Wave-absorbing sponge for ground experiment of X-band microwave charging transmitting antenna

Abstract

The invention relates to a wave-absorbing sponge for a ground experiment of an X-band microwave charging transmitting antenna. The wave-absorbing sponge is of a two-dimensional array structure, according to the geometric structure of the wave-absorbing sponge in one period, the bottom is a wave-absorbing sponge flat plate, the upper portion is a quadrangular wedge body, the lower surface of the quadrangular wedge body is a rectangular face, the other faces are triangular faces, and the width of the lower surface and the width of the periodic structure are both w, the length of the periodic structure is equal to the length l of the bottom wave-absorbing sponge flat plate in the period, the bottom edge of the lower surface is in contact with the surface of the wave-absorbing sponge flat plate and forms an included angle alpha with the surface, the thickness of the four-edge wedge body in the incident wave propagation direction is gradually increased, and the four-edge wedge body is periodically repeated in the horizontal x direction, so that the lower surfaces of the four-edge wedge body are seamlessly spliced together; the longitudinal direction periodically repeats in the y-direction. The device has the beneficial effects that ground microwave reflection in a microwave wireless energy transmission ground beam efficiency measurement experiment is inhibited, the accuracy of a measurement result is ensured not to be influenced by the ground reflection, the structure is simple, the cost is low, and the weight is light.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of microwave beam transmitting antennas in planar reflectarray antennas, and relates to a wave-absorbing sponge for ground experiment measurement. BACKGROUND

[0002] In recent years, microwave wireless energy transmission technology has attracted attention in the application scene of wireless charging of unmanned aerial vehicles for inspection of high-voltage transmission towers in mountainous areas. In the X-band, microwave wireless energy uses the approximate non-diffraction propagation characteristics of microwave beams in the radiative near-field region to achieve wireless energy transmission on the order of tens of meters, which basically meets the distance requirements for charging unmanned aerial vehicles for inspection from the ground to the top of the transmission tower. On the other hand, X-band microwaves can propagate with low loss in mountainous atmospheric environments such as strong winds, rain and fog, which meets the technical needs of effective charging under various special atmospheric conditions.

[0003] A planar reflectarray antenna is a common microwave beam transmitting antenna, and its energy transmission efficiency is usually referred to as beam efficiency, which refers to the ratio of microwave power reaching the receiving area to the microwave power emitted by the antenna. The design and debugging of a planar array antenna ultimately aims at and is based on the experimental measurement results of beam efficiency, and experimental measurement and debugging are usually carried out on the ground, so it is necessary to consider the reflection of the microwave beam on the ground. The ground has weak conductivity, and the microwave beam reaches the ground after diffraction and is reflected by the ground. The reflected wave and the axially propagating beam superimpose to form an interference space fringe. Even if the transmitting and receiving device is elevated by several meters, it is difficult to eliminate the adverse effects of this interference on measurement. Therefore, microwave absorbing materials need to be laid on the ground to reduce the power of the reflected wave. The common microwave absorbing material on the market is an angle pyramid-shaped wave-absorbing sponge, as shown in FIG. 1, which is commonly used for microwave absorption in a microwave darkroom. With multiple reflections of microwaves between the angle pyramid arrays, high power loss can be achieved. The maximum loss occurs when the microwave incidence direction is approximately parallel to the normal direction of the angle pyramid array. Lu et al. pointed out in a paper that when the microwave is incident at an angle of ~ 15° into the plane of the angle pyramid array, the number of microwave reflections is as low as 1-2 times, and the wave-absorbing ability of the angle pyramid wave-absorbing sponge array is greatly weakened. Figure 1

[0004] In view of the problem that the typical angle pyramid wave-absorbing sponge array cannot effectively absorb the grazing incidence microwave, a new two-dimensional array structure of wave-absorbing sponge is designed, which has a power loss of not less than -20 dB for horizontally and vertically polarized incident waves, and can be applied to beam efficiency test experiments of long-distance microwave wireless energy transmission. SUMMARY

[0005] The application aims to provide a wave-absorbing sponge for ground experiment of an X-band microwave charging transmitting antenna.

[0006] ​The technical solution of this invention: A microwave absorbing sponge for ground experiments of X-band microwave charging transmitting antennas. The absorbing sponge has a two-dimensional array structure and is made of pure dielectric loss sponge material. The geometric structure of the absorbing sponge within one period is a bottom absorbing sponge plate and an upper quadrangular wedge. The lower surface of the quadrangular wedge is rectangular, and the other surfaces are triangular. The width of the lower surface is equal to the width of the periodic structure. w The length of the periodic structure is equal to the length of the bottom absorbing sponge plate within the period. l The distance between the high side edge of the quadrangular wedge and the absorber foam plate is h0. The bottom edge of the lower surface contacts the surface of the absorber foam plate and forms an angle α with it. The lower surface is defined as the rear side, that is, the surface with its normal pointing inwards towards the bottom foam plate. The front side of the quadrangular wedge is defined as the surface with its normal pointing outwards away from the bottom foam plate, and does not contact the absorber foam plate. The angle between the front side and the absorber foam plate is... β The distance between the lowest point of the front side and the absorbing sponge plate is h1. The thickness of the four-sided wedge gradually increases in the direction of incident wave propagation. The horizontal x-direction repeats periodically, so that the lower surfaces of the four-sided wedge are seamlessly spliced ​​together. The longitudinal direction repeats periodically in the y-direction, forming a two-dimensional absorbing sponge array structure.

[0007] Preferably, h1 is 10 mm, and the quadrangular wedge is bonded to the bottom absorbing foam plate with a hardened sponge of low dielectric constant. The maximum thickness of the quadrangular wedge in the direction perpendicular to the incident direction in the cross-section is 2 cm.

[0008] Furthermore, slots are cut into the upper part of the four-sided wedge, and the slots are separated by air to form a periodically arranged dielectric rod waveguide, with several metal sheets attached to the front side.

[0009] Preferably, h0 is 90 mm, the longitudinal length of the quadrangular wedge projected onto the absorbing sponge plate is 150 mm, the groove depth is 13.6 mm, and the groove width is 6.3 mm.

[0010] Preferably, the metal sheet is square with a side length of 7.7 mm. The period length of the metal sheet in the wave propagation direction is 17 mm, and the period length in the horizontal direction is 14 mm. The included angle α is 37°.

[0011] The beneficial effects of this invention are: the microwave absorbing sponge structure is used to suppress ground microwave reflection in the microwave wireless power transmission ground beam efficiency measurement experiment, ensuring that the accuracy of the measurement results is not affected by ground reflection, and it is effective for any polarization case, and has the characteristics of simple structure, low cost and light weight. Attached Figure Description

[0012] Figure 1 It is a pyramidal-shaped wave-absorbing sponge array structure; Figure 2It is the reflection of incident waves from a cone-shaped absorbing sponge between the cones; Figure 3 This is a structural diagram of the microwave absorbing sponge plate and the four-sided wedge of the present invention; Figure 4 This demonstrates three different absorption paths when an incident wave enters the wave-absorbing sponge structure array of this invention. Figure 5 It refers to the distribution of the electric field in the horizontal longitudinal section and the distribution of the magnetic field in the vertical longitudinal section after slotting the dielectric rod array structure; Figure 6 The distribution of the guided wave electric field when a horizontally polarized wave is incident on a dielectric rod array is shown in the left figure: the amplitude of the tangential electric field in the cross section at the incident position; and the amplitude of the tangential electric field in the cross section at the end of the wedge of the graded dielectric. Figure 7 The distribution of the guided wave electric field when a horizontally polarized wave is incident on a dielectric rod array loaded with a thin metal sheet. Left figure: amplitude of the tangential electric field in the cross section at the incident position; Right figure: amplitude of the tangential electric field in the cross section at the end of the wedge of the graded dielectric. Figure 8 The reflection coefficient varies with the period length of the propagation direction after loading a periodically arranged square metal sheet; Figure 9 It is a wave-absorbing sponge structure with slots and metal sheet structure; Figure 10 These are the reflection coefficients of the absorbing sponges designed for different frequencies; Figure 11 It is the reflection coefficient of the absorbing sponge designed under different grazing incidence angles at 10 GHz. Detailed Implementation

[0013] A microwave absorbing sponge for ground experiments of X-band microwave charging transmitting antennas is provided. The material of the absorbing sponge is pure dielectric loss absorbing sponge, which has advantages in loss capability in the X-band compared with magnetic loss absorbing sponge. Its relative permittivity is 6 and the loss tangent is 0.1, which is the same as the dielectric properties of common pyramidal absorbing sponges.

[0014] like Figure 3 As shown, the geometric characteristics of the wave-absorbing sponge structure within one cycle are: 1. The lower surface of the square wedge is an inclined plane with a rectangular shape. Figure 3 The middle part is the occluded surface, and its width is equal to the width of the periodic structure. w The length of the periodic structure is equal to the length of the bottom absorbing sponge plate within the period. l The distance between the high side edge of the quadrangular wedge and the bottom surface is h0, and the bottom edge of the lower surface contacts the surface of the wave-absorbing sponge plate and forms an angle with it. α included angle αApproximately 37°; 2. Viewed along the microwave incident direction, the upper surface area of ​​the quadrangular wedge is very small, having degenerated into a single vertex. The entire quadrangular wedge can be considered as the spatial region swept by the lower surface during the process of minimum continuous deformation from the lower surface to the vertex; 3. The front side of the quadrangular wedge is defined as the surface normal to the bottom absorbing sponge plate and pointing outwards. Figure 3 The middle surface is the surface with its normal direction close to the positive z-axis, while the rear side surface (i.e., the aforementioned lower surface) is the surface with its normal direction pointing inwards towards the bottom absorbing foam plate. The front side surface does not contact the bottom absorbing foam plate, and the angle between the front side surface and the upper surface of the bottom absorbing foam plate is... β , β Less than α; 4. The horizontal x-direction repeats periodically, making the lower surfaces of the four-sided wedges seamlessly joined together, and the longitudinal y-direction repeats periodically, forming a two-dimensional wave-absorbing sponge array structure.

[0015] exist Figure 3 In the model, the distance between the lowest point of the front side and the absorbing plate is h1. Reducing h1 will result in a lower reflection coefficient, but simply reducing h1 cannot achieve the requirement of a reflection coefficient below -20 dB, because the thickness of the quadrangular wedge decreases as h1 decreases, leading to a decrease in the mechanical strength of the structure, which is detrimental to engineering applications. To ensure sufficient mechanical strength, h1 can be chosen to be 10 mm, and the quadrangular wedge can be bonded to the bottom absorbing sponge plate using a low dielectric constant hardened sponge. In this case, the maximum thickness of the quadrangular wedge in the perpendicular incident direction within the cross-section is approximately 2 cm. For ease of display, the non-lossy material bonded to the quadrangular wedge and the absorbing sponge plate is omitted and not shown in the model.

[0016] like Figure 4 As shown, when the angle between the wave vector direction of the incident microwave and the surface of the bottom absorbing sponge plate is small, most of the incident microwave is reflected and absorbed multiple times in the V-shaped region between the lower surface and the bottom absorbing sponge, constituting the main mechanism of microwave absorption, corresponding to... Figure 4 The incident wave is indicated by the solid arrow. Some microwaves arrive at the front side of the square wedge at an incident angle of nearly 90°. These microwaves undergo multiple propagation paths, such as transmission, reflection, and diffraction, while their propagation direction remains close to the longitudinal axis. Thus, they are "captured" by the V-shaped region of the next longitudinal cycle. Some microwaves also arrive at the left and right sides upon incident, eventually entering the V-shaped region of the next cycle as well. This secondary mechanism of microwave absorption, propagating laterally into other V-shaped regions, corresponds to... Figure 4 The incident wave is indicated by the dashed arrow. A portion of the incident microwave also enters the interior of the four-sided wedge. Looking along its propagation direction, the absorbing medium resembles a wedge structure with gradually increasing thickness. This portion of the microwave entering the absorbing medium possesses a special guided wave mode, corresponding to... Figure 4The incident wave, indicated by the dashed straight arrow and the dashed curved arrow, will lose power after traveling a certain distance inside the medium.

[0017] The main pathway for microwave power dissipation is the portion that penetrates the square wedge into free space after entering the V-shaped region, undergoing multiple reflections and absorptions. The angle between its propagation direction and the bottom surface is smaller than the grazing angle of incidence, preventing it from effectively entering the next longitudinal V-shaped absorption region and instead causing it to dissipate into free space. In addition, guided waves entering the square wedge also experience reflection during mode conversion, a natural consequence of impedance mismatch. This reflection also directly leads to microwave power dissipation into free space. Therefore, specific designs are needed to address these two types of microwave power dissipation.

[0018] Reflection is inevitable during the mode transition of a plane wave entering a square wedge. Generally, reducing reflection is achieved by improving impedance matching. However, upon entering an infinitely wide wedge structure with a gradually varying thickness, the guided wave mode is no longer the TEM mode but a TE mode relative to the propagation direction, making impedance mismatch difficult to completely avoid. However, by slotting, a transition mode can be introduced, allowing the plane wave to first match the transition mode, and then the transition mode to match the TE mode of the guided wave within the absorbing medium, thus reducing reflection during mode transition. After slotting, the absorbing medium is separated by air, forming periodically arranged dielectric rod waveguides. When these dielectric rod waveguides are excited by a plane wave, the resulting mode is a mixed EM mode, such as... Figure 5 As shown. Compared to the tangential field in free space, the amplitude of the tangential magnetic field inside the dielectric rod waveguide doubles, while the amplitude of the tangential electric field remains essentially unchanged, resulting in poor mode matching, such as... Figure 6 As shown. If on the surface of the medium, i.e. Figure 2 By attaching some thin metal sheets to the front side, the magnetic field can be effectively suppressed, while the electric field component is only affected on the surface of the medium and not inside the medium, so the suppression of the electric field is not obvious. Figure 7 A comparison of the tangential fields in two cross-sections before and after loading the metal sheet is presented. The amplitude of the tangential electric field decreased by a factor of 1.33, while the amplitude of the tangential magnetic field remained essentially unchanged, indicating improved mode matching. By changing parameters such as the slot width, the geometry of the metal patch, and the period length, the amplitude of the tangential field can be optimized to achieve better matching, thereby reducing the reflected wave power.

[0019] For the portion of microwave power entering free space from the V-shaped region through the square wedge, a square metal sheet can be fabricated and arranged in a periodic structure to guide surface waves to participate in energy coupling. Since the propagation direction of the transmitted wave is close to the front side, surface wave coupling is easily achieved. Therefore, the surface wave mode guided by the metal sheet can reduce the transmitted wave power entering free space. During the propagation of the surface wave mode, its power is absorbed within the medium, thereby improving the overall absorption capacity and reducing the reflection coefficient. When the relative permittivity of the medium is 6, the surface wave wavelength is approximately 1.2 cm. Assuming an incident angle of 20° from the medium to the air interface, the wave vector difference that the metal sheet needs to compensate for is approximately (2π / Lambda spp - k air Therefore, the period of the metal sheet is approximately 17 mm. The effect of changing the period of the metal sheet on the reflectivity is... Figure 8 As shown in the figure, it is not difficult to see that when the period length in the propagation direction is 17 mm, the reflection coefficient reaches a minimum value of -25 dB, and the reflection coefficient also reaches -24 dB under vertical polarization conditions. This result has met the design target requirements at the 10 GHz frequency point.

[0020] The power absorption characteristics of the designed structure at different frequencies, grazing incidence angles, and incident polarization directions were investigated. In the CST microwave studio, the incident wave mode and angle were set using unit cell boundaries, and the reflection coefficient and total absorbed power within the medium were examined. The established numerical simulation model is as follows: Figure 9 As shown. Figure 9 The slotted structure in Figure 5 The structure is consistent with the others, the difference being the length in the propagation direction, i.e., the finite groove depth. The overall horizontal and vertical period lengths of the array are 90 mm and 300 mm, respectively. The highest point height h0 of the lower surface of the quadrangular wedge is 90 mm, and h1 is 10 mm. The vertical length of the quadrangular wedge projected onto the absorbing sponge plate is 150 mm. The groove depth is approximately 13.6 mm, and the groove width is approximately 6.3 mm. The metal sheet is approximately square with a side length of 7.7 mm. The period length of the metal sheet in the wave propagation direction is 17 mm, and the period length in the horizontal direction is 14 mm. The reflection coefficient varies with frequency in... Figure 10 The study shows that, within the frequency range of 9.2–10.6 GHz, at a grazing incidence angle of 15°, the reflection coefficients of both horizontally and vertically polarized incident plane waves are below -20 dB. For measuring the efficiency of microwave wireless power transmission beams, the absorption characteristics of narrowband, low-reflection wave transmission are suitable for practical applications.

[0021] The reflection coefficient at 10 GHz varies with the grazing incidence angle. Figure 11The study shows that when the grazing incident angle is less than 12°, the effect of the V-shaped absorption region in "capturing" the incident plane wave is significantly reduced, resulting in an increase in the reflection coefficient. When the angle between the incident direction and the normal to the horizontal plane is less than 45°, the reflection coefficient is greater than -20 dB, which is in stark contrast to the typical pyramidal absorbing sponge array, which has the best absorption effect when the incident direction is perpendicular. Figure 11 The horizontal axis in the figure represents the angle between the incident direction and the normal to the horizontal plane.

[0022] The two-dimensional periodic array structure of absorbing sponge designed in this invention addresses the problem of reflection caused by grazing incidence of X-band microwave beams on the ground, aiming to reduce the reflection coefficient. The absorbing material is a pure dielectric loss material, and its geometry consists of two parts: a bottom absorbing sponge plate and an inclined four-sided wedge. The four-sided wedge and the absorbing sponge plate form a V-shaped absorbing structure. The thickness of the four-sided wedge gradually increases along the direction of incident wave propagation, and the top edge reflects horizontally polarized plane waves to a certain extent. The tangential field matching under horizontal polarization is improved through a slotted structure design and the loading of a metal sheet. By adjusting the slot width, the equal width of the metal sheet, and the periodic length of the metal sheet along the propagation direction, a surface wave mode is excited. This confines the microwave power from the V-shaped region that reaches the surface through the medium to the medium-air interface, dissipating this power within the medium and preventing its escape into free space. After optimizing the slot depth, slot width, metal sheet side length, and metal patch period length, simulations showed that the reflection coefficients of both polarized incident waves were less than -20 dB when the angle between the incident angle and the horizontal plane normal was within the range of 45-78° at 10 GHz. The designed absorbing array structure can be used to suppress ground microwave reflection in microwave wireless power transmission ground beam efficiency measurement experiments, ensuring that the accuracy of the measurement results is not affected by ground reflection. It is effective for any polarization case and features simple structure, low cost, and light weight.

Claims

1. An absorbing sponge for ground experiments using an X-band microwave charging transmitting antenna, characterized in that: The absorbing sponge is a two-dimensional array structure made of pure dielectric loss sponge material. The geometry of one cycle of the absorbing sponge consists of a flat absorbing sponge plate at the bottom and a four-sided wedge at the top. The lower surface of the wedge is rectangular, and the other surfaces are triangular. The width of the lower surface is equal to the width of the cycle structure. w The length of the periodic structure is equal to the length of the bottom absorbing sponge plate within the period. l The distance between the high side edge of the quadrangular wedge and the absorber foam plate is h0. The bottom edge of the lower surface contacts the surface of the absorber foam plate and forms an angle α with it. The lower surface is defined as the rear side, that is, the surface with its normal pointing inwards towards the bottom foam plate. The front side of the quadrangular wedge is defined as the surface with its normal pointing outwards away from the bottom foam plate, and does not contact the absorber foam plate. The angle between the front side and the absorber foam plate is... β The distance between the lowest point of the front side and the absorbing sponge plate is h1. The thickness of the four-sided wedge gradually increases in the direction of incident wave propagation. The horizontal x-direction repeats periodically, so that the lower surfaces of the four-sided wedge are seamlessly spliced ​​together. The longitudinal direction repeats periodically in the y-direction, forming a two-dimensional absorbing sponge array structure.

2. The absorbing sponge for ground experiments of X-band microwave charging transmitting antennas according to claim 1, characterized in that: h1 is 10 mm. The quadrangular wedge is bonded to the bottom wave-absorbing foam plate using a hardened sponge with a low dielectric constant. The maximum thickness of the quadrangular wedge in the direction perpendicular to the incident direction in the cross-section is 2 cm.

3. The absorbing sponge for ground experiments of X-band microwave charging transmitting antennas according to claim 1, characterized in that: The upper part of the four-sided wedge is slotted, and the slots are separated by air to form a periodically arranged dielectric rod waveguide. Several metal sheets are attached to the front side.

4. The absorbing sponge for ground experiments of X-band microwave charging transmitting antennas according to claim 3, characterized in that: The overall horizontal and vertical period lengths are 90 mm and 300 mm, respectively, h0 is 90 mm, the longitudinal length of the quadrangular wedge projected onto the absorbing sponge plate is 150 mm, the groove depth is 13.6 mm, and the groove width is 6.3 mm.

5. The absorbing sponge for ground experiments of X-band microwave charging transmitting antennas according to claim 3, characterized in that: The metal sheet is square with a side length of 7.7 mm. The period length of the metal sheet in the wave propagation direction is 17 mm, and the period length in the horizontal direction is 14 mm.

6. The absorbing sponge for ground experiments of X-band microwave charging transmitting antennas according to claim 3, characterized in that: The included angle α is 37°.

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