A broadband circularly polarized microstrip antenna

Abstract

The application relates to the technical field of microstrip antennas, in particular to a broadband circularly polarized microstrip antenna which comprises an upper-layer metasurface (5), a first-layer dielectric plate (1), a lower-layer metasurface (6), a second-layer dielectric plate (2), a ground plate (8), a third-layer dielectric plate (3) and a microstrip feed line (4); wherein the upper-layer metasurface (5) is an upper-layer quadrilateral array formed by uniformly arranging N*N upper-layer diamond units, the lower-layer metasurface (6) is a lower-layer quadrilateral array formed by uniformly arranging N*N lower-layer diamond units, the upper-layer diamond units and the lower-layer diamond units are different in size, and the spacing between the upper-layer diamond units is greater than the spacing between the lower-layer diamond units; the horizontal diagonal line of the upper-layer diamond unit is longer than the vertical diagonal line, and the vertical diagonal line of the lower-layer diamond unit is longer than the horizontal diagonal line; the third-layer dielectric plate (3) is provided with a cross-shaped slot (7) formed by horizontal slots and vertical slots; the horizontal slots and the vertical slots are equal in width and different in length.

Description

Technical Field

[0001] This invention relates to the field of microstrip antenna technology, and in particular to a broadband circularly polarized microstrip antenna. Background Technology

[0002] Circularly polarized antennas are widely used in wireless communication systems such as Global Navigation Satellite System (GNSS), mobile satellite communication, radio frequency identification (RFID), wireless local area network (WLAN), direct broadcast television service (DBS), and global microwave interconnection access (WIMAX) due to their unique characteristics.

[0003] Traditional circularly polarized antenna design methods are mainly divided into single-feed, multi-feed, and multi-element methods. The single-feed method refers to using one feed port to excite two orthogonal modes simultaneously to achieve circular polarization. This method has a simple structure, but the bandwidth is often narrow and it can only operate in narrowband. The multi-feed method refers to using multiple feed ports to excite different modes to achieve circular polarization. Since the 90° phase difference required to achieve circular polarization is provided by the feed phase difference, the broadband characteristics of circular polarization that can be achieved by the multi-feed method depend more on the bandwidth of the feed network. The design of broadband feed networks is more complex (see reference: Yang, L., et al. (2017). "Wideband circularly polarized antenna based on tightly coupling effect." Electronics Letters 53(7):448-450.). The multi-element method refers to using multiple linearly polarized antennas arranged in a certain order to form an array, and applying different phase excitations to each element to achieve circular polarization. The disadvantage of this method is that it occupies too much space, which is not conducive to the miniaturization of the antenna (see reference: Zou, Y., et al. (2019). "A high-gain compact circularly polarized microstrip array antenna with simplified feed network." International Journal of RF and Microwave Computer-Aided Engineering (8).).

[0004] The emergence and application of metamaterials have brought many new possibilities to the design of circularly polarized antennas. A metamaterial is an artificial structure composed of subwavelength units arranged in a periodic or aperiodic manner. Its properties are determined by the size and structure of the metamaterial. Materials that can achieve any unique properties that natural materials cannot achieve on their own belong to the category of metamaterials.

[0005] However, three-dimensional metamaterials have problems such as being difficult to process and having high losses. Metasurfaces are planar arrays composed of subwavelength scale units arranged in a certain regular periodic or non-periodic pattern, i.e., two-dimensional metamaterials. Metasurfaces have smaller structures and losses than three-dimensional metamaterials, but can still achieve arbitrary manipulation of electromagnetic waves. At the same time, the planar structure is more conducive to practical processing. Due to the unique properties of metasurfaces, the introduction of metasurfaces with different functions in antenna design has gradually become a novel and effective solution. Such antennas are called metasurface antennas (see reference: Lin, FHand ZNChen (2017). "Low-Profile Wideband Metasurface Antennas Using Characteristic Mode Analysis." IEEE Transactions on Antennas and Propagation 65(4):1706-1713.).

[0006] Currently, most broadband circularly polarized metasurface antennas adopt a single-layer structure and combine it with a complex feeding structure to further broaden the circular polarization bandwidth. This results in a complex antenna structure design and a relatively narrow circular polarization bandwidth (see references: Zhao, C. and C.-F. Wang (2018). "Characteristic Mode Design of Wide Band Circularly Polarized Patch Antenna Consisting of H-Shaped Unit Cells." IEEE Access 6:25292-25299. and references: Gao, X. et al. (2021). "A Low-Profile Broadband Circularly Polarized Patch Antenna Based on Characteristic Mode Analysis." IEEE Antennas and Wireless Propagation Letters 20(2):214-218.). Summary of the Invention

[0007] The purpose of this invention is to overcome the narrow bandwidth of existing circularly polarized microstrip array antennas and to propose a broadband circularly polarized microstrip antenna based on a rhombic metasurface.

[0008] To solve the above-mentioned technical problems, the broadband circularly polarized microstrip antenna provided by the present invention includes: an upper metasurface 5, a first dielectric substrate 1, a lower metasurface 6, a second dielectric substrate 2, a ground plane 8, a third dielectric substrate 3, and a microstrip feed line 4 arranged in parallel from top to bottom. The upper metasurface 5 is an upper quadrilateral array composed of N×N uniformly arranged upper rhombic units, where N is an odd number. The upper quadrilateral array has the upper rhombic unit at its center removed, and slots extending along the lateral diagonal of the upper rhombic unit are provided in the upper rhombic units at its four corners. The lower metasurface 6 is a lower quadrilateral array composed of N×N uniformly arranged lower rhombic units. The lower quadrilateral array has slots extending along the lateral diagonal of the lower rhombic unit in the lower rhombic units at its four corners. The dimensions of the upper layer rhombus units are different from those of the lower layer rhombus units, and the spacing between the upper layer rhombus units is greater than that between the lower layer rhombus units; the horizontal diagonal of the upper layer rhombus unit is longer than its vertical diagonal, and the vertical diagonal of the lower layer rhombus unit is longer than its horizontal diagonal; an air layer is provided between the first layer dielectric plate 1 and the second layer dielectric plate 2; the third layer dielectric plate 3 is provided with a cross-shaped slot 7 composed of horizontal slots and vertical slots; the horizontal slots and vertical slots have equal widths but different lengths; the intersection point of the cross-shaped slot 7 coincides with the center of the third layer dielectric plate 3; the microstrip feed line 4 is located on the bottom surface of the third layer dielectric plate 3, including a fan-shaped segment covering the intersection point of the cross-shaped slot 7 and an extension segment extending from the apex of the fan-shaped segment to the edge of the third layer dielectric plate 3, the angle between the extension segment and the adjacent horizontal slot is 45°.

[0009] As an improvement to the aforementioned antenna, the upper metasurface 5 is printed onto the top surface of the first dielectric substrate 1.

[0010] As an improvement to the aforementioned antenna, the lower metasurface 6 is printed onto the top surface of the second dielectric substrate 2.

[0011] As an improvement to the aforementioned antenna, the size of the upper rhombic unit and the spacing between the upper rhombic units are obtained through the following method: The horizontal and vertical diagonals of the lower rhombic unit are set to be the same, generating a pair of orthogonal characteristic modes. The size of the lower rhombic unit is adjusted, and the size of the lower rhombic unit when the mode importance coefficient of the pair of characteristic modes is maximized at the desired center frequency is used as the initial value of the upper rhombic unit size. In electromagnetic simulation software, the initial value of the upper rhombic unit size is input, and the size of the upper rhombic unit and the spacing between the upper rhombic units are adjusted. The size of the upper rhombic unit corresponding to the minimum axial ratio within the desired frequency band and the spacing between the upper rhombic units are used as their final values.

[0012] As an improvement to the aforementioned antenna, the size of the lower rhombic element is obtained through the following method: The horizontal and vertical diagonals of the lower rhombic element are set to be the same, generating a pair of orthogonal characteristic modes. The size of the lower rhombic element is adjusted, and the size of the lower rhombic element at which the mode importance coefficient of the pair of characteristic modes is maximized at the desired center frequency is taken as the initial value of the lower rhombic element's size. In electromagnetic simulation software, the initial value of the lower rhombic element's size is input, and based on the axial ratio bandwidth design parameters of the microstrip antenna, the size of the lower rhombic element is adjusted, with the size of the lower rhombic element corresponding to the minimum value of the desired frequency band being taken as its final value.

[0013] As an improvement to the aforementioned antenna, the air layer is used to adjust the antenna's impedance characteristics.

[0014] As an improvement to the aforementioned antenna, the first dielectric substrate 1, the second dielectric substrate 2, and the third dielectric substrate 3 are made of the same material.

[0015] Compared with existing technologies, the advantages of this invention are:

[0016] 1. The power supply structure is simple, avoiding the design of complex power divider feed networks. It can excite two working modes by using a simple unequal length cross gap plus microstrip feeder coupling power supply, and generate a certain phase shift between the two working modes.

[0017] 2. It adopts a double-layer metasurface design, and the antenna has a wide operating bandwidth with circular polarization;

[0018] 3. The antenna has a simple structure and is easy to expand. It can be used as a unit of an array to form a broadband circularly polarized array. Attached Figure Description

[0019] Figure 1 This is a three-dimensional structural schematic diagram of the antenna of the present invention;

[0020] Figure 2 This is a schematic diagram of the antenna side view structure of the present invention;

[0021] Figure 3 This is a schematic diagram of the first layer metasurface structure of the antenna of the present invention;

[0022] Figure 4 This is a schematic diagram of the second metasurface structure of the antenna of the present invention;

[0023] Figure 5 This is a schematic diagram of the antenna feeding structure of the present invention;

[0024] Figure 6 This is a curve showing the characteristic of the antenna return loss as a function of frequency in this invention.

[0025] Figure 7This is a graph showing the characteristic curve of the antenna axial ratio as a function of frequency in this invention;

[0026] Figure 8 This is the radiation pattern of the antenna of the present invention at 14 GHz;

[0027] Figure 9 This is a graph showing the characteristic curves of the gain of the antenna of the present invention in the normal direction as a function of frequency, specifically the gain of the main polarization and cross polarization.

[0028] Figure Labels

[0029] 1. First layer dielectric substrate 2. Second layer dielectric substrate

[0030] 3. Third-layer dielectric substrate 4. Microstrip feeder

[0031] 5. Upper metasurface 6. Lower metasurface

[0032] 7. Cross-shaped gap 8. Floor joint Detailed Implementation

[0033] The technical solutions provided by the present invention will be further illustrated below with reference to the embodiments.

[0034] The broadband circularly polarized microstrip antenna provided in this embodiment mainly includes the following technical features.

[0035] The broadband circularly polarized microstrip antenna comprises a three-layer dielectric substrate, a one-layer air layer, and two-layer metasurface structures. The feeding section uses unequal-length slot feeding and a microstrip feed line structure with a tail angle of theta.

[0036] The basic unit of the metasurface is a rhombus unit. The sizes of the rhombus units in the upper and lower layers are different. The vertical diagonal of the rhombus unit in the lower layer 6 is longer than the horizontal diagonal, while the horizontal diagonal of the rhombus unit in the upper layer 5 is longer than the vertical diagonal. The spacing between the rhombus units in the upper layer is greater than the spacing between the rhombus units in the lower layer. The lower layer 6 is a 3×3 arrangement of rhombus units, and the upper layer 5 is an array of rhombus units with the central patch removed. The two layers of metasurfaces are placed orthogonally on the front side of the first dielectric substrate 1 and the second dielectric substrate 2, respectively. There is an air layer between the first dielectric substrate 1 and the second dielectric substrate 2. The thickness of the air layer is H, which is used to adjust the impedance bandwidth of the antenna.

[0037] The dimensions of the rhombic units, which serve as the basic units of the upper and lower metasurfaces, and the thickness of the intermediate air layer can be obtained using the following method:

[0038] 1) Use characteristic mode analysis to analyze the characteristic modes of the lower metasurface 6 with rhombic units: First, set the two diagonals of the rhombic unit to be the same, adjust the size of the rhombic unit so that the resonant frequency of the two orthogonal characteristic modes is at the required center frequency, take the size of the rhombic unit at this time as the initial value, and set the initial values ​​of the two metasurfaces to be the same.

[0039] 2) Calculate the dimensions of the lower metasurface 6: Based on the antenna's impedance bandwidth design specifications, adjust the lengths px and py of the two diagonals of the rhombic element in the electromagnetic simulation software to minimize the return loss (S11) in the required frequency band.

[0040] 3) Adjust the thickness of the air layer so that S11 meets the design requirements of the antenna's impedance bandwidth;

[0041] 4) Calculate the dimensions of the upper metasurface 5: Adjust the diagonal length of the rhombic unit and the width of the gap (px_1, py_1, w1_1) in the electromagnetic simulation software to minimize the axial ratio within the required frequency band.

[0042] In conventional antenna design, the narrow operating bandwidth is often due to the difficulty in achieving broadband orthogonal mode separation in the radiating elements or the inability of the feeding structure to provide broadband phase shift characteristics over a wide frequency band. Therefore, the inventors of this application propose a hybrid phase shift strategy to guide the design of the antenna feeding structure. In the radiating structure design (i.e., a double-layer metasurface structure), rhombic elements and horizontal slots in some rhombic elements have already been used to achieve orthogonal mode separation, resulting in a certain phase shift between the orthogonal modes. To ensure the broadband characteristics of the antenna, phase shift design is also required in the feeding structure design. The hybrid phase shift strategy achieves the phase shift requirements of a broadband circularly polarized antenna through the synergistic effect of the feeding and radiating structures.

[0043] Specifically, the antenna is fed by a microstrip feed structure with unequal-length cross slots located on the third dielectric substrate 3 and a tail angle of theta. The feed structure is located below the second dielectric substrate 2, with the cross slots at the center of the third dielectric substrate 3. The two slots are of different lengths, with the longer slot parallel to the long diagonal of the rhombic element of the lower metasurface 6 and the shorter slot parallel to the short diagonal of the rhombic element of the lower metasurface. The microstrip feed is located on the back side of the third dielectric substrate 3 where the slots are located, with the head of the microstrip feed forming an angle of 45° with the slot.

[0044] To better understand the technical solution of the present invention, the present invention will be further described below in conjunction with the accompanying drawings.

[0045] like Figure 1The broadband circularly polarized microstrip array antenna element shown can be used in various common microwave frequency bands. Without loss of generality, taking the Ku-band as an example, the antenna consists of four layers: three dielectric substrates and one air layer. The layers are parallel to each other. Figure 2 As shown. The antenna's three-layer dielectric substrate is made of RO4003 laminate with a relative permittivity of 3.55, and the thicknesses from top to bottom are 0.254 mm, 1.587 mm, and 0.254 mm, respectively, with an air layer thickness of 0.5 mm.

[0046] The radiating element consists of two parallel layers. The upper metasurface 5 is located on the front of the first dielectric substrate 1, and the lower metasurface 6 is located on the front of the second dielectric substrate 2. Both metasurfaces are centered on the dielectric substrates. Notably, the vertical diagonal of the rhombic element in the lower metasurface 6 is longer than its horizontal diagonal, while the horizontal diagonal of the rhombic element in the upper metasurface 5 is longer than its vertical diagonal. The directions of the long diagonals of the two metasurfaces are orthogonal. A single metasurface has a narrow operating bandwidth, which is not conducive to widening the antenna bandwidth. To achieve a wider circular polarization bandwidth, a parasitic metasurface layer needs to be added, i.e., the lower metasurface 6 is added on top of the upper metasurface 5. The upper metasurface 5 mainly affects the high-frequency characteristics of the antenna's axial ratio, while the lower metasurface 6 mainly affects the low-frequency characteristics. The widening of the antenna is achieved through the synergistic effect of the two metasurfaces. An air layer of thickness H is placed between the first dielectric substrate 1 and the second dielectric substrate 2 to adjust the impedance matching of the antenna.

[0047] The inventors of this application discovered that although existing circularly polarized microstrip antennas also employ a double-layer metasurface design, the upper metasurface unit is 2×2 and the lower metasurface unit is 4×4. Their main function is to provide capacitive loading to achieve antenna miniaturization, but they cannot achieve the goal of increasing bandwidth. In the technical solution of this embodiment, both metasurface layers are 3×3, and their function is to allow each metasurface layer to operate in different frequency bands, thus achieving broadband operation.

[0048] Specifically, such as Figure 4 As shown, the lower metasurface 6 is composed of two-dimensionally distributed rhomboid metal patch units, wherein the lengths of the two diagonals of the rhombus are px and py respectively, and the number, period and gap of the rhomboid patches are 3×3 along the U and V directions respectively. A slot with a width of 0.2 mm is cut on the four small patches at the edge to achieve degeneracy separation of two orthogonal modes and generate a certain phase difference.

[0049] like Figure 3As shown, the upper metasurface 5, compared to the lower metasurface 6, has removed the central patch. The presence of the central patch introduces cross-polarization, thus deteriorating the antenna's axial ratio characteristics. The size of the rhombic patches on the upper metasurface 5 and the spacing between the patches are different from those on the lower metasurface. The spacing between the rhombic patches on the upper metasurface 5 is greater than that on the lower metasurface 6. The diagonal lengths of the rhombic elements on the upper metasurface 5 are px_1 and py_1, and the gap w1_1 is 1.17 mm.

[0050] The dimensions of the basic unit rhombuses of the upper and lower metasurfaces and the thickness of the intermediate air layer can be obtained using the following method:

[0051] 1) Characteristic mode analysis of the lower metasurface 6 with rhombic units: First, set the two diagonals of the rhombic unit to be the same. Since the metasurface is symmetrical at this time, there must be a pair of mutually orthogonal characteristic modes. Adjust the size of the rhombic unit so that the mode importance coefficient of this pair of characteristic modes is maximized (close to 1) at the required center frequency. Use the size of the rhombic unit at this time as the initial value. The initial size of the rhombic unit is the same for both metasurface layers. The mode importance coefficient is an important indicator for evaluating the resonance capability of a mode, and it is related to the eigenvalue λ. n Functions:

[0052] MS n =1 / |1+jλ n |

[0053] When MS n The closer it is to 1, the easier it is to activate the corresponding nth mode.

[0054] 2) Calculate the dimensions of the lower metasurface: The resonant frequencies of the two orthogonal modes are related to the lengths (px, py) of the two diagonals of the rhombus unit. According to the antenna impedance bandwidth design specifications, the lengths of the two diagonals of the rhombus are adjusted in the electromagnetic simulation software to shift the resonant frequencies of the two orthogonal modes (the longer the length, the lower the corresponding resonant frequency, and vice versa), so that S11 is minimized in the required frequency band.

[0055] 3) Adjust the thickness H of the air layer so that S11 meets the design requirements of the antenna impedance bandwidth. The thickness is approximately 0.035 wavelengths at the center frequency.

[0056] 4) Calculate the dimensions of the upper metasurface 5: The dimensions of the upper metasurface 5 mainly determine the axial ratio bandwidth of the antenna. In the electromagnetic simulation software, adjust the diagonal length of the rhombic element and the width of the slot (px_1, py_1, w1_1) to minimize the axial ratio within the required frequency band.

[0057] To excite the desired modes and achieve circular polarization over a wide bandwidth, the proposed hybrid phase-shift strategy utilizes the synergistic effect of the feeding and radiating structures to meet the phase shift requirements of a broadband circularly polarized antenna. The design process begins by introducing perturbations into the antenna's radiating structure to separate degenerate modes, ensuring the antenna meets phase shift requirements near the resonant frequency. However, when deviating from the resonant frequency, the altered electrical dimensions of the metasurface structure make it difficult to maintain the required phase difference characteristics for circular polarization on the radiating structure. Subsequently, the phase-shifting feeding structure is designed to compensate for the phase shift caused by changes in the electrical dimensions of the radiating structure, thus enabling the phase-shifting and radiating structures to work together to achieve stable phase shift characteristics.

[0058] Antenna feeding structure as follows Figure 1 and Figure 5 As shown, the third dielectric substrate 3 is coupled with a cross-shaped slot 7 of unequal length. The lengths of the two slots are sl1 and sl2, respectively, with an initial length approximately half the wavelength at the center frequency. The microstrip feed 4 below consists of a 45° bend and a fan-shaped segment. The fan-shaped segment primarily improves the impedance matching of the antenna and also allows more energy to be coupled to the cross-shaped slot 7. The characteristic impedance of the microstrip feed 4 is 50 ohms, the fan-shaped segment has an opening angle of 60°, and a length of 0.16 wavelengths. During antenna operation, the energy on the microstrip feed 4 is first coupled to the cross-shaped slot 7, and then the energy on the cross-shaped slot 7 is coupled to the two metasurface radiating elements, thus achieving energy radiation.

[0059] like Figure 6 The figure shows the return loss characteristic curve of the circularly polarized antenna of the present invention as a function of frequency. It can be seen that it has good return loss parameters in the range of 12.49-20.77GHz, and the loss is below -10dB in the entire frequency band. Figure 7 The figure shows the axial ratio characteristic curve of the circularly polarized antenna of the present invention as a function of frequency. The 3dB axial ratio bandwidth is 11.57-18.17GHz. Figure 8 As shown, the overall radiation pattern of the broadband circularly polarized microstrip antenna at the center frequency was calculated using HFSS full-wave analysis software. The circular polarization bandwidth of the antenna must consider both the impedance bandwidth and axial ratio bandwidth; therefore, this antenna achieves a circular polarization bandwidth of 37% (12.49-18.17 GHz). The maximum gain of the antenna within the frequency band is 7.87 dB.

[0060] like Figure 8 As shown, the HFSS full-wave analysis software was used to calculate the characteristic curves of the main polarization and cross-polarization gain in the normal direction of the broadband circularly polarized microstrip array antenna as a function of frequency.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A broadband circularly polarized microstrip antenna, characterized in that, include: The following components are arranged in parallel from top to bottom: an upper metasurface (5), a first dielectric substrate (1), a lower metasurface (6), a second dielectric substrate (2), a ground plane (8), a third dielectric substrate (3), and a microstrip feeder (4). The upper metasurface (5) is an upper quadrilateral array composed of N×N upper rhombic units arranged uniformly, where N is an odd number. The upper quadrilateral array removes the upper rhombic unit located at its center and sets slots extending along the horizontal diagonal of the upper rhombic unit in the upper rhombic units located at its four corners. The lower metasurface (6) is a lower quadrilateral array composed of N×N lower rhombic units arranged uniformly. The lower quadrilateral array has slots extending along the transverse diagonal of the lower rhombic unit at its four corners. The upper and lower rhomboid units have different sizes, and the spacing between the upper rhomboid units is greater than the spacing between the lower rhomboid units; the horizontal diagonal of the upper rhomboid unit is longer than the vertical diagonal, and the vertical diagonal of the lower rhomboid unit is longer than the horizontal diagonal. An air layer is provided between the first dielectric substrate (1) and the second dielectric substrate (2); The third layer medium plate (3) is provided with a cross-shaped slot (7) composed of horizontal slots and vertical slots; the horizontal slots and vertical slots are of equal width but different lengths; the intersection point of the cross-shaped slot (7) coincides with the center of the third layer medium plate (3); The microstrip feed line (4) is located on the bottom surface of the third dielectric substrate (3) and includes a fan-shaped segment covering the intersection of the cross slot (7) and an extension segment extending from the apex of the fan-shaped segment to the edge of the third dielectric substrate (3). The angle between the extension segment and the adjacent transverse slot is 45°.

2. The broadband circularly polarized microstrip antenna according to claim 1, characterized in that, The upper metasurface (5) is printed onto the top surface of the first dielectric substrate (1).

3. The broadband circularly polarized microstrip antenna according to claim 1, characterized in that, The lower metasurface (6) is printed onto the top surface of the second dielectric substrate (2).

4. The broadband circularly polarized microstrip antenna according to claim 1, characterized in that, The dimensions of the upper-layer rhombic unit and the spacing between the upper-layer rhombic units are obtained by the following method: Set the horizontal and vertical diagonals of the lower-level rhombus unit to be the same to generate a pair of orthogonal feature modules. Adjust the size of the lower-level rhombus unit and use the size of the lower-level rhombus unit when the mode importance coefficient of the pair of feature modules is at its maximum at the required center frequency as the initial size value of the upper-level rhombus unit. In the electromagnetic simulation software, input the initial value of the upper-layer rhombic unit size, and adjust the size of the upper-layer rhombic unit and the spacing between the upper-layer rhombic units. The final value is the size of the upper-layer rhombic unit corresponding to the minimum axial ratio within the required frequency band and the spacing between the upper-layer rhombic units.

5. The broadband circularly polarized microstrip antenna according to claim 1, characterized in that, The dimensions of the lower-layer rhombic unit are obtained using the following method: Set the horizontal and vertical diagonals of the lower-level rhombus unit to be the same to generate a pair of orthogonal feature modules. Adjust the size of the lower-level rhombus unit and use the size of the lower-level rhombus unit when the mode importance coefficient of the pair of feature modules is at its maximum at the required center frequency as the initial size value of the lower-level rhombus unit. In electromagnetic simulation software, the initial size of the lower-layer rhombic element is input. Based on the axial ratio bandwidth design index of the microstrip antenna, the size of the lower-layer rhombic element is adjusted. The size of the lower-layer rhombic element corresponding to the minimum value of the required frequency band is taken as its final value.

6. The broadband circularly polarized microstrip antenna according to claim 1, characterized in that, The air layer is used to adjust the impedance characteristics of the antenna.

7. The broadband circularly polarized microstrip antenna according to claim 1, characterized in that, The first layer dielectric substrate (1), the second layer dielectric substrate (2), and the third layer dielectric substrate (3) are made of the same material.

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