar detection; on the other hand, they must maintain efficient absorption even under extreme oblique incidence conditions to adapt to the conformal structural design of stealth targets and effectively cope with multi-radar network detection.

[0003] In recent years, improving absorption bandwidth and angular selectivity using novel materials or resistive loading has become a hot topic in microwave absorber design. In their paper (Truly All-Dielectric Ultrabroadband Metamaterial Absorber: Water-Based and Ground-Free[J]. IEEE Antennas and Wireless Propagation Letters, 2019, 18(3):536-540.), Jianwen Xie et al. designed an all-dielectric metamaterial absorber using a subwavelength water-based resonator, achieving 75° angular stability. However, limited by the proposed metamaterial structure, its absorption bandwidth is only 101.1%, and under 75° incident conditions, the lowest absorptivity within the operating frequency band is less than 60%, making it impossible to simultaneously achieve efficient absorption of incident electromagnetic energy across a wide frequency band and a large angular domain. In their paper (Wideband Dual-Polarized Microwave Absorber at Extremely Oblique Incidence[J]. IEEE Transactions on Antennas and Propagation, 2023, 71(3):2497-2506.), Qihao Lv et al. designed a broadband absorber capable of good absorption at an incident angle of 75° based on calculations of the equivalent permittivity and equivalent permeability using a resistance loading method. However, the absorber's operating bandwidth in the 50°~80° range is only 61.2%, failing to achieve efficient absorption in a specific angular region.

[0004] Therefore, there is an urgent need to develop a microwave absorber that can achieve efficient, stable, and wideband absorption in a specific extreme oblique incidence angle domain, in order to overcome the problems of narrow absorption bandwidth or low absorption efficiency under extreme oblique incidence conditions in existing technologies. Summary of the Invention

[0005] To overcome the shortcomings of the aforementioned technologies, this invention proposes a broadband microwave absorber for the 75°~85° limiting oblique incidence angle domain. Within each periodically arranged hybrid unit of the absorber, this invention includes two absorption structures based on asymmetric filter networks. The absorption structures, designed using composite transmission lines, achieve highly efficient electromagnetic absorption over a wide bandwidth. The two absorption structures in each hybrid unit are arranged in parallel, with one structure specifically designed for efficient absorption at a 75° incidence angle and the other for efficient absorption at an 80° incidence angle. Ultimately, this invention achieves stable electromagnetic absorption with an absorption rate of no less than 70% within a 120% fractional bandwidth and the 75°~85° limiting oblique incidence angle domain. It also offers advantages such as single-layer PCB compatibility and ease of fabrication.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A broadband microwave absorber for 75°~85° extreme oblique incidence is disclosed. The broadband microwave absorber comprises at least 3×3 identical hybrid units 1 uniformly arranged at a fixed spacing. Each hybrid unit 1 includes two absorption structures based on asymmetric filter networks, which perform broadband impedance matching for space waves at different extreme oblique incidence angles. By combining these two broadband absorption structures, the hybrid unit 1 can not only efficiently absorb electromagnetic waves at a 75° incidence angle but also maintain excellent electromagnetic absorption performance at an 80° incidence angle. Ultimately, the absorber achieves broadband absorption characteristics in the 75°~85° extreme oblique incidence angle range. Details are as follows: Furthermore, the two absorption structures in the hybrid unit 1 are a left absorption structure 11 and a right absorption structure 12. When electromagnetic waves are incident at different angles, the incident energy is distributed to the two absorption structures. Both absorption structures are single-layer printed circuit board (PCB) structures, closely arranged and integrated on the same dielectric substrate 101 and metal ground plane 102. The two absorption structures are arranged in parallel, with the left absorption structure 11 designed for a 75° incident angle and the right absorption structure 12 designed for an 80° incident angle. Both absorption structures calculate the input impedance based on an asymmetric filter network to achieve wideband impedance matching. The two absorption structures differ in the geometry of the slot lines and microstrip lines, as well as the resistance value of the resistive load, resulting in different characteristic impedances of their respective transmission lines, thus achieving impedance matching at different incident angles.

[0007] Furthermore, the left-side absorption structure 11 and the right-side absorption structure 12 are basically the same. Both absorption structures are composed of composite transmission lines, including slotted lines, four microstrip lines, resistive loads, and shorting pins. With the positive x-axis direction as the front, the negative x-axis direction as the rear, the positive y-axis direction as the right, the negative y-axis direction as the left, the positive z-axis direction as the top, and the negative z-axis direction as the bottom. Specifically: The left-side absorption structure 11 includes a left-side slot line 111, a left-side first microstrip line 112, a left-side second microstrip line 113, a left-side third microstrip line 114, a left-side fourth microstrip line 115, a left-side resistive load 116, a left-side first short-circuit pin 117, a left-side second short-circuit pin 118, and a left-side third short-circuit pin 119. The left-side slot line 111 is a slot structure located on the metal ground plane 102 behind the dielectric substrate 101. The slot structure extends from the top of the metal ground plane 102 along the negative z-axis, and its top left side is tangent to the left-side first short-circuit pin 117. The metal ground plane 102 is completely covered except for the area where the slot structure is located, and the other structures are located on the front side of the dielectric substrate 101. The left-side slot line 111 is electrically connected to the left-side first microstrip line 112 via the left-side first shorting pin 117; the left-side first to left-side fourth microstrip lines 112-115 are in close contact with the front side of the dielectric substrate 101; the end of the left-side fourth microstrip line 115 is electrically connected to the metal ground plane 102 on the rear side of the dielectric substrate 101 via the left-side second shorting pin 118, wherein the metal ground plane 102 is in close contact with the rear side of the dielectric substrate 101; the left-side resistive load 116 is connected in parallel with the left-side third to left-side fourth microstrip lines 114 and 115, and is electrically connected to the metal ground plane 102 on the rear side of the dielectric substrate 101 via the left-side third shorting pin 119. Specifically, the left-side first shorting pin 117 penetrates the dielectric substrate 101 along the x-axis direction, one end of which is electrically connected to the metal ground plane 102 and tangent to the left-side slot line 111, and the other end of which is electrically connected to the left-side first microstrip line 112. The first microstrip line 112 on the left extends horizontally in the positive y-axis direction from the connection point, bends once at 90°, and then extends in the negative z-axis direction, its end connecting to one end of the second microstrip line 113 on the left. The second microstrip line 113 on the left extends in the negative z-axis direction, bends once at 90°, and then extends in the negative y-axis direction, its end connecting to the third microstrip line 114 on the left. The third microstrip line 114 on the left extends in the negative y-axis direction, bends once at 90°, and then extends in the negative z-axis direction, its end connecting to one pole of the resistive load 116 on the left. The other pole of the resistive load 116 on the left is electrically connected to the metal ground plane 102 through the third shorting pin 119 on the left. The third shorting pin 119 on the left penetrates the dielectric substrate 101 in the x-axis direction, one end of which is electrically connected to the metal ground plane 102, and the other end is electrically connected to the resistive load 116 on the left. Simultaneously, the end of the third microstrip line 114 on the left is also connected in parallel to the fourth microstrip line 115 on the left. The fourth microstrip line 115 on the left extends horizontally towards the positive y-axis from the end of the third microstrip line 114 on the left, and after a 90° bend, it turns and extends towards the positive z-axis. The end of the fourth microstrip line 115 on the left is electrically connected to the metal ground plane 102 via the second shorting pin 118 on the left. The second shorting pin 118 on the left passes through the dielectric substrate 101 along the x-axis, with one end electrically connected to the metal ground plane 102 and the other end electrically connected to the end of the fourth microstrip line 115 on the left.

[0008] Furthermore, the left-side first microstrip line 112, left-side second microstrip line 113, left-side third microstrip line 114, and left-side fourth microstrip line 115 are connected in series end to end. The characteristic impedance of each transmission line segment is set according to the comprehensive calculation results of the filter network to achieve broadband impedance matching at a 75° incident angle. The left-side first microstrip line 112, left-side second microstrip line 113, left-side third microstrip line 114, and left-side fourth microstrip line 115 are bent to reduce the size of the absorption structure.

[0009] Furthermore, the dimensions of the left-side absorption structure 11 are 12mm~20mm along the z-axis, 3mm~6mm along the y-axis, and 0.301mm along the x-axis. The impedance relationships of the transmission lines in the left-side absorption structure 11 are as follows: impedance of left slot line 111 > impedance of left second microstrip line 113 > impedance of left fourth microstrip line 115 > impedance of left first microstrip line 112 > impedance of left third microstrip line 114; the thickness of the dielectric substrate 101 ranges from 0.2mm to 4mm.

[0010] The right-side absorption structure 12 includes a right-side slot line 121, a right-side first microstrip line 122, a right-side second microstrip line 123, a right-side third microstrip line 124, a right-side fourth microstrip line 125, a right-side resistive load 126, a right-side first short-circuit pin 127, a right-side second short-circuit pin 128, and a right-side third short-circuit pin 129. The right-side slot line 121 is a slot structure located on the rear metal ground plane 102 of the dielectric substrate 101. The slot structure extends from the top of the rear metal ground plane 102 along the negative z-axis, and its top left side is tangent to the right-side first short-circuit pin 127. The metal ground plane 102 is completely covered except for the area where the slot structure is located, and the other structures are located on the front side of the dielectric substrate 101. The right-side slot line 121 is electrically connected to the right-side first microstrip line 122 via the right-side first shorting pin 127; the right-side first to right-side fourth microstrip lines 122-125 are in close contact with the front side of the dielectric substrate 101; the right-side fourth microstrip line 125 is electrically connected to the metal ground plane 102 on the rear side of the dielectric substrate 101 via the right-side second shorting pin 128; the right-side resistive load 126 is connected in parallel with the right-side third to right-side fourth microstrip lines 124 and 125, and is electrically connected to the metal ground plane 102 on the rear side of the dielectric substrate via the right-side third shorting pin 129. Specifically, the right-side first shorting pin 127 penetrates the dielectric substrate 101 along the x-axis direction, with one end electrically connected to the metal ground plane 102 and tangent to the right-side slot line 121, and the other end electrically connected to the right-side first microstrip line 122. The first microstrip line 122 on the right extends horizontally in the positive y-axis direction from the connection point, bends once at 90°, and then extends in the negative z-axis direction, its end connecting to one end of the second microstrip line 123 on the right. The second microstrip line 123 on the right extends in the negative z-axis direction, bends once at 90°, and then extends in the negative y-axis direction, its end connecting to the third microstrip line 124 on the right. The third microstrip line 124 on the right extends in the negative y-axis direction, bends once at 90°, and then extends in the negative z-axis direction, its end connecting to one pole of the resistive load 126 on the right. The other pole of the resistive load 126 on the right is electrically connected to the metal ground plane 102 through the third shorting pin 129 on the right. The third shorting pin 129 on the right penetrates the dielectric substrate 101 in the x-axis direction, one end of which is electrically connected to the metal ground plane 102, and the other end is electrically connected to the resistive load 126 on the right. Simultaneously, the end of the third microstrip line 124 on the right is also connected in parallel to the fourth microstrip line 125 on the right. The fourth microstrip line 125 on the right extends horizontally towards the positive y-axis from the end of the third microstrip line 124 on the right, and after a 90° bend, it turns and extends towards the positive z-axis. The end of the fourth microstrip line 125 on the right is electrically connected to the metal ground plane 102 via the second shorting pin 128 on the right. The second shorting pin 128 on the right passes through the dielectric substrate 101 along the x-axis, with one end electrically connected to the metal ground plane 102 and the other end electrically connected to the end of the fourth microstrip line 125 on the right.

[0011] Furthermore, the right-side first microstrip line 122, right-side second microstrip line 123, right-side third microstrip line 124, and right-side fourth microstrip line 125 are connected in series end to end. The characteristic impedance of each transmission line segment is set according to the comprehensive calculation results of the asymmetric filter network to achieve broadband impedance matching at an 80° incident angle. The right-side first microstrip line 122, right-side second microstrip line 123, right-side third microstrip line 124, and right-side fourth microstrip line 125 are bent to reduce the size of the absorption structure.

[0012] Furthermore, the dimensions of the right-side absorption structure 12 are 12mm~20mm along the z-axis, 3mm~6mm along the y-axis, and 0.301mm along the x-axis. The impedance relationships of the transmission lines in the right-side absorption structure 12 are as follows: right-side slot line 121 impedance = right-side fourth microstrip line 125 impedance > right-side second microstrip line 123 impedance > right-side first microstrip line 122 impedance = right-side third microstrip line 124 impedance; the thickness of the dielectric substrate 101 ranges from 0.2mm to 4mm.

[0013] Furthermore, the differences between the right absorption structure 12 and the left absorption structure 11 are as follows: the right slot line 121 and the left slot line 111, the right first microstrip line 122 and the left first microstrip line 112, the right second microstrip line 123 and the left second microstrip line 113, the right third microstrip line 124 and the left third microstrip line 114, the right fourth microstrip line 125 and the left fourth microstrip line 115, and the right resistive load 126 and the left resistive load 116 are different in size and impedance. Furthermore, the composite transmission line form in the two absorption structures is not limited; slotted lines and microstrip lines can be replaced with other types of transmission lines, such as single-wire lines, parallel double lines, etc.

[0014] The beneficial effects of this invention are: This invention utilizes a slotted-microstrip hybrid transmission line structure to achieve quantitative design of broadband electromagnetic absorption characteristics under limiting oblique incidence conditions. The invention integrates two absorption structures optimized for 75° and 80° incident angles respectively into a single hybrid unit. The left absorption structure is independently optimized for the 75° angle, and the right absorption structure is independently optimized for the 80° angle. The two absorption structures are arranged side-by-side in space without overlapping, achieving broadband impedance matching for electromagnetic waves incident at 75° and 80° through their respective slotted-microstrip hybrid transmission lines. This structure eliminates the absorption gap at the limiting large angles of 75° and 80°, enabling the absorber to achieve stable broadband absorption performance in the 75°~85° limiting oblique incidence angle range. Simultaneously, the hybrid unit employs a single-layer printed circuit board structure, which is thin and eliminates the need for multi-layer stacking, facilitating processing and mass production. This invention can achieve average absorption rates of 84.3%, 84.1%, and 79.6% for electromagnetic energy incident at 75°, 80°, and 85° within a 120% fractional bandwidth, respectively, providing a technical method for the predictable design of structurally conformal stealth equipment. Attached Figure Description

[0015] Figure 1 This is a diagram of the overall structure of the absorber.

[0016] Figure 2 This is a three-dimensional structural diagram of a hybrid unit.

[0017] Figure 3 This is a front view of a mixed unit.

[0018] Figure 4 This is the rear view of the mixed unit.

[0019] Figure 5 The simulation curves of the reflection coefficient, transmission coefficient, and absorptivity of the hybrid unit under 0° incident conditions are shown.

[0020] Figure 6 The simulated reflection coefficient, transmission coefficient, and absorptivity curves of the hybrid element under 75° incident conditions are shown.

[0021] Figure 7 The simulated reflection coefficient, transmission coefficient, and absorptivity curves of the hybrid element under 80° incident angle are shown.

[0022] Figure 8 The simulated reflection coefficient, transmission coefficient, and absorptivity curves of the hybrid element under an 89° incident angle are shown.

[0023] Figure 9 The curve showing the variation of the average absorptivity within the simulation band with the incident angle is shown.

[0024] In the figure: 1 is a hybrid unit, 11 is the left absorption structure, and 12 is the right absorption structure; 101 Dielectric substrate, 102 Metal ground plane, 111 Left groove line, 112 Left first microstrip line, 113 Left second microstrip line, 114 Left third microstrip line, 115 Left fourth microstrip line, 116 Left resistive load, 117 Left first shorting pin, 118 Left second shorting pin, 119 Left third shorting pin. 121 Right side slot line, 122 Right side first microstrip line, 123 Right side second microstrip line, 124 Right side third microstrip line, 125 Right side fourth microstrip line, 126 Right side resistive load, 127 Right side first shorting pin, 128 Right side second shorting pin, 129 Right side third shorting pin. Detailed Implementation

[0025] The specific embodiments of the present invention will be described in detail below with reference to the technical solutions and accompanying drawings.

[0026] A broadband microwave absorber for 75°~85° limiting oblique incidence has the following overall structure: Figure 1 As shown, it consists of 3×5 hybrid units 1. The hybrid units 1 are uniformly arranged in a two-dimensional plane, and all hybrid units have completely identical structures, arranged periodically at fixed intervals to form the final broadband microwave absorber. Each hybrid unit 1 has a thickness of 0.301 mm, and the distance between the centers of adjacent hybrid units 1 is determined by the actual operating frequency. Each hybrid unit 1 includes two absorption structures, such as... Figure 2 As shown, the left absorption structure 11 and the right absorption structure 12 are designed for limiting oblique incidence angles of 75° and 80°, respectively. The two absorption structures differ in the geometry of the slot line, microstrip line, and resistive load, resulting in different characteristic impedances of their respective transmission lines, thus achieving broadband impedance matching at their respective angles. Since the left absorption structure 11 has a broadband high absorption response near the 75° incident angle, and the right absorption structure 12 has a broadband high absorption response near the 80° incident angle, their absorption responses naturally connect and complement each other within the 75°~85° angle range. This allows each hybrid unit to achieve efficient electromagnetic absorption with a 120% fractional bandwidth in the limiting oblique incidence angle domain of 75°~85°. Furthermore, since the average absorptivity at 75°, 80°, and 85° incident angles reaches 84.3%, 84.1%, and 79.6%, respectively, the microwave absorber composed of hybrid units 1 simultaneously possesses both limiting angle domain absorption and broadband absorption characteristics. The specific structure of one of the hybrid units 1 is described in detail below: like Figure 3 and Figure 4As shown, the two absorption structures are composed of composite transmission lines. The left absorption structure 11 includes a left slot line 111, a left first microstrip line 112, a left second microstrip line 113, a left third microstrip line 114, a left fourth microstrip line 115, a left resistive load 116, a left first shorting pin 117, a left second shorting pin 118, and a left third shorting pin 119. The right absorption structure 12 includes a right slot line 121, a right first microstrip line 122, a right second microstrip line 123, a right third microstrip line 124, a right fourth microstrip line 125, a right resistive load 126, a right first shorting pin 127, a right second shorting pin 128, and a right third shorting pin 129. The two absorption structures have similar structural compositions; the left absorption structure 11 will be used as an example. The first microstrip line 112 to the fourth microstrip line 115 on the left are located on the front side of the dielectric substrate 101, and the left groove line 111 is located on the rear side of the dielectric substrate 101. The dielectric substrate 101 is made of IT-988GSE material, with a relative permittivity of 3.26 and a thickness of 0.301 mm. The metal ground plane 102 on the rear side of the dielectric substrate 101 has a width of 3 mm and a length of 3 mm. The left resistive load 116 is a resistive film material with a sheet resistance of 50 ohms. The left-side slot line 111 is 0.8 mm wide and 6 mm long; the left-side first microstrip line 112 is 0.12 mm wide and 4.8 mm long; the left-side second microstrip line 113 is 0.12 mm wide and 4.8 mm long; the left-side third microstrip line 114 is 0.55 mm wide and 4.7 mm long; the left-side fourth microstrip line 115 is 0.15 mm wide and 4.8 mm long; the left-side resistive load 116 is 0.37 mm wide and 0.54 mm long; and the diameter of the left-side first to third shorting pins 117-119 is 0.1 mm. The left-side first shorting pin 117 vertically penetrates the dielectric substrate 101, connecting the left-side slot line 111 and the left-side first microstrip line 112. The first microstrip line 112 on the left is bent and connected in series with the second microstrip line 113 on the left; the second microstrip line 113 on the left is bent and connected in series with the third microstrip line 114 on the left; the third microstrip line 114 on the left is bent and connected to the resistive load 116 on the left. The other end of the resistive load 116 on the left is grounded through the third shorting pin 119 on the left. The end of the third microstrip line 114 on the left is also connected in parallel with the fourth microstrip line 115 on the left, which is bent and grounded through the second shorting pin 118 on the left. The first microstrip line 112, the second microstrip line 113, the third microstrip line 114, and the fourth microstrip line 115 on the left are connected in series end to end. The characteristic impedance of each transmission line segment is set according to the integrated design results of the asymmetric filter network to achieve broadband impedance matching at a 75° incident angle. The first microstrip line 112, the second microstrip line 113, the third microstrip line 114, and the fourth microstrip line 115 on the left side each undergo a right-angle bend, which significantly reduces the height of the absorption structure. The impedances of the left slot line 111, the first microstrip line 112, the second microstrip line 113, the third microstrip line 114, the fourth microstrip line 115, and the left resistive load 116 are 153.1Ω, 89.9Ω, 101.2Ω, 72.8Ω, 100.3Ω, and 73.2Ω, respectively.

[0027] The differences between the right-side absorption structure and the left-side absorption structure are as follows: the right-side groove line 121 is 0.4 mm wide and 5.7 mm long; the right-side first microstrip line 122 is 0.3 mm wide and 4.7 mm long; the right-side second microstrip line 123 is 0.15 mm wide and 4.7 mm long; the right-side third microstrip line 124 is 0.3 mm wide and 4.7 mm long; the right-side fourth microstrip line 125 is 0.1 mm wide and 4.8 mm long; the right-side resistive load 126 is 0.46 mm wide and 0.6 mm long; and the diameter of the right-side first to right-side third shorting pins 127-129 is 0.1 mm. The impedances of the right-side slot line 121, right-side first microstrip line 122, right-side second microstrip line 123, right-side third microstrip line 124, right-side fourth microstrip line 125 and right-side resistive load 126 are 95.2Ω, 63.1Ω, 78.7Ω, 63.1Ω, 95.2Ω and 65.4Ω, respectively.

[0028] Furthermore, taking the left-side absorption structure 11 as an example, its working principle is as follows: a plane electromagnetic wave incident from free space is first captured by the left-side slot line 111 and converted into a guided wave in the left-side slot line 111. Next, the guided wave energy is transmitted through the left-side first short-circuit pin 117 to the left-side first microstrip line 112, and then along the series-connected second microstrip line 113 to the third microstrip line 114. It is then transmitted to the parallel-short-circuited fourth microstrip line 115 and the left-side resistive load 116, and is finally completely absorbed by the resistive load 116. The working principle of the right-side absorption structure 12 is similar to that of the left-side absorption structure 11, except that the dimensions and impedances of the slot line, microstrip line, and resistive load in the composite transmission line are different.

[0029] Furthermore, the working principle of the hybrid unit 1 to achieve electromagnetic absorption in the limiting angular domain is as follows: First, based on the incident angle... θ Calculate the input impedance of electromagnetic waves Z in The calculation formula is as follows: (1) in, The impedance of electromagnetic waves in free space is 377 ohms.

[0030] The left-side absorption structure 11 is impedance matched at an incident angle of 75°, at which point the input impedance of the electromagnetic wave is... Z in = Z 0cos75°, where Z 0 = 377Ω is the free-space wave impedance. By optimizing the characteristic impedance of the composite transmission line in the left-side absorption structure 11 to meet the theoretical calculation value of the asymmetric filter network, broadband electromagnetic absorption at a 75° incident angle is finally achieved.

[0031] The right-side absorption structure 12 is impedance matched at an 80° incident angle, at which point the input impedance of the electromagnetic wave is... Z in = Z 0cos80°, where Z 0 = 377Ω is the free-space wave impedance. By optimizing the characteristic impedance of the composite transmission line in the right-side absorption structure 12 to meet the theoretical calculation value of the asymmetric filter network, broadband electromagnetic absorption at an incident angle of 80° is finally achieved.

[0032] The hybrid unit 1 integrates both the left-side absorption structure 11 and the right-side absorption structure 12, thus enabling simultaneous electromagnetic absorption at 75° and 80°. Furthermore, the absorption responses of the two structures overlap and complement each other near the 85° angle, ultimately covering the extreme angular range of 75° to 85°.

[0033] The simulation curves of the absorber hybrid unit at incident angles of 0°, 75°, 80°, and 89° are shown below. Figures 5 to 8 As shown. For the 4~16GHz operating frequency band, at incident angles of 75° and 80°, the in-band main polarization reflection coefficient, main polarization transmission coefficient, cross-polarization reflection coefficient, and cross-polarization transmission coefficient are below -13.8dB and -11.2dB, respectively, with the highest absorptivity reaching 98.2% and 97.5%. At incident angles of 0° and 89°, the in-band main polarization reflection coefficient is significantly improved, and the lowest absorptivity decreases to 27.3% and 20.7%, respectively.

[0034] The curve showing the variation of the average absorptivity within the simulation band of the aforementioned absorber hybrid unit with the incident angle is as follows: Figure 9 As shown in the figure, the horizontal axis represents the incident angle, and the vertical axis represents the average absorptivity within the band. At 0° incident angle, the average absorptivity within the band is 43.8%; at 75° incident angle, it reaches 84.3%; at 80° incident angle, it is 84.1%; and at 89° incident angle, it is 37%. Since the left-side absorption structure 11 and right-side absorption structure 12 in the hybrid unit 1 are designed for 75° and 80° incident angles, the average absorptivity within the band is no higher than 43.8% at both 0° and 89° incident angles. Therefore, this invention exhibits electromagnetic absorption angle selectivity within the range of 0° to 90°.

[0035] Based on the above results, the present invention can maintain a fractional bandwidth of 120% in the limiting oblique incidence angle range of 75° to 85°, and the in-band average absorption rate under 75° and 80° incidence is not less than 84.1%, thus achieving broadband high-efficiency electromagnetic absorption under limiting oblique incidence conditions.

[0036] In the hybrid unit 1, the number of absorption structures can be flexibly adjusted according to performance requirements, such as using one, three, or other numbers of absorption structures. The absorption characteristics of the absorption structures can also be modified according to the target incident angle; for example, all structures can be designed for a 75° oblique incidence angle to improve absorption efficiency at that angle, or all structures can be designed for an 80° incident angle to optimize absorption performance in the corresponding angular region. The form of the composite transmission line in the absorption structure is not limited; slotted lines and microstrip lines can be replaced with other types of transmission lines, such as single-wire lines, parallel double lines, etc. Furthermore, the hybrid unit can also be expanded into a dual-polarization unit.

[0037] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed 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 modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A broadband microwave absorber for 75°~85° limiting oblique incidence, characterized in that, The broadband microwave absorber is composed of no less than 3×3 identical hybrid units (1) arranged uniformly at a fixed interval; each hybrid unit (1) includes two absorption structures based on asymmetric filter networks, which perform broadband impedance matching for space waves under different polar angles of incidence. The two absorption structures in the hybrid unit (1) are the left absorption structure (11) and the right absorption structure (12), respectively. When electromagnetic waves are incident at different angles, the incident energy is distributed to the two absorption structures. Both absorption structures are single-layer printed circuit board structures, closely arranged and integrated on the same dielectric substrate (101) and metal ground plane (102). The two absorption structures are arranged in parallel, wherein the left absorption structure (11) is designed for a 75° incident angle and the right absorption structure (12) is designed for an 80° incident angle. Both absorption structures calculate the input impedance based on the asymmetric filter network to achieve wideband impedance matching. The left absorption structure (11) and the right absorption structure (12) are basically the same. Both absorption structures are composed of composite transmission lines, including slot lines, four microstrip lines, resistive loads and shorting pins; with the positive x-axis direction as the front side, the negative x-axis direction as the rear side, the positive y-axis direction as the right side, the negative y-axis direction as the left side, the positive z-axis direction as the upper side, and the negative z-axis direction as the lower side.

2. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 1, characterized in that, The two absorption structures differ in the geometry of the slot line and microstrip line and the resistance value of the resistive load, resulting in different characteristic impedances of their respective transmission lines, thus achieving impedance matching at different incident angles. By combining these two broadband absorption structures, the hybrid unit (1) can not only efficiently absorb electromagnetic waves at an incident angle of 75°, but also maintain excellent electromagnetic absorption performance at an incident angle of 80°, ultimately achieving broadband absorption characteristics in the 75°~85° limiting oblique incident angle range.

3. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 1, characterized in that, The left absorption structure (11) includes a left groove line (111), a left first microstrip line (112), a left second microstrip line (113), a left third microstrip line (114), a left fourth microstrip line (115), a left resistive load (116), a left first shorting pin (117), a left second shorting pin (118), and a left third shorting pin (119); The left groove line (111) is a slot structure located on the metal floor (102) behind the dielectric substrate (101). The slot structure extends from the top of the metal floor (102) along the negative z-axis. Its top left side is tangent to the left first short-circuit pin (117). The metal floor (102) is completely covered except for the area where the slot structure is located. Other structures are located on the front side of the dielectric substrate (101). The left groove line (111) is electrically connected to the left first microstrip line (112) through the left first shorting pin (117); the left first to left fourth microstrip lines (112)-(115) are closely attached to the front side of the dielectric substrate (101); the end of the left fourth microstrip line (115) is electrically connected to the metal ground plane (102) on the rear side of the dielectric substrate (101) through the left second shorting pin (118); the left resistive load (116) is connected in parallel with the left third to left fourth microstrip lines (114) and (115), and is electrically connected to the metal ground plane (102) on the rear side of the dielectric substrate (101) through the left third shorting pin (119); The left first microstrip line (112), left second microstrip line (113), left third microstrip line (114) and left fourth microstrip line (115) are connected in series from beginning to end; the characteristic impedance of each transmission line is set according to the comprehensive calculation results of the filter network to achieve broadband impedance matching at an incident angle of 75°.

4. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 3, characterized in that, In the left-side absorption structure (11): The first shorting pin (117) on the left penetrates the dielectric substrate (101) along the x-axis. One end of the pin is electrically connected to the metal ground plane (102) and tangent to the left groove line (111). The other end of the pin is electrically connected to the first microstrip line (112) on the left. The first microstrip line (112) on the left extends horizontally in the positive y-axis direction from the connection point of the first shorting pin (117) on the left. After a 90° bend, it turns to extend in the negative z-axis direction. Its end is connected to one end of the second microstrip line (113) on the left. The second microstrip line (113) on the left extends in the negative z-axis direction. After a 90° bend, it turns to extend in the negative z-axis direction. Extending in the negative y-axis direction, the end is connected to the third microstrip line (114) on the left; the third microstrip line (114) on the left extends in the negative y-axis direction, and after a 90° bend, it turns to extend in the negative z-axis direction, and its end is connected to one pole of the resistive load (116) on the left; the other pole of the resistive load (116) on the left is electrically connected to the metal ground plane (102) through the third shorting pin (119) on the left; the third shorting pin (119) on the left passes through the dielectric plate 101 in the x-axis direction, and one end is electrically connected to the metal ground plane (102), and the other end is electrically connected to the resistive load (116) on the left; Meanwhile, the end of the third microstrip line (114) on the left is also connected in parallel with the fourth microstrip line (115) on the left; the fourth microstrip line (115) on the left extends horizontally from the end of the third microstrip line (114) on the left towards the positive y-axis, and after a 90° bend, it turns to extend in the positive z-axis direction; the end of the fourth microstrip line (115) on the left is electrically connected to the metal ground plane (102) through the second shorting pin (118) on the left; the second shorting pin (118) on the left passes through the dielectric plate 101 along the x-axis direction, with one end electrically connected to the metal ground plane (102) and the other end electrically connected to the end of the fourth microstrip line (115) on the left.

5. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 4, characterized in that, In the left-side absorption structure (11): The left absorption structure (11) has a dimension of 12mm~20mm along the z-axis, a dimension of 3mm~6mm along the y-axis, and a dimension of 0.301mm along the x-axis. The impedance relationship of each transmission line in the left absorption structure (11) is as follows: impedance of the left slot line (111) > impedance of the left second microstrip line (113) > impedance of the left fourth microstrip line (115) > impedance of the left first microstrip line (112) > impedance of the left third microstrip line (114).

6. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 5, characterized in that, The right-side absorption structure (12) includes a right-side groove line (121), a right-side first microstrip line (122), a right-side second microstrip line (123), a right-side third microstrip line (124), a right-side fourth microstrip line (125), a right-side resistive load (126), a right-side first short-circuit pin (127), a right-side second short-circuit pin (128), and a right-side third short-circuit pin (129); The right-side groove (121) is a slot structure located on the metal floor (102) behind the dielectric substrate (101). The slot structure extends from the top of the rear metal floor (102) along the negative z-axis. Its top left side is tangent to the right first short-circuit pin (127). The metal floor (102) is completely covered except for the area where the slot structure is located. Other structures are located on the front side of the dielectric substrate (101). The right-side slot line (121) is electrically connected to the right-side first microstrip line (122) via the right-side first shorting pin (127); the right-side first to right-side fourth microstrip lines (122)-(125) are closely attached to the front side of the dielectric substrate (101); the right-side fourth microstrip line (125) is electrically connected to the metal ground plane (102) on the rear side of the dielectric substrate (101) via the right-side second shorting pin (128); the right-side resistive load (126) is connected in parallel with the right-side third to right-side fourth microstrip lines (124) and (125), and is electrically connected to the metal ground plane (102) on the rear side of the dielectric substrate via the right-side third shorting pin (129); The right first microstrip line (122), right second microstrip line (123), right third microstrip line (124) and right fourth microstrip line (125) are connected in series from beginning to end; the characteristic impedance of each transmission line is set according to the comprehensive calculation results of the asymmetric filter network to achieve wideband impedance matching at an 80° incident angle.

7. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 6, characterized in that, In the right-side absorption structure (12): The first short-circuit pin (127) on the right penetrates the dielectric substrate (101) along the x-axis. One end of the pin is electrically connected to the metal ground plane (102) and tangent to the right groove line (121). The other end of the pin is electrically connected to the first microstrip line (122) on the right. The first microstrip line (122) on the right extends horizontally from the connection point of the first short-circuit pin (127) on the right in the positive y-axis direction. After a 90° bend, it turns to extend in the negative z-axis direction. Its end is connected to one end of the second microstrip line (123) on the right. The second microstrip line (123) on the right extends in the negative z-axis direction. After a 90° bend, it turns to extend in the negative z-axis direction. The right-side third microstrip line (124) extends in the negative y-axis direction and then turns to the negative z-axis direction after a 90° bend, with its end connected to one pole of the right-side resistive load (126). The other pole of the right-side resistive load (126) is electrically connected to the metal ground plane (102) through the right-side third shorting pin (129). The right-side third shorting pin (129) passes through the dielectric plate 101 in the x-axis direction, with one end electrically connected to the metal ground plane (102) and the other end electrically connected to the right-side resistive load (126). Meanwhile, the end of the third microstrip line (124) on the right is also connected in parallel with the fourth microstrip line (125) on the right; the fourth microstrip line (125) on the right extends horizontally from the end of the third microstrip line (124) on the right in the positive y-axis direction, and after a 90° bend, it turns to extend in the positive z-axis direction; the end of the fourth microstrip line (125) on the right is electrically connected to the metal ground plane (102) through the second shorting pin (128) on the right. The second short-circuit pin (128) on the right side passes through the dielectric plate 101 along the x-axis direction. One end of the pin is electrically connected to the metal ground plate (102), and the other end is electrically connected to the end of the fourth microstrip line (125) on the right side.

8. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 7, characterized in that, In the right-side absorption structure (12): The right-side absorption structure (12) has a dimension of 12mm~20mm along the z-axis, a dimension of 3mm~6mm along the y-axis, and a dimension of 0.301mm along the x-axis. The impedance relationship of each transmission line in the right-side absorption structure (12) is as follows: right-side slot line (121) impedance = right-side fourth microstrip line (125) impedance > right-side second microstrip line (123) impedance > right-side first microstrip line (122) impedance = right-side third microstrip line (124) impedance.

9. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 8, characterized in that, The thickness of the dielectric substrate (101) ranges from 0.2 mm to 4 mm.

10. A broadband microwave absorber for 75°~85° limiting oblique incidence as described in claim 9, characterized in that, The differences between the right absorption structure (12) and the left absorption structure (11) are as follows: the right slot line (121) and the left slot line (111), the right first microstrip line (122) and the left first microstrip line (112), the right second microstrip line (123) and the left second microstrip line (113), the right third microstrip line (124) and the left third microstrip line (114), the right fourth microstrip line (125) and the left fourth microstrip line (115), and the right resistive load (126) and the left resistive load (116) are different in size and impedance; The composite transmission line configuration in the two absorption structures is unrestricted, allowing slotted lines and microstrip lines to be replaced with other types of transmission lines, including single-wire or parallel double-wire or other types.

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