CN107086374B - Miniaturized low-profile ultra-wide passband frequency selective surface and design method thereof - Google Patents

Abstract

The invention discloses a miniaturized low-profile ultra-wide passband frequency selective surface and a design method thereof, comprising the following steps: the first metal patch layer, the middle layer and the second metal patch layer are sequentially pressed together, the units of the first metal patch layer are rectangular with the same length and width, four corners of the rectangular are respectively provided with a quarter square ring with outward openings, the center of each unit is a complete rectangular ring, and after the plane period is prolonged, a staggered square ring array is formed; the second metal patch layer is rectangular, the size of the unit is the same as that of the first metal patch layer, the center of the second metal patch layer is a cross-shaped metal wire, and rectangular metal patches are connected with the cross-shaped metal wire at the midpoint of each side of the unit. The frequency selection structure has ultra-wide passband characteristics, and simultaneously, the overall thickness of the frequency selection is greatly reduced. The frequency selective structure provided by the invention has extremely low section, and can be freely combined with structures such as skins, shells, protective covers and the like with most thicknesses, so that the unique electrical performance of the frequency selective structure is exerted.

Description

A miniaturized low-profile ultra-wide passband frequency selective surface and its design method

technical field

The invention belongs to the field of electromagnetic field and microwave technology, and specifically refers to a miniaturized low-profile ultra-wide passband frequency selective surface and a design method thereof.

Background technique

Modern warfare has entered the information age, and the target information detected by radar determines the fate of the target. The radar antenna system on the aircraft is an important source of scattering, and has a high radar cross section (RCS) in certain frequency and viewing angle ranges. Reducing the RCS of the antenna system is an important issue for the aircraft to achieve stealth. The traditional aircraft dielectric radome is "transparent" in the whole frequency band and has no stealth effect. Therefore, the design of the wave-transparent/stealth multifunctional integrated radome is extremely important. For example, how to reduce the backscatter of the radar antenna on the head of the aircraft has become an influence One of the key factors of aircraft stealth performance. Ordinary dielectric radome cannot reduce RCS. Although the application of wave-absorbing materials can reduce backscattering, it will affect the normal communication of the aircraft at the same time. The application of frequency selective surface structure in the dielectric radome, that is, frequency selective surface technology (Frequency Selective Surface, FSS) can overcome the above defects. Because FSS has spatial filtering characteristics, it can effectively control the reflection and transmission performance of electromagnetic waves.

The frequency selective surface structure is a two-dimensional structure composed of a large number of periodically arranged metal patch units with specific shapes or gaps between metal planes. When the frequency of the incident electromagnetic wave is at the resonant frequency of the unit, FSS presents total reflection (sticker sheet type) or full transmission (aperture type), electromagnetic waves of other frequencies can pass through FSS (patch type) or be totally reflected (aperture type), so FSS is essentially a special spatial filter that can effectively control Transmission properties of electromagnetic waves. By applying the FSS technology to the radome, the radome can obtain the function of frequency selection and perform frequency selective wave transmission. The radome maintains normal wave penetration within the design frequency band; outside the design frequency band, the radome is equivalent to a metal cover to shield electromagnetic waves. Its function is to make the antenna cabin of the aircraft exhibit different RCS characteristics inside and outside the designed frequency band.

Journal "Telecommunications Technology" 2012, 52(3): 371-374, Li Yuqing, Pei Zhibin, Qu Shaobo et al proposed "the design of a bandpass frequency selective surface with broadband characteristics"; the simulation in the journal paper shows that the frequency selective surface The three poles are 6.44GHz, 8.80GHz and 10.97GHz. The three poles are coupled to form a flat-top wide passband with very small central insertion loss, the maximum central insertion loss is only 0.45dB, the 3dB operating bandwidth is 5.40-11.47GHz at this time, the absolute bandwidth is 6.07GHz, and the relative bandwidth reaches 72%. And outside the passband, S2l can drop rapidly to below -20dB and keep it all the time. The frequency selective surface structure has good sideband selection and out-of-band suppression characteristics. However, there is an objective problem in this technical solution. Due to the use of a multi-layer cascaded structure, the frequency selective surface has a relatively large thickness and a relatively high profile. As a microwave passive material, the thickness of the structure directly determines its applicability. Frequency selective surfaces are mainly used in the vicinity of high RCS parts of aircraft in the form of skins or external covers. Therefore, frequency selective surface structures with large thickness will pose considerable difficulties to the design of skins and covers, and often cannot be practically applied.

Journal literature: Liang B Y, Xue Z H, Li W M, et al.Ultra-wideband frequencyselective surface at K and Ka band[C].IEEE International Conference on Microwave Technology&Computational Electromagnetics.IEEE,2013:55-57, the purpose of which is to design a The new ultra-wide passband frequency selective surface structure provides a structural solution for the filter and wave penetration requirements of ultra-wideband electromagnetic applications. The unit size is 2.898mm*4.733mm, the thickness of the single layer of the structure is 2mm, the thickness of the adhesive layer between the dielectric boards is about 0.05mm, and the overall thickness is about 6.1mm. The -3dB bandwidth is from 17.83GHz to 45.66GHz, and the relative bandwidth reaches 88%, which belongs to the ultra-wide passband frequency selection surface. However, this technical solution still has the problem of high profile and large thickness. Although the number of layers is only three, the total thickness still reaches 6.1mm, which greatly affects the application range.

Contents of the invention

Aiming at the shortcomings of the above-mentioned prior art, the object of the present invention is to provide a miniaturized low-profile ultra-wide passband frequency selective surface and a design method thereof, so as to solve the above-mentioned defects in the prior art. The invention ensures that the frequency selective surface structure has ultra-wide passband characteristics, and at the same time greatly reduces the overall thickness of the frequency selective surface; it can be freely combined with structures such as skins, shells, and protective covers with most thicknesses, So as to exert its unique electrical properties.

In order to achieve the above object, a miniaturized low-profile ultra-wide passband frequency selective surface of the present invention is composed of three layers, namely: the first metal patch layer, the middle layer and the second metal patch layer, the three Pressed together in order, the unit of the first metal patch layer is a rectangle with the same length and width, and a quarter square ring with an opening outward is set on the four corners of the rectangle, and the center of the unit is a complete rectangular ring. After the plane period is extended, it presents a staggered array of square rings; the unit of the second metal patch layer is a rectangle with the same size as the unit of the above-mentioned first metal patch layer, and the center is a cross-shaped metal wire. At the midpoint of each side, there is a rectangular metal patch connected to the cross-shaped metal wire. After the plane period is extended, a grid-like array appears in which the cross-shaped metal wire and the rectangular metal patch alternately appear.

Preferably, the middle layer uses a high-frequency microwave circuit board.

A design method of a miniaturized low-profile ultra-wide passband frequency selective surface of the present invention comprises the following steps:

1) According to the bandwidth and section requirements of the required frequency selection surface, select a suitable microwave filter and give an equivalent circuit;

2) Following the principle of impedance matching, the equivalent circuit of the microwave filter is approximately transformed to the form of matching frequency selective surface design, and the basic structure of the frequency selective surface is obtained;

3) Deduce the parameter range in the basic structure of the above-mentioned design frequency selection surface through the transformation formula of the distributed parameter electric element and the lumped parameter electric element and the parallel circuit resonant frequency formula;

4) Utilize the metal patch and dielectric layer structure in the frequency selective surface to realize the electrical properties of the capacitance, inductance and transmission line in the equivalent circuit of the above step 2), and clarify the specific frequency selective surface structure;

5) Select the processing material, and use the copper clad laminate technology to produce the designed frequency selection surface finished product.

Preferably, the working bandwidth of the microwave filter in the above step 1) and the working bandwidth of the corresponding frequency selective surface structure should be of the same order of magnitude, for example, an ultra wideband filter should be selected as a reference for the ultra wideband frequency selective surface structure. The order of the frequency response curve of the microwave filter determines the structural thickness of the frequency selective surface, that is, the profile. The higher the order of the microwave filter, the higher the profile of the frequency selective surface.

Preferably, the equivalent circuit of the microwave filter in the above step 1) is a first-order parallel capacitor-inductance combined resonant circuit, and the resistor Z1 and the resistor Z2 are respectively the input and output impedances of the two-port network in the equivalent circuit; the inductor L1 and the capacitor C1 , Inductor L3 and capacitor C3, in the two groups of inductor-capacitor series resonant circuits, the values of inductors L1 and L3 are high, and the values of capacitors C1 and C3 are low; The value of high, low value of inductance L2.

Preferably, in the above step 2), firstly, the input and output impedances of the two-port network are substituted with the free impedance Z ₀ =377Ω of free space, and secondly, the capacitors C ₁ , C ₃ and inductance L ₂ with low values are ignored, according to the transmission line Theoretically, transform the original inductance-capacitance-inductance T-type network L ₁ -C ₂ -L ₃ into a capacitance-inductance-capacitance π-type network C'-L'-C'; see the equivalent circuit of the short transmission line Make a parallel capacitance-inductance circuit formed by a specific resistor in series, replace a capacitance C' in the π-type network with a short transmission line Z ₁₂ , and modify the remaining capacitance C' and inductance L' in the network. C and L to match the overall impedance of the π-type network C'-L'-C'circuit; the equivalent circuit corresponds to the frequency selective surface, Z ₀ is replaced by the free impedance of free space, and the capacitor C uses a layer of metal patch To achieve its capacitance characteristics, the transmission line Z ₁₂ uses a layer of dielectric layer to realize its impedance characteristics, and the inductance L uses a layer of metal patch to realize its inductance characteristics; the first and third layers are thin metal patch layers, and the second layer has a certain thickness of the dielectric layer.

Preferably, the above step 3) the value of the electric element in the circuit corresponds to the specific parameter range of the patch and the dielectric layer obtained in the frequency selective surface structure by the following formula:

得到，谐振频率正比于单元周期尺寸p，反比于金属贴片间隔宽度s、金属栅格线宽w。Among them, C is the capacitance value of the final equivalent circuit, L is the inductance value of the final equivalent circuit, ε ₀ ≈8.85*10^(-12), μ ₀ ≈1.26*10^(-6) and π≈3.14 are Constant constant, ε _r is the dielectric constant of the selected specific dielectric layer, p is the unit period size of the frequency selective surface, s is the interval width of the first layer of metal patches, and w is the width of the second layer of metal grid lines; s and w represent the overall size after the periodic extension of the unit structure, which does not appear in the unit parameters alone; from the formula (1)(2), it can be seen that increasing the unit period size p and reducing the metal patch interval width s Or metal grid line width w, increase the value of C and L; according to the resonant frequency of the parallel circuit

It is obtained that the resonant frequency is proportional to the unit period size p, and inversely proportional to the spacing width s of the metal patch and the line width w of the metal grid.

Preferably, the above step 4) uses the metal patch of the metal patch of the capacitance and inductance element in the equivalent circuit in the step 2) to match the electrical performance with excellent conductivity, and realizes the lumped capacitance C' in the structure of distributed capacitance. The performance in the circuit; similarly, using the metal of the same material to realize the characteristics of the inductance L' in a grid structure; relying on a high-frequency microwave circuit board with a suitable dielectric constant and loss tangent instead of a short transmission line to obtain a capacitive metal Frequency selective surface structure of patch layer-impedance matching dielectric plate layer-inductive metal patch layer.

Preferably, the processing sample of the frequency selective surface in the above step 5) contains at least 3*3 unit arrays, the first layer and the second layer of metal patches are selected from metals with excellent electrical conductivity, and the thickness of the patches is controlled at 35um-70um The dielectric board in the inner and middle layers needs to meet the relative permittivity requirements derived during design, while ensuring a low loss tangent. During processing, the dielectric board and the metal patch layer need to be closely connected, and the copper-clad laminate technology is used for lamination.

Preferably, the metal with excellent electrical conductivity selected in the above step 5) is silver or copper.

Beneficial effects of the present invention:

The present invention greatly reduces the overall thickness of the frequency selective surface while ensuring that the frequency selective surface structure has an ultra-wide passband characteristic. The cross section of the frequency selective surface structure proposed by the present invention is extremely low, and can be freely combined with structures such as skins, shells, and protective covers of most thicknesses, so as to exert its unique electrical properties. It has high practical value in the fields of aircraft stealth, electromagnetic compatibility, and radiation shielding.

Description of drawings

Figure 1 is a top view of the complete structure of the frequency selective surface.

Figure 2 is a side view of the complete structure of the frequency selective surface.

Fig. 3 is a top view of the unit structure of the first layer of the frequency selective surface.

Fig. 4 is a top view after planar period extension of the unit structure of the first layer of the frequency selective surface.

Fig. 5 is a top view of the unit structure of the third layer of the frequency selective surface.

Fig. 6 is a top view after planar period extension of the unit structure of the third layer of the frequency selective surface.

Fig. 7 is an equivalent circuit diagram of the microwave filter in the embodiment.

Fig. 8 is an equivalent circuit diagram suitable for frequency selective surface design after approximate transformation in the embodiment.

Fig. 9 is a graph showing reflection curves and transmission curves of a frequency selective surface under normal incidence conditions.

Fig. 10 is a reflection curve and a transmission curve diagram of a frequency selective surface under a certain angle of incidence.

Fig. 11 is a distribution diagram of surface electric field intensity at the resonant frequency of the first metal patch on the frequency selective surface.

Fig. 12 is the surface electric field intensity distribution of the second layer metal patch on the frequency selective surface at the resonant frequency.

Detailed ways

In order to facilitate the understanding of those skilled in the art, the present invention will be further described below in conjunction with the embodiments and accompanying drawings, and the contents mentioned in the embodiments are not intended to limit the present invention.

Referring to Figures 1 to 6, a miniaturized low-profile ultra-wide passband frequency selective surface of the present invention is composed of three layers, namely: the first metal patch layer 1, the intermediate dielectric layer 2 and the second The metal patch layer 3, the three are pressed together in sequence, the unit of the first metal patch layer 1 is a rectangle with the same length and width, and a quarter square ring with an opening outward is respectively set on the four corners of the rectangle, and the unit A complete rectangular ring is set in the center, and after the plane period is extended, it presents a staggered array of square rings; the second metal patch layer 3 has a unit of the same size as the unit of the first metal patch layer. The center is a cross-shaped metal wire, and there are rectangular metal patches connected to the cross-shaped metal wire at the midpoint of each side of the unit. After the plane cycle is extended, it presents a similar pattern in which the cross-shaped metal wire and the rectangular metal patch appear alternately. grid array.

Wherein, the middle layer adopts a high-frequency microwave circuit board, and its type selection should match the impedance of the short transmission line in the equivalent circuit in step 2), and the Rogers series high-frequency microwave circuit board can be selected. It should be noted that n*n (n is a positive integer and greater than or equal to 3) units are generally selected to form a complete structure in the actual processing of the frequency selective surface to reflect its periodic characteristics. The size of the high-frequency circuit board in the middle layer always matches the size of the upper and lower layers. That is (n*p)mm*(n*p)mm, where p is the frequency selective surface element period.

A design method of a miniaturized low-profile ultra-wide passband frequency selective surface of the present invention comprises the following steps:

1) According to the bandwidth and section requirements of the required frequency selection surface, select a suitable microwave filter and give an equivalent circuit;

Wherein, the operating bandwidth of the microwave filter in the above-mentioned step 1) should be similar to the corresponding frequency selection surface structure, the frequency response curve order of this microwave filter determines the structural thickness, i.e. the profile, of the frequency selection surface, and the higher the order of the microwave filter is Higher, the higher the profile of the frequency selective surface. In order to meet the requirements of ultra-broadband and low profile, the first-order broadband bandpass microwave filter is selected as the design prototype, and the equivalent circuit of the filter can be obtained according to the basic theory of circuit analysis. This circuit is only used to illustrate the working principle of the microwave filter, so the values of these electrical components do not need to be quantified, only qualitative description is required.

The equivalent circuit of the filter in the present embodiment is as accompanying drawing 7, and the equivalent circuit of visible this microwave filter is a first-order parallel capacitor inductance combined resonant circuit; Resistance Z ₁ and resistance Z ₂ are two ports in the equivalent circuit respectively The input and output impedance of the network; inductance L ₁ and capacitance C ₁ , inductance L ₃ and capacitance C ₃ in the two groups of inductance-capacitance series resonant circuits, the inductance values L ₁ and L ₃ are high, and the capacitance values C ₁ and C ₃ are low ; and in the capacitor-inductor parallel resonant circuit of capacitor C ₂ and inductor L ₂ , the capacitor value C ₂ is high and the inductance value L ₂ is low.

2) Following the principle of impedance matching, the equivalent circuit of the microwave filter is approximately transformed into a form suitable for the design of the frequency selective surface, and the basic structure of the frequency selective surface is obtained;

Firstly, since the frequency selective surface is a spatial structure, the input and output impedances of the two-port network are substituted by the free impedance Z ₀ =377Ω of free space, and secondly, the capacitors C ₁ , C ₃ and inductance L ₂ with low values are ignored, according to Transmission line theory, transforming the original inductance-capacitance-inductance T-type network L ₁ -C ₂ -L ₃ into a capacitance-inductance-capacitance π-type network C'-L'-C'; the equivalent circuit of a short transmission line As a parallel capacitance-inductance circuit formed by a specific resistor in series, replace a capacitance C' in the π-type network with a short transmission line Z ₁₂ , and modify the remaining capacitance C' and inductance L' in the network. C and L to match the overall impedance of the π-type network C'-L'-C'circuit; so far the π-type capacitor-resistance-inductance network CZ ₁₂ -L in Figure 8 can be obtained, which shows that this circuit is a first-order resonance structure. Corresponding the equivalent circuit to the frequency selective surface, Z ₀ is replaced by the free impedance of free space, the capacitor C uses a layer of metal patch to realize its capacitance characteristics, the transmission line Z ₁₂ uses a layer of dielectric layer to realize its impedance characteristics, and the inductance L uses One layer of metal patch realizes its inductance characteristics; it can be seen that the frequency selection surface corresponding to the equivalent circuit in Figure 8 is a three-layer structure, and the first and third layers are thin metal patch layers, and the second layer is a dielectric layer with a certain thickness .

3) Deduce the parameter range in the basic structure of the above-mentioned design frequency selection surface through the transformation formula of the distributed parameter electric element and the lumped parameter electric element and the parallel circuit resonant frequency formula;

Above-mentioned step 3) the value of the electric element in the circuit corresponds to the specific parameter range of the patch and the dielectric layer obtained in the frequency selection surface structure by the following formula:

得到，谐振频率正比于单元周期尺寸p，反比于金属贴片间隔宽度s、金属栅格线宽w。Among them, C is the capacitance value of the final equivalent circuit, L is the inductance value of the final equivalent circuit, ε ₀ ≈8.85*10^(-12), μ ₀ ≈1.26*10^(-6) and π≈3.14 are Constant constant, ε _r is the dielectric constant of the selected specific dielectric layer, p is the unit period size of the frequency selective surface, s is the interval width of the first layer of metal patches, and w is the width of the second layer of metal grid lines; s and w represent the overall size after the periodic extension of the unit structure, which does not appear in the unit parameters alone; from the formula (1)(2), it can be seen that increasing the unit period size p and reducing the metal patch interval width s Or metal grid line width w, increase the value of C and L; according to the resonant frequency of the parallel circuit

It is obtained that the resonant frequency is proportional to the unit period size p, and inversely proportional to the spacing width s of the metal patch and the line width w of the metal grid.

It should be noted that although the parameter h representing the thickness of the frequency selective surface structure does not appear in the formula derivation, the value of h determines the spatial coupling strength between the first layer and the third layer of the frequency selective surface, because the design in this embodiment It is a first-order resonance structure, the range of h is controlled between 0.5-1.5mm, and it has a relatively low section thickness.

The parameters of the electrical components in the equivalent circuit after filter conversion are C=6.34*10^(-13)F, L=1.43*10^(-8)H. First examine the requirement of designing the lowest frequency point (6GHz) of expected passband, by f _l =c/λ _l , we can know that the wavelength at this frequency point is 50mm, and the value of p in the embodiment is at λ ₀ /5 (controlled at 9.95mm -10.05mm), compared with the half-wavelength structure of the traditional frequency selective surface, it has the characteristics of miniaturization. Then, substituting the value of p into formula (1)(2), the value of parameter s is between 2.15mm-2.25mm, and the value of parameter w is between 0.25-0.35mm. These two parameters are in the design structure does not appear directly, but will determine the value of specific parameters. For example, in this embodiment, the value of parameter s is equal to (p-2a ₂ -a ₄ )/2, and the value of parameter w is equal to (a ₃ *(a ₄ -a ₁ )/a ₄ +2a ₁ *a ₂ /a ₄ ). Finally, according to the coupling situation of the upper and lower metal patches of the surface structure according to the actual frequency, the value of the optimization parameter h is adjusted between 0.95mm-1.05mm.

4) Utilize the metal patch and dielectric layer structure in the frequency selective surface to realize the electrical properties of the capacitance, inductance and transmission line in the equivalent circuit of the above step 2), and clarify the specific frequency selective surface structure;

The above step 4) utilizes the electrical performance with excellent conductivity to match the metal patch of the capacitance and inductance element in the equivalent circuit in step 2), and realizes the performance of the lumped capacitance C' in the circuit with the structure of a distributed capacitance; similarly , use the metal of the same material to realize the characteristics of the inductance L' in a grid structure; rely on the RogersRT5880 high-frequency microwave circuit board with a suitable dielectric constant and loss tangent instead of a short transmission line, and obtain a capacitive metal patch layer-impedance matching Dielectric plate layer - frequency selective surface structure of inductive metal patch layer. The shape and arrangement of distributed capacitance and inductance directly affect the effect and stability of the frequency selective surface when it works in free space. After selecting the graphics and optimizing the parameters, a design with good performance can be obtained.

The specific parameters of the miniaturized low-profile ultra-wide passband frequency selective surface proposed in this embodiment are shown in Table 1. The values of specific parameters _εr , p and h refer to the derivation results in step 3), and _εr determines the dielectric layer material selection, p determines the size of the frequency selective surface unit, h determines the overall thickness of the frequency selective surface; and the value range of s and w guides all the specific parameters in the two-layer metal patch (a ₁ -a ₄ , b ₁ - The value of b ₄ ). The designed shape is not unique, and the structure of this embodiment is the result of parameter optimization. Fig. 1 is a top view schematic diagram of the complete structure obtained after the period extension of the frequency selective surface unit proposed by the present invention, which intuitively reflects the periodic characteristics of the structure. Here, an array structure composed of 9 units is selected, and the specific number of units can be determined depending on the occasion. . Table 1 is as follows:

Table 1

parameter name a1 a2 a3 a4 p parameter value 0.4mm 1.4mm 0.4mm 2.8mm 10.0mm parameter name b1 b2 b3 b4 h parameter value 2.8mm 1.4mm 0.3mm 10.0mm 1.0mm

5) Select suitable processing materials, and use copper-clad laminate technology to produce the designed frequency-selective surface finished products.

The processing sample of the frequency selective surface in the above step 5) contains at least 3*3 unit arrays, the first layer and the second layer of metal patches are selected from metals with excellent electrical conductivity, and the best material is silver (resistivity 15.86ρ /nΩ·m), generally choose copper (resistivity 16.78ρ/nΩ·m) to have a better effect, the thickness of the patch is controlled within 35um-70um, and has no obvious impact on the electrical properties of the structure, the pattern of the patch The shape is etched using the standard process of the national specification for printed circuit boards (QJ3103-99). The dielectric plate of the middle layer needs to meet the relative permittivity requirement derived during design, while ensuring that the loss tangent is as low as possible, the high-frequency microwave circuit board of Rogers is generally selected for use, and the structure proposed in the present invention selects Rogers-RT5880 material ( The relative permittivity is 2.2, the relative magnetic permeability is 1.0, and the loss tangent is 0.0009), and the effect is good. During processing, the dielectric board and the metal patch layer need to be closely connected, and the standard technology of copper-clad laminates in the national specification for printed circuit boards (GB4722-84) is used for lamination.

It can be seen from Fig. 1 that the frequency selective surface has complete rotational symmetry as a whole, which endows it with certain polarization stability. At the same time, because the first layer of metal patches on the frequency selective surface uses multiple miniaturized resonant units arranged in a staggered manner, the second layer of metal patches uses a non-resonant structure to match the first layer of metal patches, and its unit size is much smaller than that of ordinary frequencies. Selecting one-half of the wavelength required by the surface unit, the angular stability is also improved compared to traditional frequency selective surfaces. At the same time, due to its miniaturization characteristics, taking 3*3 units as an example, the overall structure size is only 3cm*3cm, which shows that its minimum working size is sufficient to meet the needs of most occasions.

With the help of CST STUDIO SUITE 2016 software for simulation, it can be seen in Figure 9 that the frequency selective surface structure is a first-order resonance structure, and the reflection curve near the resonance frequency point is relatively smooth, corresponding to a low Q value in the equivalent circuit, that is, the 3dB passband is from 5.27GHz covers to 22.62GHz, the bandwidth is 17.35GHz, and the relative bandwidth of the center frequency point 13.95Ghz reaches 124.4%, covering the entire 6-18GHz common frequency band of aviation antennas, far exceeding the relative bandwidth of the ultra-wideband frequency selection surface in the prior art . At the same time, the frequency selective surface also has a certain angle stability. As shown in Fig. 10, under the oblique incidence angle of 15 degrees, the 3dB passband covers from 5.40GHz to 21.42GHz, the bandwidth is 16.02GHz, and the relative bandwidth is 119.5%. Under the oblique incidence at an angle of 30 degrees, the 3dB passband covers from 5.83GHz to 18.89GHz, the bandwidth is 13.06GHz, and the relative bandwidth is 105.7%. It can be seen that in the case of oblique incidence, the frequency selective surface can still guarantee more than 100% relative bandwidth, with considerable angular stability.

Figure 11 and Figure 12 show the distribution of the surface electric field at the resonant frequency point (12.8 GHz) of the two-layer metal patch with a frequency selective surface structure under the condition of vertical incidence of electromagnetic waves. It can be seen that the surface electric field distribution is relatively uniform, and the patch Each part excites a strong electric field (the black area has a higher field strength), which proves that the bandpass frequency selective surface has low loss in transmission energy, high transmission efficiency, and reasonable design.

There are many specific application approaches of the present invention, and the above description is only a preferred embodiment of the present invention. It should be pointed out that for those of ordinary skill in the art, some improvements can also be made without departing from the principles of the present invention. Improvements should also be regarded as the protection scope of the present invention.

Claims (9)

1. A miniaturized low-profile ultra-wide passband frequency selective surface, which is composed of three layers, is characterized in that it is respectively: the first metal patch layer, the middle layer and the second metal patch layer, and the three are in order Pressed together, the unit of the first metal patch layer is a rectangle with the same length and width, and a quarter square ring with an opening outward is set on the four corners of the rectangle, and the center of the unit is a complete rectangular ring. After continuation, a staggered array of square rings is presented; the unit of the second metal patch layer is a rectangle with the same size as the unit of the above-mentioned first metal patch layer, and the center is a cross-shaped metal wire. At the midpoint, there are rectangular metal patches connected with cross-shaped metal wires. After the plane cycle is extended, a grid-like array of cross-shaped metal wires and rectangular metal patches appears alternately. 2. The miniaturized low-profile ultra-wide passband frequency selective surface according to claim 1, characterized in that, the middle layer adopts a high-frequency microwave circuit board. 3. a design method of a miniaturized low-profile ultra-wide passband frequency selective surface, based on the miniaturized low-profile ultra-wide passband frequency selective surface according to claim 1, it is characterized in that, comprising the following: 1) According to the bandwidth and section requirements of the required frequency selection surface, select a suitable microwave filter and give an equivalent circuit; 2) Following the principle of impedance matching, the equivalent circuit of the microwave filter is approximately transformed to the form of matching frequency selective surface design, and the basic structure of the frequency selective surface is obtained; 3) Deduce the parameter range in the basic structure of the above-mentioned design frequency selection surface through the transformation formula of the distributed parameter electric element and the lumped parameter electric element and the parallel circuit resonant frequency formula; 4) Utilize the metal patch and dielectric layer structure in the frequency selective surface to realize the electrical properties of the capacitance, inductance and transmission line in the equivalent circuit of the above step 2), and clarify the specific frequency selective surface structure; 5) Select the processing material and use the laminate technology to produce the designed frequency selective surface finished product. 4. the design method of miniaturized low-profile ultra-wide passband frequency selective surface according to claim 3, it is characterized in that, the operating bandwidth of the microwave filter in the above-mentioned steps 1) and the corresponding frequency selective surface structure should belong to the same order of magnitude , the order of the frequency response curve of the microwave filter determines the structural thickness of the frequency selective surface, that is, the profile. The higher the order of the microwave filter, the higher the profile of the frequency selective surface. 5. the design method of miniaturized low-profile ultra-wide passband frequency selective surface according to claim 3, is characterized in that, above-mentioned step 2) in, at first, the input and output impedance of two-port network use the free impedance of free space Substitute Z ₀ =377Ω, and then ignore the capacitors C ₁ , C ₃ and inductor L ₂ with low values, and transform the original inductor-capacitor-inductor T-shaped network L ₁ -C ₂ -L ₃ into capacitors according to the transmission line theory -Inductance-capacitance π-type network C'-L'-C'; the equivalent circuit of a short transmission line is regarded as a parallel capacitor-inductance loop formed by a specific resistor in series, and a capacitor C' in the π-type network Replace it with a short transmission line Z ₁₂ , and at the same time correct the component values of the remaining capacitor C' and inductor L' in the network to C and L to match the overall impedance of the π-type network C'-L'-C'circuit; the equivalent circuit Corresponding to the frequency selection surface, the capacitor C uses a layer of metal patch to realize its capacitance characteristics, the transmission line Z ₁₂ uses a layer of dielectric layer to realize its impedance characteristics, and the inductance L uses a layer of metal patches to realize its inductance characteristics; The third layer is a thin metal patch layer, and the second layer is a dielectric layer with a certain thickness. 6. the design method of miniaturized low-profile ultra-wide passband frequency selective surface according to claim 3, it is characterized in that, above-mentioned step 3) the value of electric element in the circuit is corresponding to obtain in the frequency selective surface structure by following formula The specific parameter range of the sheet and the dielectric layer:

Among them, C is the capacitance value of the final equivalent circuit, L is the inductance value of the final equivalent circuit, ε ₀ ≈8.85*10^(-12), μ ₀ ≈1.26*10^(-6) and π≈3.14 are Constant constant, ε _r is the dielectric constant of the selected dielectric layer, p is the unit period size of the frequency selective surface, s is the interval width of the first layer of metal patches, and w is the width of the second layer of metal grid lines; s and w represents the overall size of the unit structure after periodic extension, and does not appear in the unit parameters alone; from the formula (1)(2), it can be seen that increasing the unit period size p, reducing the metal patch interval width s or Metal grid line width w, increase the value of C and L; according to the resonant frequency of the parallel circuit

It is obtained that the resonant frequency is proportional to the unit period size p, and inversely proportional to the spacing width s of the metal patch and the line width w of the metal grid. 7. the design method of miniaturized low-profile ultra-wide passband frequency selective surface according to claim 5, it is characterized in that, above-mentioned step 4) utilizes the excellent electric performance matching step 2) of electrical conductivity in the equivalent circuit internal capacitance and The metal patch of the inductance element realizes the performance of the lumped capacitance C' in the circuit with the structure of the distributed capacitance; similarly, the characteristics of the inductance L' are realized by using the metal of the same material in the grid structure; relying on the dielectric A high-frequency microwave circuit board with a constant and loss tangent replaces a short transmission line to obtain a frequency-selective surface structure of a capacitive metal patch layer-impedance matching dielectric plate layer-inductive metal patch layer. 8. The design method of the miniaturized low-profile ultra-wide passband frequency selective surface according to claim 3, characterized in that, the processing sample of the frequency selective surface in the above-mentioned step 5) comprises at least 3*3 unit arrays, the first The first and second layers of metal patches are made of metal with excellent electrical conductivity, and the thickness of the patch is controlled within 35um-70um. The dielectric plate of the middle layer needs to meet the relative permittivity requirements derived during design, and at the same time ensure the loss tangent Low, the dielectric board and the metal patch layer need to be closely connected during processing, and the copper-clad laminate technology is used for lamination. 9. The method for designing a miniaturized low-profile ultra-wide passband frequency selective surface according to claim 8, characterized in that the metal with excellent electrical conductivity is selected as silver or copper in the above step 5).

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