A low-pass absorptive frequency selective structure with narrow transition band

Abstract

The present invention proposes a low-pass absorption frequency selection structure with a narrow transition band. The transition bandwidth of the currently reported low-pass absorption frequency selection structure is narrow, and the selectivity is not excellent enough. The present invention is a periodic structure. The structural unit is composed of two parts, one is a loss layer, and the other is a three-dimensional band-stop structure. The loss layer uses radio frequency resistors and bent metal wires with large equivalent inductance values, and the three-dimensional band-stop structure uses metal strips printed on a dielectric substrate in the vertical direction. The entire structure has the characteristics of high-frequency wave absorption and low-frequency wave transmission, and a narrow transition band. The present invention perfectly matches the performance indicators of antenna covers of a large number of ultra-wideband antennas and spectrum monitoring systems working in the VHF / UHF frequency bands used in the military field, and has strong applicability.

Description

Technical Field

[0001] The invention belongs to the field of microwave technology and relates to a low-pass absorption frequency selection structure with a narrow transition band and insensitivity to polarization. Background Art

[0002] In the field of military equipment, frequency selective surface / structure (FSS) is often used as a radome to reduce the radar cross section (RCS) and improve the stealth performance of combat platforms. The traditional FSS achieves the reduction of the RCS of a single station by deviating the radar reflection wave from the incident direction. However, it cannot solve the problem of reducing the RCS of dual stations / multi-stations. In order to solve this problem, a new artificial electromagnetic structure combining FSS and microwave absorbers, the absorptive frequency selective surface / structure (AFSS), came into being. AFSS can achieve the function of transmitting waves within the passband and absorbing waves outside the passband.

[0003] At present, most AFSS are bandpass type, which has a passband and an absorption band on one or both sides of the passband. However, due to the limitation of the passband bandwidth, the bandpass AFSS cannot meet the requirements of VHF / UHF band ultra-wideband antennas for stealth antenna covers. In this context, low-pass AFSS was proposed. Its main feature is "low pass and high absorption", that is, high-frequency absorption and low-frequency transmission. The general definition of f p is the cut-off frequency of the passband (usually expressed as |S 21 |≥-1dB), f a1 and f a2 is the absorption band (|S 21 |≤-10dB,|S 11 |≤-10dB) start and end frequencies. p to f a1 The transition band is between them. Since the low-pass AFSS allows frequencies lower than f p The electromagnetic wave transmission of the antenna is as long as the working frequency band is lower than f p , the low-pass AFSS can meet the stealth radome requirements of the antenna.

[0004] The existing low-pass AFSS transition band is too wide, making the transition from wave transmission to wave absorption very slow. Therefore, it is urgent to develop a low-pass AFSS with a transition band as narrow as possible (the transition from wave transmission to wave absorption is as fast as possible) and good frequency selection characteristics. When used as a radome, the working frequency band of the antenna can be very close to the absorbing frequency band of the radome, so it has better applicability. Summary of the invention

[0005] The purpose of the present invention is to provide a polarization-insensitive low-pass AFSS with a narrow transition band to address the deficiencies of the prior art. The low-pass AFSS allows frequencies lower than the cutoff frequency (f p ) electromagnetic waves pass through, and the absorption frequency is located in its absorption band (f a1 to f a2 ) in the electromagnetic wave, the transition band between the passband and the absorption band (f p to f a1 ) is very narrow, so it has better applicability. In addition, the low-pass AFSS structure is symmetrical, so it has polarization insensitivity (it can respond to any polarization incident electromagnetic waves).

[0006] The low-pass absorption frequency selection structure of the narrow transition band of the present invention is a periodic structure, each unit includes a loss layer and a three-dimensional band-stop structure; an air gap is left between the loss layer and the three-dimensional band-stop structure;

[0007] The loss layer comprises a first dielectric substrate, and a first absorbing surface and a second absorbing surface respectively arranged on the upper and lower surfaces of the first dielectric substrate; the first absorbing surface and the second absorbing surface are arranged vertically in space;

[0008] The first absorbing surface includes a first metal wire, a first radio frequency resistor, and a second metal wire connected in series in sequence; the second absorbing surface includes a third metal wire, a second radio frequency resistor, and a fourth metal wire connected in series in sequence; the centers of the first radio frequency resistor, the second radio frequency resistor, and the first dielectric substrate are located in the same straight line;

[0009] The equivalent circuit of the loss layer is a series LC circuit, and its equivalent inductance value is 6.1-14.05nH, and the corresponding equivalent capacitance value is 0.064-0.135pF. The equivalent inductance value is related to the center frequency of the absorption band.

[0010] The three-dimensional band-stop structure includes the second to fifth dielectric substrates and four metal strips; it is located below the loss layer and is arranged vertically with the loss layer.

[0011] The second dielectric substrate, the third dielectric substrate, the fourth dielectric substrate, and the fifth dielectric substrate are enclosed to form a hollow rectangular parallelepiped structure with two open ends; two metal strips are respectively arranged on the inner sides of two adjacent dielectric substrates of the rectangular parallelepiped structure; the two metal strips of each dielectric substrate are located at the end of the dielectric substrate close to the loss layer and the end far from the loss layer;

[0012] The distance between the farthest ends of two metal strips located on the same dielectric substrate is 0.4λ0-0.6λ0, preferably 0.5λ0, wherein λ0 represents the wavelength corresponding to the central frequency point of the absorption band.

[0013] Preferably, the first metal wire, the second metal wire, the third metal wire, and the fourth metal wire have the same structure, and each of them includes a folded line portion and a straight line portion; one end of the folded line portion is connected to one end of the straight line portion, and the other end of the straight line portion is connected to a radio frequency resistor located on the same absorbing surface;

[0014] Compared with the horizontally placed absorbing surface, the 45° inclined absorbing surface makes full use of the limited area within the periodic unit, and the bending method makes the metal wire longer, thereby obtaining a larger equivalent inductance value. The metal wire near the RF resistor is not bent to prevent coupling between the metal wires of the upper and lower absorbing surfaces.

[0015] Preferably, the lengths of the first metal wire, the second metal wire, the third metal wire and the fourth metal wire are all 0.3λ0-0.5λ0, wherein λ0 represents the wavelength corresponding to the central frequency point of the absorption band.

[0016] Preferably, the thickness of the air gap is 0.15λ0-0.35λ0, preferably 0.25λ0, wherein λ0 represents the wavelength corresponding to the center frequency of the absorption band.

[0017] Preferably, the first and second radio frequency resistors are the same, and the specific resistance value needs to be determined by analyzing the specific situation.

[0018] Preferably, the first, second, third and fourth metal wires have the same structure and have rotational symmetry.

[0019] Preferably, the equivalent inductance value of a single metal strip ranges from 8.1 to 17.7 nH, and the corresponding equivalent capacitance value ranges from 0.06 to 0.11 pF. The equivalent inductance value is related to the center frequency of the absorption band.

[0020] Preferably, the length of a single metal strip is 0.3λ0-0.5λ0, wherein λ0 represents the wavelength corresponding to the center frequency of the absorption band.

[0021] Preferably, the metal strip is a C-shaped structure, with its opening facing the center of the dielectric substrate;

[0022] Preferably, the metal strip adopts a bending line structure;

[0023] Preferably, the first to fifth dielectric substrates are FR4 dielectric substrates;

[0024] Preferably, the four metal strips in the three-dimensional band-stop structure have the same length and line width.

[0025] Preferably, the second dielectric substrate, the third dielectric substrate, the fourth dielectric substrate and the fifth dielectric substrate have the same length, width and thickness.

[0026] Preferably, the size of the first dielectric substrate is the same as the size of the open end of the rectangular parallelepiped structure enclosed by the second dielectric substrate, the third dielectric substrate, the fourth dielectric substrate, and the fifth dielectric substrate.

[0027] Working principle:

[0028] The present invention is different from the traditional two-dimensional low-pass AFSS. It adopts a combination of two-dimensional and three-dimensional methods to cleverly construct a low-pass absorption frequency selection structure with a narrow transition band. Theoretical analysis based on the equivalent circuit model shows that there are two key points to achieve the narrow transition band of the low-pass AFSS: 1. The inductance value of the loss layer equivalent LC resonant circuit needs to be as large as possible; 2. The frequency selection characteristics of the three-dimensional band-stop structure are as high as possible. Based on this principle, the present invention realizes a larger equivalent inductance within a periodic unit of a limited area by constructing a bent metal wire, but at the same time ensures the performance of the absorption band. In addition, a three-dimensional band-stop structure is constructed to achieve its higher frequency selection characteristics. Then the two are properly combined to finally achieve a low-pass AFSS with a narrow transition band.

[0029] The metal strips on the three-dimensional band-stop structure resonate to produce transmission zeros, thereby generating a high-frequency stopband and a low-frequency passband. The combination of two metal strips on the inner side of the three-dimensional band-stop structure dielectric substrate widens the stopband bandwidth. When electromagnetic waves are incident on the surface of the structure, low-frequency electromagnetic waves pass through the loss layer almost unaffected, and after reaching the three-dimensional band-stop structure, they pass through the three-dimensional band-stop structure losslessly or with low insertion loss. At high frequencies, the loss layer and the three-dimensional band-stop structure together achieve wave absorption. The principle of the three-dimensional band-stop structure is that the metal strips on the inner side of the dielectric substrate are equivalent to an LC series circuit. By designing the value of LC, it resonates at a specific frequency to generate a stopband. Since the structure is exactly the same in the x and y directions, it has the same induction characteristics for TE and TM waves, that is, it has dual polarization (polarization insensitive) characteristics.

[0030] The above-mentioned TE wave refers to an electromagnetic wave whose electric field direction is parallel to the y-axis when the electromagnetic wave is incident along the incident direction (-z direction); the TM wave refers to an electromagnetic wave whose electric field direction is parallel to the x-axis when the electromagnetic wave is incident along the incident direction (-z direction).

[0031] The novel narrow transition band low-pass AFSS structure proposed by the present invention has the following advantages:

[0032] (1) The present invention has the function of low insertion loss or even lossless wave transmission in a wide frequency band at low frequencies, and has a wide absorption band at high frequencies. The transition band between the transmission band and the absorption band is narrow, and the selectivity is good, that is, the applicability is strong. It is perfectly applied to a large number of ultra-wideband antennas and spectrum monitoring systems in the military field working in the VHF / UHF frequency band. On the one hand, it reduces the RCS of the antenna, and on the other hand, it does not reduce the gain of the antenna, thereby achieving good stealth performance. The low-pass, high-absorption, and narrow transition band performance achieved by the entire structure is jointly determined by the performance of the loss layer and the three-dimensional band-stop structure.

[0033] (2) The present invention has a simple structure and can be realized using a simple PCB process, which greatly reduces the cost. The loss layer only requires welding of RF resistors, which is easy to operate.

[0034] (3) The low-pass AFSS proposed in the present invention is processed using a PCB board with FR4 medium, and has a simple structure, small size, low cost, and is easy to process.

[0035] (4) The low-pass AFSS proposed in the present invention can be used as an application on the stealth radome of VHF / UHF band antennas. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a schematic diagram of a three-dimensional structural unit of the present invention;

[0037] Figure 2 is a top view of a lossy layer unit of the present invention;

[0038] Figure 3 is a detailed top view of a sacrificial layer unit of the present invention and its annotations;

[0039] Figure 4 It is a front view and annotation of the three-dimensional band-stop structure unit of the present invention;

[0040] Figure 5 It is a simulation result diagram of transmission and reflection coefficient (|S21| and |S11|) of the present invention;

[0041] Figure 6 is a diagram of the simulation result of the wave absorption rate of the present invention;

[0042] Figure 7 are the reflection and transmission coefficients of the unit structure of the present invention when the electromagnetic wave is incident obliquely; (a) and (b) correspond to TE and TM polarizations, respectively. DETAILED DESCRIPTION

[0043] The present invention is further analyzed below in conjunction with specific embodiments.

[0044] like Figure 1As shown, the low-pass AFSS with narrow transition band proposed by the present invention includes two parts. One part is a loss layer, and the other part is a three-dimensional band-stop structure. An air gap is introduced between the loss layer and the three-dimensional band-stop structure.

[0045] The top view of the loss layer is shown in Figure 2 As shown, the loss layer unit size is 10×10 mm, and the details are shown in Figure 3 As shown. The upper layer is the first absorbing surface, including the first bending metal wire 2, the second bending metal wire 3 and the first RF resistor 6 welded between the two metal wires plated on the first dielectric substrate 1. The lower layer is the second absorbing surface, which has the same structure as the first absorbing surface. The first absorbing surface is vertical to the second absorbing surface in space, including the third bending metal wire 4, the fourth bending metal wire 5 and the second RF resistor 7 welded between the two metal wires. The first absorbing surface and the second absorbing surface are respectively distributed on the two sides of the RF4 first dielectric substrate 1 with a thickness of 0.3 mm.

[0046] Three-dimensional band-stop structures such as Figure 4 As shown, the three-dimensional band-stop structure is composed of a second dielectric substrate 8, a third dielectric substrate 9, a fourth dielectric substrate 10, a fifth dielectric substrate 11, a first metal strip 12, a second metal strip 13, a third metal strip 14 and a fourth metal strip 15. The second dielectric substrate 8, the third dielectric substrate 9, the fourth dielectric substrate 10 and the fifth dielectric substrate 11 have the same structure, with a length of 10 mm, a thickness of 0.3 mm and a height of 26 mm; the first metal strip 12, the second metal strip 13, the third metal strip 14 and the fourth metal strip 15 have the same structure. The first metal strip 12 and the third metal strip 14 are located on the inner surface of the fifth dielectric substrate 11, the second metal strip 13 and the fourth metal strip 15 are located on the inner surface of the second dielectric substrate 8, and the second dielectric substrate 8 is adjacent to the fifth dielectric substrate 11.

[0047] The specific structural geometric parameters are as follows:

[0048] Among them, the length of the lossy layer unit is a, the thickness of the dielectric substrate is t, the length and resistance of the RF resistor are a1 and R respectively, the bending depth of the bent metal wire is d, l 1、 l 2、 l 3、 l 4、 l 5、 l 6、 l7 is the specific size of the metal line, the line width of the lossy layer metal line is w. The distance of the air gap introduced between the lossy layer and the three-dimensional band-stop structure is h. The width of a metal strip unit of the three-dimensional band-stop structure is d1, and the depth of the bending groove is z. l The concave width of the bending groove is b, the length of the metal rings on both sides of the bending groove is d2, and the width of the metal rings on both sides of the bending groove is x l, the line width of the three-dimensional band-stop structure metal strip is w1. The distance from the lower end of the first metal strip 12 to the upper end of the third metal strip 14 of the three-dimensional band-stop structure is h1.

[0049]

[0050]

[0051] Figure 5 , Figure 6 and Figure 7 This is the low-pass AFSS simulation result. Figure 5 are the reflection and transmission coefficients. It can be seen that the absorption band of the structure (|S 11 |≤-10dB, |S 21 |≤-10dB) is between 4.6GHz and 7.2GHz, with a relative bandwidth of 44%. Below 2.9GHz is the passband (|S 21 |≥-1dB). The transition band is between 2.9 GHz and 4.6 GHz, and the bandwidth is 1.58:1. Compared with the narrowest bandwidth of 2.6:1 of the currently reported polarization-insensitive low-pass AFSS, the transition band bandwidth of the present invention is reduced by 40%.

[0052] Figure 6 As the absorption rate changes with frequency, it is obvious that the absorption efficiency in the absorption band is as high as over 80%, and the absorption effect is good. Figure 7 are the transmission and reflection coefficients at oblique incidence. It can be seen that when the incident angle of the electromagnetic wave reaches 30°, the low-pass AFSS can still demonstrate basically stable performance.

[0053] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A low-pass absorption frequency selective structure with a narrow transition band, which is a periodic structure, characterized in that Each unit includes a lossy layer and a three-dimensional band-stop structure; an air gap is left between the lossy layer and the three-dimensional band-stop structure; The loss layer comprises a first dielectric substrate, and a first absorbing surface and a second absorbing surface respectively arranged on the upper and lower surfaces of the first dielectric substrate; the first absorbing surface and the second absorbing surface are arranged vertically in space; The first absorbing surface comprises a first metal wire, a first radio frequency resistor, and a second metal wire connected in series in sequence; the second absorbing surface comprises a third metal wire, a second radio frequency resistor, and a fourth metal wire connected in series in sequence; the centers of the first radio frequency resistor, the second radio frequency resistor, and the first dielectric substrate are located in the same straight line; the first metal wire, the second metal wire, the third metal wire, and the fourth metal wire have the same structure, and they all comprise a folded line portion and a straight line portion; one end of the folded line portion is connected to one end of the straight line portion, and the other end of the straight line portion is connected to the radio frequency resistor located on the same absorbing surface; The equivalent circuit of the loss layer is a series LC circuit, and its equivalent inductance value is 6.1-14.05nH, and the corresponding equivalent capacitance value is 0.064-0.135pF; The three-dimensional band-stop structure is located below the loss layer and is vertically arranged with respect to the loss layer; it includes the second to fifth dielectric substrates and four metal strips; The second dielectric substrate, the third dielectric substrate, the fourth dielectric substrate, and the fifth dielectric substrate are enclosed to form a hollow rectangular parallelepiped structure with two open ends; two metal strips are respectively arranged on the inner sides of two adjacent dielectric substrates of the rectangular parallelepiped structure; the two metal strips of each dielectric substrate are located at the end of the dielectric substrate close to the loss layer and the end far from the loss layer; The farthest distance between two metal strips located on the same dielectric substrate is 0.4λ0-0.6λ0, where λ0 represents the wavelength corresponding to the center frequency of the absorption band.

2. A narrow transition band low-pass absorption frequency selective structure according to claim 1, characterized in that The farthest distance between two metal strips located on the same dielectric substrate is 0.5λ0, where λ0 represents the wavelength corresponding to the center frequency of the absorption band.

3. A narrow transition band low-pass absorption frequency selective structure according to claim 1, characterized in that The lengths of the first metal wire, the second metal wire, the third metal wire, and the fourth metal wire are all 0.3λ0-0.5λ0, wherein λ0 represents the wavelength corresponding to the central frequency point of the absorption band.

4. A narrow transition band low-pass absorption frequency selective structure according to claim 1, characterized in that The thickness of the air gap is 0.15λ0-0.35λ0, wherein λ0 represents the wavelength corresponding to the center frequency of the absorption band.

5. A narrow transition band low-pass absorption frequency selective structure according to claim 1, characterized in that The equivalent inductance of a single metal strip ranges from 8.1 to 17.7 nH, and the corresponding equivalent capacitance ranges from 0.06 to 0.11 pF.

6. A narrow transition band low-pass absorption frequency selective structure according to claim 1, characterized in that The length of a single metal strip is 0.3λ0-0.5λ0, where λ0 represents the wavelength corresponding to the center frequency of the absorption band.

7. A narrow transition band low-pass absorption frequency selective structure according to claim 1, characterized in that The metal strip is a C-shaped structure, and its opening faces the center of the dielectric substrate.

8. A narrow transition band low-pass absorption frequency selective structure according to claim 7, characterized in that The metal strip adopts a bending line structure.

9. A narrow transition band low-pass absorption frequency selective structure according to claim 1, characterized in that The size of the first dielectric substrate is the same as the size of the open end of the rectangular parallelepiped structure formed by the second dielectric substrate, the third dielectric substrate, the fourth dielectric substrate, and the fifth dielectric substrate.

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