A hybrid cascaded bandpass frequency selective surface with flexible design of passband and stopband

Abstract

The present invention discloses a hybrid cascade bandpass frequency selective surface with flexible design of passband and stopband, which belongs to the field of electromagnetic field and microwave technology. The frequency selective surface described in the present invention is composed of a two-layer structure: the upper layer is a grid square ring structure, and the lower layer is a double square ring structure. Without introducing an additional electromagnetic structure, the present invention increases the equivalent interlayer spacing of the upper and lower layer structures by staggering the upper and lower layer units, thereby improving the large-angle stability of the frequency selective surface. The invention can achieve stable transmission performance of electromagnetic waves in the 3.3-3.8GHz frequency band under different polarizations and angles of incidence. At the same time, the frequency band where the passband and stopband of the invented frequency selective surface are located can adjust the side length of the relevant square ring structure to achieve flexible design. The invented frequency selective surface has a transmission zero point outside the passband, which can achieve good stopband characteristics.

Description

Technical Field

[0001] The invention belongs to the field of electromagnetic field and microwave technology, and in particular relates to a hybrid cascade bandpass frequency selective surface with flexibly designed pass / stop bands. Background Art

[0002] Fifth-generation (5G) communication systems will be widely used in various fields due to their high data transmission rates and low latency. However, their predecessors, such as 4G, 3G, and even 2G, will continue to play a role for a considerable period of time. Even though some older-generation networks may be canceled in the future, the frequency bands allocated for them may still be used. Therefore, the 5G era requires dual-band / multi-band base station antenna arrays to cover these frequency bands. To save installation space and costs, antennas typically operate on different frequency bands, which are often interleaved. However, in multi-band antenna arrays, the electromagnetic environment of the antennas in each band becomes quite complex. Electromagnetic coupling in multi-band antenna arrays always affects the radiation pattern, input impedance, and port isolation of antennas operating in different frequency bands. In a multi-antenna solution, multiple antennas need to be developed to cover the required frequency bands. The size of the low-frequency antenna is usually larger than that of the high-frequency antenna. When the high-frequency antenna is working, due to the obstruction of the low-frequency antenna, the electromagnetic waves radiated by the high-frequency antenna will stimulate induced currents on its metal structure when passing through the low-frequency antenna, generating secondary radiation, thereby distorting the radiation pattern of the high-frequency antenna and affecting the matching effect. Therefore, it is often necessary to specially design the low-frequency antenna to improve its impact on the antenna operating in the high-frequency band. Introducing frequency selective surfaces in multi-band antennas can achieve the cross-band decoupling effect of multi-band antennas without changing the structure of the low-frequency antenna.

[0003] A frequency selective surface (FSS) is a single-layer or multi-layer planar or three-dimensional structure formed by a periodic arrangement of scattering units or aperture structures. Due to its filtering properties for electromagnetic waves of different operating frequency bands, incident angles, and polarization states, FSS can be considered as spatial filters and is widely used in radar radome applications. An ideal FSS radome can protect the internal antenna from the external physical environment, fully scatter electromagnetic waves outside the antenna's operating frequency band, and allow electromagnetic waves within the operating frequency band to pass through with low loss, while also meeting the normal scanning requirements of the antenna array. This requires that the design of the FSS must have low loss within the passband, wide-angle stability, and be easy to flexibly adjust in different frequency bands to adapt to similar applications in different frequency bands, thereby reducing design costs.

[0004] To enhance the wide-angle stability of frequency selective surfaces, the paper "Compact Ultra-Wide Band Frequency Selective Surface With High Selectivity" uses meander lines to miniaturize the frequency selective surface, thereby improving wide-angle stability. The paper "A Novel Fractal Inspired Iterated Four-Legged Loaded Loop Elements Based 2.5-D Miniaturized Frequency Selective Surface" enhances the wide-angle stability of the frequency selective surface by introducing metal vias in a dielectric substrate. The paper "On the Improvement of Angular Stability of the 2nd-order Miniaturized FSS Structure" achieves scan-independent characteristics of the frequency selective surface by introducing a thick dielectric layer. To achieve wide-angle stability of frequency selective surfaces, existing frequency selective surfaces often incorporate additional physical structures or complex graphical structures, increasing design and manufacturing costs. Furthermore, their angular stability often manifests as ensuring that the designed frequency point remains stable when electromagnetic waves are incident at different angles, rather than ensuring that the transmission characteristics remain stable at different angles within the desired frequency band.

[0005] In response to the above problems, the present invention discloses a hybrid cascaded bandpass frequency selective surface with flexibly designed pass and stop bands. Summary of the Invention

[0006] Building on the aforementioned background technology and addressing the shortcomings of existing technologies, this invention proposes a hybrid cascaded bandpass frequency selective surface (FSS) with flexibly designed passbands and rejection bands. This FSS, composed of two layers of printed metal dielectric plates, exhibits excellent transmission characteristics for electromagnetic waves of varying polarizations and incident angles within the operating frequency band, while exhibiting excellent reflection characteristics for electromagnetic waves outside the operating frequency band. This design covers the n78 band of 5G communications and can be applied to multi-band, common-aperture antennas for 5G communications.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A hybrid cascade bandpass frequency selective surface with flexibly designed pass / stop bands comprises a dielectric substrate with a printed grid square ring structure and a dielectric substrate with a printed square square ring structure.

[0009] Furthermore, the dielectric substrate of the printed grid square ring is composed of a square ring and a short branch perpendicular to the edge at the center of each edge.

[0010] Furthermore, the two square rings printed on the dielectric substrate for printing the two square rings are designed to be staggered, and appear as double right-angle structures distributed on the four corners of the substrate unit in the same period as the grid square rings. After the periodic arrangement, the double right-angle structure can be spliced ​​with the double right-angle structure of the adjacent unit to restore the double square ring structure.

[0011] The beneficial effects brought about by the frequency selective surface of the present invention are: 1. It has good transmission characteristics for electromagnetic waves in the designed frequency band of 3.3-3.8GHz, and has good reflection characteristics outside the band. 2. Under the conditions of different polarizations and different angles of oblique incidence, the FSS still has good transmission characteristics within the designed frequency band. When the angle of incidence is 60°, the minimum value of the FSS transmission coefficient is -0.8dB. 3. The FSS adopts two layers of dielectric plates, which has a simple structure, is easy to manufacture, and has low production cost. 3. For the same type of applications with other required frequency bands, the side lengths of the square rings of the grid square ring structure and the square square ring structure can be adjusted to achieve flexible design of the pass-band and stop-band frequency bands and reduce design costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a schematic diagram of the frequency selective surface structure of the present invention;

[0013] Figure 2 A top view of the square ring layer structure of the frequency selective surface grid of the present invention;

[0014] Figure 3 A top view of the square ring layer structure of the frequency selective surface of the present invention;

[0015] Figure 4 1. This is a graph showing the transmission characteristics of the frequency selective surface TE and TM polarized waves at 0° incidence in the 1.0-4.5 GHz frequency band according to an embodiment of the present invention;

[0016] Figure 5 Graph showing the transmission characteristics of the frequency selective surface TE and TM polarized waves at 60° incidence in the 1.0-4.5 GHz frequency band according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] The technical solution of the present invention is described in detail below with reference to the accompanying drawings, but the protection scope of the present invention is not limited to the implementation examples.

[0018] like Figure 1The figure shows a hybrid cascaded bandpass frequency selective surface unit with flexible pass / stop band design. This unit has a period of 30mm and is constructed from two layers of printed dielectric substrates. The upper layer is a 1mm thick dielectric substrate with a grid of square rings and a dielectric constant of 3.0. The composite square ring structure etched on the substrate consists of a square ring 1 and four short rectangular branches 2 located at the center of each side of the square ring and perpendicular to the ring edge. The lower layer is a 1mm thick dielectric substrate with a dielectric constant of 3.0. The square rings printed on the substrate are arranged as double right-angle structures distributed at the four corners of the dielectric substrate. The double right-angle structures are composed of inner right-angles 3 and outer right-angles 4. After periodic arrangement, these double right-angle structures merge with the double right-angle structures of adjacent units to form a double right-angle structure. Air is 18mm thick between the upper and lower dielectric substrates.

[0019] like Figure 2 As shown, the first layer of the grid square ring structure has an outer side length of l1 = 22 mm, an inner side length of l2 = 18 mm, a short rectangular branch length of l3 = 3 mm, and a width of l4 = 1 mm. This grid square ring structure is equivalent to a parallel LC circuit. Adjusting the side length of the grid square ring can change the resonant frequency of the equivalent parallel LC circuit, achieving a certain degree of wave transmission performance within the designed frequency band.

[0020] like Figure 3As shown, the second-layer square ring structure appears as a double right-angle structure distributed at the four corners of the dielectric plate. The outer right angle has an outer side length of L1 = 13.75mm and a width of L2 = 0.5mm. The outer right angle has an outer side length of L3 = 8.25mm and a width of L4 = 2.1mm. The equivalent circuit of the double square ring structure is two parallel series LC circuits. The resonant frequencies of the two series LC circuits differ due to the size difference between the inner and outer square rings. When the two series LC circuits resonate, two transmission zeros are generated at their resonant frequencies. The inner and outer rings control the transmission zeros at high and low frequencies, respectively, forming stopbands around these two resonant frequencies. Furthermore, because the reactance of the series LC circuit is capacitive before resonance and inductive after resonance, within the frequency band between the two transmission zeros, the outer ring of the square ring structure is in a resonant state, while the inner ring has not yet resonated. At this point, its equivalent circuit can be simplified to a parallel LC circuit, thus forming a passband. Adjusting the side length of the square rings can change the resonant frequency of the series LC circuit in the equivalent parallel branch, thereby adjusting the passband and stopband frequency band of the square ring structure. The bandwidth formed by the grid square ring layer and the square ring layer is relatively narrow, but when cascaded, the bandwidth can be expanded, thereby achieving a passband design within the desired frequency band. At the same time, the parallel series LC circuit equivalent to the square ring structure can also introduce transmission zeros outside the passband, achieving out-of-band suppression.

[0021] Figure 4 and Figure 5 Figure 2 shows the transmission characteristics of a frequency selective surface (FSS) for TE and TM polarized waves at incident angles of 0° and 60°, respectively, in a specific embodiment of the present invention. It can be seen that, at both 0° and 60° incidence, the transmission coefficients for TE and TM polarized waves remain above -0.8 dB in the 3.3-3.8 GHz range, demonstrating that the invented FSS exhibits excellent angular and polarization stability. At high angles of incidence, it exhibits good passband characteristics for both TE and TM polarized waves. Beyond the passband, due to the presence of a transmission zero, the FSS also exhibits strong out-of-band suppression.

[0022] The above description and implementation methods are only some preferred examples of the present invention and do not constitute any limitation to the present invention. For professionals in this field, the present application may have various changes and variations, but the modifications and changes based on the ideas of the present invention are still within the scope of protection of the claims of the present invention.

Claims

1. A hybrid cascaded bandpass frequency selective surface with flexibly designed pass / stop bands, characterized in that: The frequency selective surface is composed of a mixed cascade of a grid square ring structure and a double square ring structure. The double square ring layer is cascaded below the grid square ring layer after being staggered. The grid square ring structure is formed by etching a composite square ring groove on a single-sided copper-clad dielectric board. The composite square ring is composed of a square ring and a short branch perpendicular to the edge at the center of each side. The double square ring structure is a double right-angle structure distributed at the four corners of the dielectric substrate. After periodic arrangement, the double right-angle structure and adjacent units can be spliced ​​to form a double square ring structure.

2. The hybrid cascaded bandpass frequency selective surface with flexibly designed pass / stop bands according to claim 1, characterized in that: The thickness of the upper and lower dielectric plates is 1 mm, and the space between the dielectric plates is air with a thickness of 18 mm.

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