Air cavity partition type magnetic induction antenna structure for suppressing eddy current loss

Abstract

The application provides an air cavity partition type magnetic induction antenna structure for inhibiting eddy current loss, and belongs to the technical field of magnetic induction antennas.The application constructs a theoretical model, deduces from a magnetic field diffusion equation, and determines that the radius of the cylindrical air cavity is a key design parameter for inhibiting eddy current loss, and the cavity length has little effect on the inhibiting effect, thereby establishing a core design criterion of "optimizing only the radius".The antenna designed according to the criterion comprises a coaxial sealed cylindrical air cavity, which is filled with air, and physically separates the magnetic induction coil from external seawater, fundamentally inhibits the eddy current loss by reducing the effective medium volume for generating the eddy current.The application avoids the energy consumption and stability problems caused by complex circuit compensation, has a simple structure, and provides a high-efficiency and reliable solution for underwater magnetic induction communication.

Description

Technical Field

[0001] This invention relates to the field of magnetic induction antenna technology, and in particular to an air cavity isolated magnetic induction antenna structure for suppressing eddy current loss. Background Technology

[0002] Magnetic induction antennas transmit energy and information based on Faraday's law of electromagnetic induction, using induced magnetic fields as the transmission medium. Compared with traditional antennas that rely on electromagnetic waves for communication, they have significant advantages such as low cost, low power consumption, miniaturization, and stable channels. Furthermore, due to the shorter communication distance, the signal is less affected by Doppler shift and multipath effects, showing significant application potential in near-field communication in complex environments such as underground, underwater, and tunnels, and has become an important research direction in the field of underwater communication.

[0003] However, when magnetic induction antennas are applied to high-conductivity media such as seawater, their performance is severely limited by eddy current losses: the time-varying induced magnetic field induces eddy currents in the seawater medium around the antenna, and some of the energy is dissipated as heat. This eddy current loss not only reduces the radiation efficiency of the magnetic induction antenna and significantly reduces the underwater communication distance, but also causes problems such as antenna detuning and quality factor degradation, becoming the core technical bottleneck restricting the practical application of magnetic induction antennas in seawater environments.

[0004] To address the aforementioned eddy current loss problem, existing technologies have proposed various suppression methods, primarily focusing on circuit compensation. For example, some studies introduce an equivalent eddy current loss impedance model to quantify eddy current loss and add compensating inductors to counteract the antenna detuning effect caused by eddy currents. Other studies reduce eddy current loss by controlling the current phase in the coil to decrease the superposition of eddy current electric fields. Still others employ dual-transmitter coil structures and variable magnetic couplers to dynamically optimize the magnetic field distribution. While these circuit compensation methods can alleviate eddy current loss to some extent, they all optimize at the circuit level, inevitably increasing the antenna system's energy consumption and the complexity of the circuit structure, which is detrimental to the miniaturization design and practical engineering implementation of magnetic induction antennas. Summary of the Invention

[0005] This invention provides a design method for eddy current suppression of an air cavity isolated magnetic induction antenna, which addresses the deficiencies in the prior art.

[0006] In a first aspect, the present invention provides a design method for eddy current suppression of an air cavity-isolated magnetic induction antenna, comprising: constructing an eddy current loss model of a cylindrical air cavity coaxial with the magnetic induction coil, equating the eddy current concentration region in seawater to a cylinder coaxial with the magnetic induction coil, introducing a vector magnetic potential based on Maxwell's equations and deriving the magnetic field diffusion equation; simplifying the magnetic field diffusion equation using cylindrical coordinates, and decomposing the vector magnetic potential into a product of a radial function and an axial function using the method of separation of variables, and solving for the general solutions of the axial and radial functions; determining the undetermined coefficients in the general solution based on the boundary conditions of continuous tangential components of the magnetic field and electric field, analyzing the radiation characteristics of the main mode of the magnetic field, in which the magnetic field is uniformly distributed along the axial direction of the cylindrical air cavity under the main mode, the magnetic field attenuation is caused only by the dielectric loss of the seawater and is independent of the length of the cylindrical air cavity, the magnetic field strength under the main mode increases with the increase of the radius of the cylindrical air cavity, and the effective integral volume of eddy current loss decreases with the increase of the radius of the cylindrical air cavity; and determining, based on the radiation characteristics of the main mode of the magnetic field, that only the radius of the cylindrical air cavity needs to be optimized to achieve eddy current loss suppression.

[0007] According to the eddy current suppression design method of the air cavity isolated magnetic induction antenna provided by the present invention, the undetermined coefficients in the general solution are determined based on the boundary condition of continuous tangential components of magnetic field and electric field, and the radiation characteristics of the main mode of magnetic field are analyzed. Specifically, the method includes: assuming seawater as an ideal conductor and deriving the discretization result of eigenvalues; distinguishing the radiation characteristics of the main mode and higher-order modes of magnetic field, clarifying that the energy decay rate of higher-order modes is faster than that of the main mode, and that the magnetic field energy is dominated by the main mode.

[0008] The eddy current suppression design method for the air cavity isolated magnetic induction antenna provided by the present invention optimizes the radius of the cylindrical air cavity, including: analyzing the attenuation characteristics of eddy currents in seawater and deriving the exponential attenuation law of eddy current density with distance from the field source; and determining the radius of the cylindrical air cavity based on the exponential attenuation law and the actual performance indicators of the antenna, so as not to increase the size of the air cavity beyond the effective range of exponential attenuation of eddy currents.

[0009] According to the eddy current suppression design method of the air cavity isolated magnetic induction antenna provided by the present invention, the attenuation characteristics of eddy currents in seawater are analyzed, and the exponential attenuation law of eddy current density with distance from the source is derived. Specifically, the method includes: simplifying the radially propagating eddy current electric field in the seawater medium into a plane wave model to obtain the simplified electric field equation; solving the simplified electric field equation to obtain the exponential attenuation relationship of the eddy current electric field intensity amplitude with distance from the source; and deriving the exponential attenuation law of eddy current density with distance from the source from the exponential attenuation relationship of the electric field intensity based on the relationship between eddy current density and electric field intensity.

[0010] The eddy current suppression design method for an air cavity isolated magnetic induction antenna provided by the present invention further includes: calculating the energy loss composition of the magnetic induction antenna for the operating frequency band of the magnetic induction antenna, ignoring radiation loss and only considering ohmic loss and equivalent eddy current loss, and completing the theoretical verification of the eddy current suppression effect based on the total loss formula; using simulation software to plot the theoretical curve and simulation curve of eddy current loss changing with the air cavity diameter in the frequency band, and completing the simulation verification of the eddy current suppression effect through the consistency of the curve trends.

[0011] The eddy current suppression design method for the air cavity isolated magnetic induction antenna provided by the present invention further includes a magnetic induction antenna fabrication step, specifically: fabricating an antenna frame and forming a magnetic induction coil on the antenna frame; selecting a cavity body as the main body of the cylindrical air cavity, fixing the magnetic induction coil to the center of the cavity body through the antenna frame and ensuring that the two are coaxially arranged; sealing the ends of the cavity body with a sealing component, installing waterproof terminals on the cavity body to achieve sealing of the cylindrical air cavity; configuring a tuning capacitor for the magnetic induction coil and forming a series resonant structure, and using a coaxial cable to complete the assembly of the feeding structure for the magnetic induction coil.

[0012] The present invention also provides an air cavity isolated magnetic induction antenna, designed using any of the above methods, comprising: a sealed cylindrical air cavity, a magnetic induction coil, and an antenna frame; the magnetic induction coil is coaxially fixed at the center position inside the cylindrical air cavity through the antenna frame; the radius of the cylindrical air cavity is designed according to the eddy current exponential attenuation law and the actual performance indicators of the antenna, and its length is not used as an optimization parameter for eddy current suppression.

[0013] According to the air cavity isolation magnetic induction antenna provided by the present invention, the antenna frame is made of insulating material; the cylindrical air cavity is a sealed cavity structure, which includes a cavity body and a sealing element; the sealing element is disposed at the end of the cavity body.

[0014] The air-cavity isolated magnetic induction antenna provided by the present invention further includes: a tuning capacitor, a coaxial cable, and a waterproof terminal; the coaxial cable passes through the waterproof terminal and is electrically connected to the magnetic induction coil; the tuning capacitor is matched with the magnetic induction coil to ensure that the antenna achieves series resonance within the operating frequency band.

[0015] According to the air cavity isolated magnetic induction antenna provided by the present invention, the magnetic induction coil is wound with copper enameled wire, and the cavity body is a transparent plastic tube structure.

[0016] The air cavity isolated magnetic induction antenna structure for suppressing eddy current loss provided by this invention has the following advantages compared with the prior art: (1) This invention provides clear and verifiable design principles, significantly reducing design complexity and uncertainty. Starting from the magnetic field diffusion equation, this invention clarifies the core design principle of "optimizing only the radius of the cylindrical air cavity while excluding its length from the key optimization parameters" through rigorous theoretical derivation and simulation verification. This provides a direct and clear theoretical basis for engineering practice, enabling designers to determine key dimensions (radius) in a targeted manner without the need for tedious length optimization or the introduction of complex compensation circuits, thereby simplifying the design process and improving design efficiency.

[0017] (2) While achieving efficient eddy current suppression, the simplicity and reliability of the antenna structure are maintained. The antenna structure described in this invention physically isolates the eddy current path through a coaxially sealed cylindrical air cavity designed according to the above criteria, directly reducing the effective integral volume of eddy current loss from the physical source. Simulation results show that this method can effectively suppress eddy current loss. Compared with circuit methods such as adding compensating inductors, controlling current phase, or using dual transmitting coils, this invention does not add additional active circuits, does not increase system power consumption, and does not require complex phase synchronization control. Therefore, the structure is simpler, the energy consumption is lower, and the operation is more stable and reliable, making it particularly suitable for underwater near-field communication scenarios with high requirements for power consumption, size, and reliability. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a flowchart illustrating the eddy current suppression design method for the air cavity isolated magnetic induction antenna provided by the present invention. Figure 2 This is a schematic diagram of the cylindrical air cavity eddy current loss model provided by the present invention; Figure 3 This is a schematic diagram showing the attenuation trend of eddy current density with distance at different frequencies, provided by the present invention. Figure 4 This is a schematic diagram of the air cavity isolation antenna provided by the present invention; Figure 5 This is a schematic diagram of the air cavity isolation antenna provided by the present invention; Figure 6 This is a schematic diagram of the improvement effect of the present invention at different frequencies as a function of the air cavity diameter. Figure 7This is a schematic diagram of the improvement effect at different frequencies as a function of air cavity length, provided by the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0021] It should be noted that, in the description of the embodiments of the present invention, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Those skilled in the art can understand the specific meaning of the above terms in the present invention according to the specific circumstances.

[0022] When magnetic induction communication systems are applied in seawater environments, their performance is significantly affected by electromagnetic properties. Seawater has a conductivity of 4-6 S / m, which causes severe eddy current losses in the time-varying magnetic field, shortening the communication distance. In seawater, the energy loss of magnetic induction antennas mainly includes radiation loss, ohmic loss, and equivalent eddy current loss. However, for low to high frequency bands, the radiation loss of magnetic induction antennas is very small and can be ignored compared to the other two types of resistance. When calculating ohmic loss, the AC ohmic resistance needs to be calculated based on the signal frequency. (AC resistance) The calculation formula is: (1) in, For frequency, The duty cycle of the conductor. The electrical conductivity of the material, S Let be the cross-sectional area of ​​the conductor. Let be the length of the conductor. The duty cycle of the conductor can be calculated using equation (2): (2) in For wire diameter, This represents the spacing between adjacent coils.

[0023] Equivalent eddy current resistance The calculation formula is: (3) in, and These are the dielectric conductivity and magnetic permeability, respectively. For the excitation current angular frequency, The number of coil turns. Where is the radius of the coil.

[0024] In summary, the total loss of a magnetic induction antenna It can be written as: (4) in, Given the current fed into the antenna, the proportion of the equivalent eddy current loss in the total coil loss can be calculated according to equation (4). When the proportion of the equivalent eddy current loss is too high, it is necessary to take eddy current suppression measures.

[0025] The following is combined Figures 1-7 This invention describes an air-cavity isolated magnetic induction antenna structure for suppressing eddy current losses, provided by an embodiment of the present invention.

[0026] Figure 1 This is a flowchart illustrating the eddy current suppression design method for the air-cavity isolated magnetic induction antenna provided by the present invention, as shown below. Figure 1 As shown, including but not limited to the following steps: Step 101: Construct a cylindrical air cavity eddy current loss model coaxial with the magnetic induction coil. The concentrated eddy current region in the seawater is equivalent to a cylinder coaxial with the magnetic induction coil. Based on Maxwell's equations, vector magnetic potential is introduced and the magnetic field diffusion equation is derived.

[0027] Figure 2 This is a schematic diagram of the cylindrical air cavity eddy current loss model provided by the present invention, as shown below. Figure 2 As shown, the present invention regards the region in the seawater where the eddies are relatively concentrated as a cylinder coaxial with the magnetic induction coil, and calculates the eddy current loss power by calculating the volume fraction of the cylindrical region using equation (5).

[0028] (5) in, This represents the eddy current loss power, and E represents the magnetic field strength generated by the energized coil. Indicates the integration region. This indicates the electrical conductivity of seawater.

[0029] The introduction of the cylindrical air cavity replaces the seawater around the coil with air with a conductivity of approximately 0, thereby reducing the effective integral volume in equation (5) and achieving the eddy current suppression effect.

[0030] To facilitate the analysis of the influence of the radius and length of the air cavity on the eddy current suppression effect, a structure was constructed as follows: Figure 2 The cylindrical coordinate system shown places the coil in... xOy A plane, coaxial with the air cavity and located at its center, is given the length of the air cavity as... , radius is , Let be the distance from any point in cylindrical coordinates to the z-axis. Since magnetic induction antennas primarily communicate in the near-field region, a vector magnetic potential can be introduced. And the magnetic field diffusion equation is derived using Maxwell's equations: (6) in, For the excitation current angular frequency, The magnetic permeability of seawater.

[0031] Since the coil and the cylindrical air cavity are axisymmetric, the magnetic field has no azimuth component. Under Coulomb gauge, the vector magnetic potential... It can be simplified to a form containing only the azimuth component: (7) Step 102: The magnetic field diffusion equation is simplified using cylindrical coordinates. The vector magnetic potential is decomposed into the product of radial and axial functions using the method of separation of variables. The general solutions of the axial and radial functions are then obtained.

[0032] Equation (6) can be written in cylindrical coordinates as: (8) Using the method of separation of variables, let the vector magnetic potential be... Radial function and axial function The product of: (9) Substitute equation (9) into equation (8) and let Simplifying, we get: (10) To make equation (10) work for any and If true, then both sides of the equation should be equal to the same AND. and An irrelevant constant, let this constant be... We obtain the following two equations: (11) By rearranging and simplifying the axial equation, we can obtain a simple harmonic oscillation equation: (12) The general solution to this equation is in trigonometric function form, taking into account the geometric structure of the model regarding... Symmetry usually means that the solution is even-symmetric, so only the cosine terms in the general solution are retained: (13) in These are undetermined coefficients. For the radial function, let... By rearranging the terms, we can obtain the modified Bessel equation: (14) The two linearly independent solutions of equation (14) are respectively the modified Bessel functions of the first kind. Second type modified Bessel function Its general solution can be written as: (15) in, and These are undetermined coefficients.

[0033] Step 103: Determine the undetermined coefficients in the general solution based on the boundary conditions of continuous tangential components of magnetic and electric fields, and analyze the radiation characteristics of the main mode of magnetic field. Under the main mode, the magnetic field is uniformly distributed along the axial direction of the cylindrical air cavity. The magnetic field attenuation is caused only by the loss of seawater medium and is independent of the length of the cylindrical air cavity. Under the main mode, the magnetic field strength increases with the increase of the radius of the cylindrical air cavity, and the effective integral volume of eddy current loss decreases with the increase of the radius of the cylindrical air cavity.

[0034] Among them, the undetermined coefficients in the general solution are determined based on the boundary conditions of continuous tangential components of magnetic and electric fields, and the radiation characteristics of the main mode of magnetic field are analyzed. This includes: assuming seawater as an ideal conductor and deriving the discretization results of eigenvalues; distinguishing the radiation characteristics of the main mode and higher-order modes of magnetic field, clarifying that the energy decay rate of higher-order modes is faster than that of the main mode, and that the energy of magnetic field is dominated by the main mode.

[0035] Specifically, after finding the general solutions for the axial and radial functions, the values ​​of the undetermined coefficients need to be determined based on the boundary conditions. At the interface, it is necessary to ensure the continuity of the tangential components of the magnetic and electric fields. Combining the mathematical relationship between the magnetic and electric fields and the vector magnetic potential, the following boundary conditions can be obtained: (16) (17) , These are the tangential components of the vector magnetic potential at the interface in the air and seawater media, respectively. Equations (16) and (17) lead to complex transcendental equations, making it difficult to solve for the undetermined coefficients. To facilitate the analysis of the effect of the change in air cavity length on the axial function, we assume that seawater is an ideal conductor, and then we can obtain the following from the above two equations: To make the undetermined coefficients in equation (13) If it is not 0, we can solve for: (18) As can be seen from equation (18), under the constraints of the boundary conditions, the eigenvalues ​​are discretized. Specifically, for When the eigenvalues ​​and eigenfunctions are both 0, this mode is denoted as the principal mode. In the principal mode, the axial magnetic field is uniformly distributed, and the attenuation of the magnetic field is entirely caused by the loss of the seawater medium, independent of the length of the air cavity. However, when… Sometimes: (19) in, The radial propagation constant representing the higher-order mode. Let be the complex wave number in the seawater medium. It is evident that, in addition to losses due to the seawater medium, there are also losses caused by the mismatch between the air cavity geometry and the mode. Therefore, the energy of higher-order modes decays faster than that of the dominant mode. When the communication distance is much greater than the air cavity size, it can be assumed that the axial magnetic field energy is mainly generated by the dominant mode, and in this mode, changes in the length of the air cavity will not significantly affect the magnetic field distribution in the seawater.

[0036] Analyzing the radial function under the principal modulus, when... At that time, due to And the magnetic field in this mode is similar to Irrelevant, that is In summary, equation (8) simplifies to the following form: (20) The general solution is: (twenty one) exist When the general solution's second term does not converge, therefore the coefficients... Furthermore, it is necessary to consider the special solutions introduced by the field source. The interpretation of time is: (twenty two) in, A special solution introduced for the field source.

[0037] when When the general solution is the Bessel function shown in equation (15), but considering that the magnetic field will always decay to 0 in seawater, the first kind of modified Bessel function... along with It increases and then diverges, therefore it must be made The general solution is: (twenty three) exist Then, the system of equations is obtained from the boundary conditions described in equations (16) and (17): (twenty four) Solving for: (25) in, For the second kind of zeroth-order modified Bessel function, since and All follow The increase exhibits an exponential decay, while Follow It grows linearly, therefore the numerator of equation (25) increases with... Increase and decrease along with Increasing the radius of the air cavity enhances the strength of the radial magnetic field. Based on equations (9) and (23), the vector magnetic potential under the principal mode is expressed as follows: (26) in, This indicates that the axial component of the vector magnetic potential under the master mode is uniformly distributed. In summary, since the magnetic field at a point in seawater is dominated by the master mode, the magnetic field in this mode is uniformly distributed axially and is only affected by losses in the seawater medium, not by changes in the air cavity length; however, the magnetic field in this mode increases with the increase of the air cavity radius. In other words, changes in the air cavity height do not significantly affect the eddy current suppression effect, while changes in the air cavity radius enhance the eddy current suppression effect.

[0038] More intuitively, the present invention can directly derive the expression for eddy current loss through the analysis of the above-mentioned vector magnetic potential. When an air cavity is provided, the electric field expression is calculated based on the vector magnetic potential: (27) Substituting equation (27) into equation (5), we obtain the eddy current loss after setting the air cavity as follows: (28) In equation (28), although the vector magnetic potential increases with the increase of the air cavity radius, the cavity radius changes the starting point of the integration for the whole integral, reducing the effective integration region. This causes the eddy current loss power to decrease with the increase of the air cavity radius.

[0039] Step 104: Based on the radiation characteristics of the main magnetic field mode, optimize only the radius of the cylindrical air cavity to suppress eddy current loss.

[0040] The optimization of the radius of the cylindrical air cavity includes: The attenuation characteristics of eddies in seawater are analyzed, and the exponential decay law of eddy current density with distance from the source is derived. Specifically, this includes: simplifying the radially propagating eddy electric field in the seawater medium into a plane wave model to obtain a simplified electric field equation; solving the simplified electric field equation to obtain the exponential decay relationship of the eddy electric field intensity amplitude with distance from the source; and deriving the exponential decay law of eddy current density with distance from the source from the relationship between eddy current density and electric field intensity based on the exponential decay relationship of the electric field intensity.

[0041] Based on the exponential decay law and the actual performance indicators of the antenna, the radius of the cylindrical air cavity is determined so as not to increase the size of the air cavity beyond the effective range of eddy current exponential decay.

[0042] Specifically, to clarify the theoretical basis for the design of the air cavity size, it is necessary to analyze the attenuation characteristics of the eddy current in seawater.

[0043] For a plane wave propagating along any defined radial direction, its electric field equation can be simplified to: (29) Due to the symmetry of the field, equation (29) can be written as: (30) The calculated eigenvalues ​​are: (31) Let be the propagation constant. For skin depth.

[0044] Therefore, the decrease in field strength amplitude with distance is as follows: (32) Depend on The decrease in eddy current density with distance can be derived as follows: (33) Eddy current density exhibits an exponential decay law as the distance from the source increases. Figure 3 This is a schematic diagram illustrating the attenuation trend of eddy current density with distance at different frequencies, as provided by the present invention. Figure 3 As shown, when considering a closed loop concentric with the excitation coil, the current density on it decreases rapidly as the loop radius increases. This trend is consistent with the analytical conclusion of equation (33). Therefore, in engineering design, there is no need to blindly pursue an excessively large air cavity size. Instead, a reasonable cavity size should be determined based on the attenuation relationship revealed by equation (33) and the specific performance requirements.

[0045] Based on the above embodiments, as an optional embodiment, the eddy current suppression design method for the air cavity isolated magnetic induction antenna provided by the present invention further includes: (1) Fabricate the antenna frame and form a magnetic induction coil on the antenna frame; (2) Select the cavity body as the main body of the cylindrical air cavity, fix the magnetic induction coil to the center of the cavity body through the antenna frame and ensure that the two are set coaxially; (3) Use sealing components to seal the ends of the cavity body and install waterproof terminals on the cavity body to achieve the sealing of the cylindrical air cavity; (4) Configure a tuning capacitor for the magnetic induction coil and form a series resonant structure. Use a coaxial cable to complete the power supply structure assembly for the magnetic induction coil.

[0046] The eddy current suppression design method for an air cavity isolated magnetic induction antenna provided by this invention further includes: calculating the energy loss composition of the magnetic induction antenna for its operating frequency band, ignoring radiation loss and only considering ohmic loss and equivalent eddy current loss, and completing the theoretical verification of the eddy current suppression effect based on the total loss formula; using simulation software to plot the theoretical curve and simulation curve of eddy current loss changing with air cavity diameter in this frequency band, and completing the simulation verification of the eddy current suppression effect through the consistency of the curve trends.

[0047] Figure 4 This is a schematic diagram of the air cavity isolation antenna provided by the present invention. Figure 5 This is a schematic diagram of the air cavity isolation antenna provided by the present invention, as shown below. Figure 4 The magnetic induction antenna is wound around an antenna frame made of polylactic acid (insulating material). The frame structure allows the antenna to be stably positioned within a transparent plastic tube (the cavity body). Figure 4 The center position of the acrylic cylinder shown. A silicone sealing cap (sealant) and waterproof terminals improve the airtightness of the air cavity, preventing seawater ingress. The magnetic induction antenna is fed by a coaxial cable. (As shown...) Figure 5 As shown, the coil is made of 35 turns of 0.5mm diameter copper enameled wire with a radius of 4cm. In addition, a tuning capacitor is included to ensure series resonance of the antenna at the operating frequency.

[0048] To verify the eddy current suppression effect of the air cavity and the influence of its size variation, simulation analysis was conducted by changing the air cavity diameter at operating frequencies of 1–3 MHz. Theoretical and simulated curves of eddy current loss as a function of air cavity diameter in this region were plotted using MATLAB and COMSOL, respectively. The results are as follows: Figure 6 As shown, Figure 6This is a schematic diagram showing the improvement effect of the present invention at different frequencies as a function of the air cavity diameter. The trends of both curves are consistent, indicating that as the frequency increases, the eddy current loss increases; while as the air cavity diameter increases, the eddy current loss decreases significantly.

[0049] Similarly, the effect of air cavity length on the suppression effect was studied within the same frequency band. Figure 7 This is a schematic diagram of the improvement effect at different frequencies as a function of air cavity length, provided by the present invention. It shows the theoretical and simulation results under the change of air cavity length, and both show that the change in length has little effect on the suppression effect.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for suppressing eddy currents in an air-cavity isolated magnetic induction antenna, characterized in that, include: A cylindrical air cavity eddy current loss model coaxial with the magnetic induction coil is constructed. The eddy current concentration region in seawater is equivalent to a cylinder coaxial with the magnetic induction coil. Based on Maxwell's equations, vector magnetic potential is introduced and the magnetic field diffusion equation is derived. The magnetic field diffusion equation is simplified by using cylindrical coordinates. The vector magnetic potential is decomposed into the product of radial and axial functions by combining the method of separation of variables. The general solutions of the axial and radial functions are obtained by solving the equation. Based on the boundary condition that the tangential components of the magnetic field and electric field are continuous, the undetermined coefficients in the general solution are determined, and the radiation characteristics of the main mode of the magnetic field are analyzed. Under the main mode, the magnetic field is uniformly distributed along the axis of the cylindrical air cavity. The magnetic field attenuation is caused only by the loss of seawater medium and is independent of the length of the cylindrical air cavity. Under the main mode, the magnetic field strength increases with the increase of the radius of the cylindrical air cavity, and the effective integral volume of eddy current loss decreases with the increase of the radius of the cylindrical air cavity. Based on the radiation characteristics of the main magnetic field mode, the radius of the cylindrical air cavity is optimized only to suppress eddy current losses.

2. The eddy current suppression design method for an air-cavity isolated magnetic induction antenna according to claim 1, characterized in that, Based on the boundary condition that the tangential components of the magnetic and electric fields are continuous, the undetermined coefficients in the general solution are determined, and the radiation characteristics of the principal mode of the magnetic field are analyzed, specifically including: Seawater is assumed to be an ideal conductor, and the discretization results of eigenvalues ​​are derived. Distinguish the radiation characteristics of the primary mode and higher-order modes of the magnetic field, and clarify that the energy decay rate of the higher-order mode is faster than that of the primary mode, and that the magnetic field energy is dominated by the primary mode.

3. The eddy current suppression design method for an air-cavity isolated magnetic induction antenna according to claim 1, characterized in that, Optimize the radius of the cylindrical air cavity, including: The attenuation characteristics of eddies in seawater were analyzed, and the exponential attenuation law of eddy current density with distance from the source was derived. Based on the exponential decay law and the actual performance indicators of the antenna, the radius of the cylindrical air cavity is determined.

4. The eddy current suppression design method for the air cavity isolated magnetic induction antenna according to claim 3, characterized in that, The attenuation characteristics of eddies in seawater are analyzed, and the exponential decay law of eddy current density with distance from the source is derived, specifically including: The radially propagating eddy current electric field in seawater is simplified into a plane wave model, resulting in the simplified electric field equation. Solving the simplified electric field equations yields an exponential decay relationship between the amplitude of the eddy current electric field intensity and the distance from the source. Based on the relationship between eddy current density and electric field strength, and from the exponential decay relationship of the electric field strength, the exponential decay law of eddy current density with distance from the field source can be derived.

5. The eddy current suppression design method for an air-cavity isolated magnetic induction antenna according to claim 1, characterized in that, Also includes: For the operating frequency band of the magnetic induction antenna, the energy loss composition of the magnetic induction antenna is calculated. Ignoring radiation loss, only ohmic loss and equivalent eddy current loss are considered. The theoretical verification of the eddy current suppression effect is completed based on the total loss formula. Theoretical and simulated curves of eddy current loss as a function of air cavity diameter in this frequency band were plotted using simulation software. The simulation verification of the eddy current suppression effect was completed by the agreement of the curve trends.

6. The eddy current suppression design method for an air-cavity isolated magnetic induction antenna according to claim 1, characterized in that, It also includes the fabrication steps of the magnetic induction antenna, specifically: Fabricate the antenna frame and form a magnetic induction coil on the antenna frame; The cavity body is selected as the main body of the cylindrical air cavity, and the magnetic induction coil is fixed to the center of the cavity body through the antenna frame, ensuring that the two are set coaxially. A sealing element is used to seal the end of the cavity body, and a waterproof terminal is installed on the cavity body to achieve the sealing of the cylindrical air cavity; A tuning capacitor is configured for the magnetic induction coil to form a series resonant structure, and a coaxial cable is used to complete the assembly of the feeding structure for the magnetic induction coil.

7. An air-cavity isolated magnetic induction antenna, characterized in that, The method described in any one of claims 1 to 6 is designed to include: a sealed cylindrical air cavity, a magnetic induction coil, and an antenna frame; The magnetic induction coil is coaxially fixed at the center position inside the cylindrical air cavity via the antenna frame; The radius of the cylindrical air cavity is designed based on the eddy current exponential decay law and the actual performance indicators of the antenna, and its length is not used as an optimization parameter for eddy current suppression.

8. The air-cavity isolated magnetic induction antenna according to claim 7, characterized in that, The antenna frame is made of insulating material; The cylindrical air cavity is a sealed cavity structure, which includes a cavity body and a sealing element; the sealing element is located at the end of the cavity body.

9. The air-cavity isolated magnetic induction antenna according to claim 8, characterized in that, It also includes: tuning capacitors, coaxial cables, and waterproof terminals; The coaxial cable passes through a waterproof terminal and is electrically connected to the magnetic induction coil; The tuning capacitor is matched with the magnetic induction coil to ensure that the antenna achieves series resonance within the operating frequency band.

10. The air-cavity isolated magnetic induction antenna according to claim 7, characterized in that, The magnetic induction coil is made of copper enameled wire, and the cavity body is a transparent plastic tube structure.

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