An extremely low frequency magnetic field communication receiver based on atomic magnetometer

Abstract

The present invention discloses an extremely low frequency magnetic field communication receiver based on an atomic magnetometer in the field of wireless communication technology. The core sensor of the extremely low frequency magnetic field communication receiver is a scalar atomic magnetometer based on the free induction decay working principle and the pulsed light pumping working mode, including a meter head and a controller electrically connected to the meter head; the meter head includes a VCSEL laser, a polarization magnetic field coil, a magnetic field gradient coil, an atomic gas chamber, a λ / 4 wave plate, a photodiode and an insulated box; the controller includes a meter head drive circuit and an FPGA-based signal processor that runs an automatic working algorithm to control the drive circuit. The receiver of the present invention has a higher sensitivity to extremely low frequency magnetic field detection under the same probe volume conditions, and has the advantages of miniaturization and lightweight. The scalar detection is suitable for dynamic carrier platforms and can be easily formed into a magnetic field gradiometer or array, thereby improving the receiver sensitivity, increasing the communication distance, and enhancing the covert communication capability.

Description

Technical Field

[0001] The present invention relates to the field of wireless communication technology, and in particular to an extremely low frequency magnetic field communication receiver based on an atomic magnetometer. Background Art

[0002] High-frequency electromagnetic signals experience high propagation losses in media such as metal, solid materials (concrete, rock, soil, etc.), and water, resulting in short propagation distances and poor penetration. Since the penetration depth of electromagnetic waves in a given medium is inversely proportional to the square root of the frequency—for example, a 1kHz frequency has a penetration depth three orders of magnitude greater than a 1GHz frequency—extremely low frequency (3-30Hz) electromagnetic waves have low propagation losses, strong seawater penetration, and long propagation distances. However, a major drawback of extremely low frequency communication is that underwater platforms such as submersibles require towed buoyant cables hundreds of meters long for reception, hindering the concealment and maneuverability of these platforms.

[0003] For underwater extremely low frequency (ELF) magnetic field communications, the receiver traditionally used is a magnetic induction coil receiver. Magnetic induction coil receivers are limited by the law of electromagnetic induction in the ELF band, resulting in long and heavy sensor probes, making them unsuitable for applications requiring portability. Furthermore, the reduced induced signal strength reduces sensor sensitivity. Furthermore, the magnetic induction coil is a vector magnetic field sensor, and its magnetic field detection output is sensitive to the sensor probe's orientation. Therefore, it is significantly affected by the motion and vibration-induced noise of the dynamic platform. Furthermore, when multiple receivers are combined to form a gradiometer or array, the directions of the receiver probes must be strictly aligned to effectively suppress common-mode environmental magnetic field noise, increasing the difficulty of installation. Therefore, it is of great significance to provide a highly sensitive, miniaturized, lightweight ELF magnetic field communication receiver based on a scalar magnetic sensor. Summary of the Invention

[0004] The purpose of the present invention is to provide an extremely low frequency magnetic field communication receiver based on an atomic magnetometer, which solves the technical problems in the existing technology that the magnetic field sensor of the magnetic induction coil receiver for extremely low frequency communication is large and heavy and is limited by the movement of the dynamic carrier platform and the large vibration induction noise. It meets the requirements of high sensitivity, miniaturization and lightweight, and is suitable for application scenarios that need to be portable or installed on a dynamic carrier platform or a mechanical vibration platform.

[0005] In order to achieve the above object, the technical solution of the present invention is as follows:

[0006] An extremely low frequency magnetic field communication receiver based on an atomic magnetometer, wherein the core sensor of the extremely low frequency magnetic field communication receiver is a scalar atomic magnetometer, and the extremely low frequency magnetic field communication receiver based on the atomic magnetometer includes a meter head and a controller electrically connected to the meter head;

[0007] The header includes:

[0008] A VCSEL laser with an integrated beam expander lens, which emits laser light for optical pumping of rubidium-87 atoms and detection of FID (free induction decay) signals, wherein the VCSEL laser contains a non-magnetic heater for temperature control and a temperature measuring element; and

[0009] Atomic gas cells, miniature alkali metal atomic vapor cells of rubidium 87 and buffer gas mixture; and

[0010] An insulation box, which is a box for placing the atomic gas chamber and maintaining a constant temperature environment, and contains a non-magnetic heating plate and a temperature measuring element; and

[0011] A λ / 4 wave plate, which converts the laser light into a circularly polarized beam; and

[0012] a photodiode for detecting a light intensity signal of the light beam after passing through the atomic gas chamber; and

[0013] Polarized magnetic field coils, a set of uniform magnetic field coils with polarized magnetic fields along the x-direction surrounded by the atomic gas chamber; and

[0014] The magnetic field gradient coils are a group of magnetic field gradient coils along the z direction and a group of magnetic field gradient coils along the y direction surrounded by the atomic gas chamber.

[0015] Furthermore, the controller includes a meter drive circuit and an FPGA-based signal processor ARM+FPGA that runs an automatic working algorithm and controls the drive circuit using a digital feedback loop; the drive circuit includes:

[0016] A photoelectric tube preamplifier circuit, used for converting the current signal of the photoelectric tube PD into a voltage signal; and

[0017] Laser driver circuit, providing and feedback controlling the VCSEL laser's injection current and temperature-controlled heating current to stabilize optical power and wavelength; and

[0018] Atomic gas temperature control circuit, providing and feedback controlling the temperature control heating current of the incubator; and

[0019] a polarization magnetic field driving circuit for providing a polarization magnetic field coil current; and

[0020] The magnetic field gradient compensation circuit provides magnetic field gradient coil current.

[0021] Furthermore, the FPGA-based signal processor ARM+FPGA includes:

[0022] A frequency meter measures the frequency of the rubidium-87 atomic FID signal output by the photoelectric tube preamplifier circuit and outputs a magnetic field value proportional to the frequency value; and

[0023] The lock-in amplifier further demodulates the magnetic field signal output by the frequency meter according to the known signal magnetic field frequency to obtain the magnetic field amplitude and phase information, thereby recovering the code element information of the amplitude modulated or phase modulated magnetic field signal.

[0024] Furthermore, the FPGA-based signal processor ARM+FPGA also includes a laser control program, an atomic gas room temperature control program, a polarization magnetic field driver and a magnetic field gradient compensation program; the laser control program feedback controls the laser drive circuit, the atomic gas room temperature control program feedback controls the atomic gas room temperature control circuit, the polarization magnetic field driver controls the polarization magnetic field drive circuit, and the magnetic field gradient compensation program controls the magnetic field gradient compensation circuit.

[0025] Furthermore, when the ambient magnetic field gradient is too large, the Larmor precession signal decays quickly and the frequency meter cannot effectively measure it. The magnetic field gradient compensation program controls the magnetic field gradient compensation circuit to provide magnetic field gradient coil current through a compensation algorithm to compensate for the ambient magnetic field gradient and obtain a Larmor precession signal that can be effectively measured.

[0026] Furthermore, the extremely low frequency magnetic field communication receiver can detect signal magnetic fields of any vibration direction. When the signal magnetic field vibration direction is consistent with the geomagnetic field direction at the location of the extremely low frequency magnetic field communication receiver head, the change in the detected total magnetic field amplitude is the largest, that is, the detected signal magnetic field is the strongest; when the signal magnetic field vibration direction is perpendicular to the geomagnetic field direction at the location of the extremely low frequency magnetic field communication receiver head, the change in the detected total magnetic field amplitude is the smallest, that is, the detected signal magnetic field is the weakest.

[0027] Furthermore, the two extremely low frequency magnetic field communication receivers are combined together so that the two extremely low frequency magnetic field communication receiver heads are arranged in a manner orthogonal to each other in the direction of the optical axis, which can ensure that the extremely low frequency magnetic field communication receiver can detect the magnetic field to be measured in any direction and eliminate the measurement "dead zone".

[0028] Furthermore, two extremely low frequency magnetic field communication receivers separated by a certain distance can constitute a magnetic field gradiometer. The directions of the two extremely low frequency magnetic field communication receivers do not need to be strictly aligned. The magnetic field gradient of the transmitted signal magnetic field is detected by differential calculation to offset the environmental magnetic field noise, improve the sensitivity of the extremely low frequency magnetic field communication receiver, and increase the communication distance.

[0029] Furthermore, multiple extremely low frequency magnetic field communication receivers are arranged at a certain interval, wherein the header directions of each extremely low frequency magnetic field communication receiver do not need to be strictly aligned, and a receiver array can be formed. The coherent accumulation and beam synthesis methods are used to enhance the detection signal-to-noise ratio, improve the sensitivity of the extremely low frequency magnetic field communication receiver, and increase the communication distance.

[0030] By adopting the above technical solution, the present invention has the following advantages:

[0031] The present invention provides an extremely low frequency (ELF) magnetic field communication receiver based on an atomic magnetometer. The core sensor of this receiver is a scalar atomic magnetometer. Compared to traditional magnetic induction coil receivers, this receiver is not limited by the laws of electromagnetic induction and the thermal noise limit of the magnetic induction coil. It has higher sensitivity to ELF magnetic field detection under the same probe volume conditions and offers the advantages of miniaturization and lightweight design. The receiver's magnetic field detection output is insensitive to the direction of the meter head and is less affected by dynamic platform motion and vibration-induced noise. It is suitable for applications requiring portability or installation on dynamic carrier platforms or mechanical vibration platforms. Multiple receivers can be combined without strict meter head alignment, easily forming a magnetic field gradiometer or array. This improves receiver sensitivity, increases communication distance, and enhances covert communication capabilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A structural diagram of a header of an extremely low frequency magnetic field communication receiver of the present invention;

[0033] Figure 2 A structural diagram of a controller of an extremely low frequency magnetic field communication receiver of the present invention;

[0034] Figure 3 This is a working process diagram of the extremely low frequency magnetic field communication receiver of the present invention;

[0035] Figure 4 The meter head arrangement mode of the combination of two extremely low frequency magnetic field communication receivers for eliminating the measurement "dead zone" of the present invention;

[0036] Figure 5 The extremely low frequency magnetic field communication receiver of the present invention detects signal magnetic fields in any vibration direction. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is described in detail below with reference to the accompanying drawings. It should be noted that, in this document, relational terms such as first and second, etc., are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include," "comprise," or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article, or apparatus.

[0038] An extremely low frequency magnetic field communication receiver based on an atomic magnetometer, wherein the core sensor includes but is not limited to a scalar atomic magnetometer with a free induction decay (FID) and other types of working principles. This embodiment is a scalar atomic magnetometer based on the free induction decay working principle and a pulsed light pumping working mode. The receiver of the present invention comprises a meter head and a controller electrically connected to the meter head. The structure of the meter head is as follows: Figure 1 As shown, it includes a VCSEL laser 1, a λ / 4 wave plate 2, a polarization magnetic field coil A, magnetic field gradient coils B and C, an atomic gas chamber 3, an insulation box 4, and a photodiode PD5. All components in the meter head are made of non-magnetic materials.

[0039] The meter operates as follows: VCSEL laser 1 emits 795nm linearly polarized light, resonating with the D1 line of rubidium-87 atoms. This light is converted to circularly polarized light after passing through a λ / 4 quartz wave plate 2. Through the laser driver circuit in the controller, VCSEL laser 1's output optical power is approximately 0.2mW. The internal non-magnetic heater and temperature measurement element ensure a temperature control accuracy of less than 10mK.

[0040] Circularly polarized light is irradiated along the x-direction onto the micro atomic chamber 3 filled with a mixture of rubidium 87 and buffer gas, for optical pumping of rubidium 87 atoms and detection of FID signals. The insulated box 4 maintains the atomic chamber 3 in a constant temperature environment. The internal non-magnetic heating plate and temperature measuring element keep the temperature at around 80°C with a temperature control accuracy of less than 10mK. The x-direction polarization magnetic field coil A generates a uniform strong polarization magnetic field along the x-direction. Close to 1mT; the magnetic field gradient coils B and C in the z and y directions generate a gradient magnetic field of a certain size and direction to compensate for the environment; the photodiode PD5 detects the light intensity signal of the light beam after passing through the atomic gas chamber 3.

[0041] The controller is as follows Figure 2 As shown, the ARM+FPGA-based signal processor includes a drive circuit and an FPGA-based signal processor that controls the drive circuit. The drive circuit includes the following modules: a photoelectric tube preamplifier circuit, a laser drive circuit, an atomic atmosphere temperature control circuit, a polarization magnetic field drive circuit, and a magnetic field gradient compensation circuit. The ARM+FPGA-based signal processor runs the automatic operation algorithm of the entire receiver and uses a digital feedback loop to control all drive circuits. It includes the following modules: a frequency counter, a lock-in amplifier, a laser control program, an atomic atmosphere temperature control program, a polarization magnetic field drive program, and a magnetic field gradient compensation program.

[0042] The controller works as follows: First, the laser drive circuit provides and feedback controls the VCSEL laser's injection current and temperature-controlled heating current to stabilize the optical power and wavelength; the atomic atmosphere temperature-controlled circuit provides and feedback controls the incubator's temperature-controlled heating current.

[0043] Next, the working process is divided into two phases, polarization phase and measurement phase, which are repeated every 10ms cycle time. Figure 3 shown.

[0044] In t=0-5ms, the polarization magnetic field driver controls the polarization magnetic field drive circuit to provide current to the polarization magnetic field coil to generate a strong polarization magnetic field. Under the combined action of laser and longitudinal polarization field, the rubidium 87 atoms are spin polarized along the direction of the polarization field, and the initial polarization rate is According to the Bloch equation describing the spin dynamics The solution is the polarization rate along the polarization direction, that is, the x-axis direction Polarizability P x The change of (t) is reflected in the change of the absorption of light intensity by the atomic gas chamber 3. The photoelectric tube preamplifier circuit converts the current signal of the photodiode PD5 into a voltage signal and outputs the change signal of the polarization stage of the rubidium 87 atom as shown in Figure 3 Shown in the left half.

[0045] At t = 5ms, the polarizability P x (t) After stabilization, the polarization magnetic field driver controls the polarization magnetic field driving circuit to quickly turn off the polarization field within a few μs. The measurement phase starts at t = 5-10ms. Assume that the total magnetic field to be measured is Along the z-axis, is the Earth's magnetic field, is the signal magnetic field, according to the Bloch equation, we can get That is, the spin of the rubidium 87 atom will rotate around The exponentially decaying Larmor precession is called free induction decay (FID). The precession frequency ω0 is proportional to the magnitude of the magnetic field B0 to be measured, that is, ω0 = γB0. The proportional coefficient γ = 6.998nT / Hz is the gyromagnetic ratio of the rubidium 87 atom. The photoelectric tube preamplifier circuit outputs the FID signal of the rubidium 87 atom measurement stage as follows: Figure 3 Shown in the right half.

[0046] A frequency meter then uses a high-precision frequency measurement algorithm to measure the frequency ω0 of the rubidium-87 atomic FID signal output by the photoelectric tube preamplifier circuit. The precise value of the total magnetic field to be measured, B0, is calculated and output based on the gyromagnetic ratio γ. After the measurement phase of the current cycle ends, the polarization phase of the next cycle begins, and the cycle repeats.

[0047] The frequency ω0 of the FID signal of rubidium 87 atoms depends only on the total magnetic field Rather than depending on the individual field components (B x ,B y ,B z), so the core sensor of this receiver is a scalar atomic magnetometer. The magnetic field detection output of the receiver is insensitive to the direction of the meter head and is less affected by the dynamic platform motion and vibration induced noise.

[0048] Since the sampling period of magnetic field measurement is 10ms, that is, the sampling rate is 100Hz, the receiver bandwidth is 50Hz. Assume that the receiver is used to detect a signal magnetic field with a known frequency of 10Hz. Next, a phase-locked amplifier is used to demodulate the signal of the total magnetic field B0 value output by the frequency meter for a period of time to obtain the signal magnetic field The amplitude and phase information of the signal can be used to recover the communication code element information.

[0049] The receiver has a magnetic field measurement range. The lower limit of the range is limited by the fact that the precession frequency of the FID signal must be greater than the relaxation rate; the upper limit of the range is limited by the limited signal processing capabilities of modules such as the frequency meter.

[0050] When the ambient magnetic field gradient is excessively high, such as 1000nT / cm, the FID signal decays rapidly during the measurement phase, making it difficult for the frequency counter to effectively measure the signal. At this point, the magnetic field gradient compensation process is activated. A compensation algorithm uses feedback to control the magnetic field gradient compensation circuit, providing a specific current to the magnetic field gradient coil. This compensates the ambient magnetic field gradient to, for example, below 100nT / cm, ensuring a measurable FID signal. After compensating for the ambient magnetic field gradient, the polarization phase of the next operating cycle begins.

[0051] The measurement "dead zone" of the extremely low frequency magnetic field communication receiver is within a cone with a few degrees of angle with the optical axis direction of the extremely low frequency magnetic field communication receiver as the axis. That is, when the optical axis direction of the extremely low frequency magnetic field communication receiver is nearly parallel to the direction of the magnetic field to be measured, the extremely low frequency magnetic field communication receiver cannot detect the magnetic field. Two extremely low frequency magnetic field communication receivers (① and ②) can be combined together so that the two heads are arranged in a mutually orthogonal manner in the direction of the optical axis, as shown in the following example. Figure 4 As shown, this ensures that the extremely low frequency magnetic field communication receiver can detect the magnetic field to be measured in any direction, that is, eliminates the measurement "dead zone".

[0052] The extremely low frequency magnetic field communication receiver can detect the signal magnetic field in any vibration direction. Figure 5 When the signal magnetic field vibration direction is consistent with the direction of the geomagnetic field at the location of the ELF magnetic field communication receiver, the detected total magnetic field amplitude changes the most, that is, the detected signal magnetic field is the strongest; when the signal magnetic field vibration direction is perpendicular to the direction of the geomagnetic field at the location of the ELF magnetic field communication receiver, the detected total magnetic field amplitude changes the least, that is, the detected signal magnetic field is the weakest.

[0053] Place two extremely low frequency magnetic field communication receivers, for example, 10 meters apart. The directions of the two receiver heads do not need to be strictly aligned, and they can form a magnetic field gradiometer. Then, perform differential calculations on the outputs of the two extremely low frequency magnetic field communication receivers to detect the magnetic field gradient of the transmitted signal. This can offset the ambient magnetic field noise, improve the receiver sensitivity, and increase the communication distance.

[0054] Multiple extremely low frequency magnetic field communication receivers are arranged into a matrix with vertical and horizontal intervals of, for example, 10m. The header directions of each extremely low frequency magnetic field communication receiver do not need to be strictly aligned, and a receiver array can be formed. The coherent accumulation and beam synthesis methods are used to enhance the detection signal-to-noise ratio, improve the receiver sensitivity, and increase the communication distance.

[0055] Finally, it should be pointed out that although the present invention has been described with reference to the current specific embodiments, ordinary technicians in this technical field should realize that the above embodiments are only used to illustrate the present invention and are not used to limit the present invention. Various equivalent changes or substitutions can be made without departing from the concept of the present invention. Therefore, as long as the changes and modifications to the above embodiments are within the scope of the essential spirit of the present invention, they will fall within the scope of the claims of the present invention.

Claims

1. An extremely low frequency magnetic field communication receiver based on an atomic magnetometer, characterized in that: The core sensor of the extremely low frequency magnetic field communication receiver is a scalar atomic magnetometer. The extremely low frequency magnetic field communication receiver based on the atomic magnetometer includes a meter head and a controller electrically connected to the meter head. The header includes: A VCSEL laser with an integrated beam expander lens, which emits laser light for optical pumping of rubidium-87 atoms and detection of FID (free induction decay) signals, wherein the VCSEL laser contains a non-magnetic heater for temperature control and a temperature measuring element; and Atomic gas cells, miniature alkali metal atomic vapor cells of rubidium 87 and buffer gas mixture; and An insulation box, which is a box for placing the atomic gas chamber and maintaining a constant temperature environment, and contains a non-magnetic heating plate and a temperature measuring element; and A λ / 4 wave plate, which converts the laser light into a circularly polarized beam; and a photodiode for detecting a light intensity signal of the light beam after passing through the atomic gas chamber; and Polarized magnetic field coils, a set of uniform magnetic field coils with polarized magnetic fields along the x-direction surrounded by the atomic gas chamber; and The magnetic field gradient coils are a group of magnetic field gradient coils along the z direction and a group of magnetic field gradient coils along the y direction surrounded by the atomic gas chamber.

2. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 1, characterized in that: The controller includes a meter drive circuit and an FPGA-based signal processor ARM+FPGA that runs an automatic working algorithm and controls the drive circuit using a digital feedback loop; the drive circuit includes: A photoelectric tube preamplifier circuit, used for converting the current signal of the photoelectric tube PD into a voltage signal; and Laser driver circuit, providing and feedback controlling the VCSEL laser's injection current and temperature-controlled heating current to stabilize optical power and wavelength; and Atomic gas temperature control circuit, providing and feedback controlling the temperature control heating current of the incubator; and a polarization magnetic field driving circuit for providing a polarization magnetic field coil current; and The magnetic field gradient compensation circuit provides magnetic field gradient coil current.

3. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 2, characterized in that: The FPGA-based signal processor ARM+FPGA includes: A frequency meter measures the frequency of the rubidium-87 atomic FID signal output by the photoelectric tube preamplifier circuit and outputs a magnetic field value proportional to the frequency value; and The lock-in amplifier further demodulates the magnetic field signal output by the frequency meter according to the known signal magnetic field frequency to obtain the magnetic field amplitude and phase information, thereby recovering the code element information of the amplitude modulated or phase modulated magnetic field signal.

4. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 3, characterized in that: The FPGA-based signal processor ARM+FPGA also includes a laser control program, an atomic gas room temperature control program, a polarization magnetic field driver program and a magnetic field gradient compensation program; the laser control program feedback controls the laser drive circuit, the atomic gas room temperature control program feedback controls the atomic gas room temperature control circuit, the polarization magnetic field driver program controls the polarization magnetic field drive circuit, and the magnetic field gradient compensation program controls the magnetic field gradient compensation circuit.

5. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 4, characterized in that: When the ambient magnetic field gradient is too large, the Larmor precession signal decays quickly and the frequency meter cannot effectively measure it. The magnetic field gradient compensation program controls the magnetic field gradient compensation circuit to provide magnetic field gradient coil current through a compensation algorithm to compensate for the ambient magnetic field gradient and obtain a Larmor precession signal that can be effectively measured.

6. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 5, characterized in that: The extremely low frequency magnetic field communication receiver can detect signal magnetic fields of any vibration direction. When the vibration direction of the signal magnetic field is consistent with the direction of the geomagnetic field at the location of the extremely low frequency magnetic field communication receiver head, the change in the detected total magnetic field amplitude is the largest, that is, the detected signal magnetic field is the strongest; when the vibration direction of the signal magnetic field is perpendicular to the direction of the geomagnetic field at the location of the extremely low frequency magnetic field communication receiver head, the change in the detected total magnetic field amplitude is the smallest, that is, the detected signal magnetic field is the weakest.

7. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 6, characterized in that: The two extremely low frequency magnetic field communication receivers are combined together so that the two extremely low frequency magnetic field communication receiver heads are arranged in a manner orthogonal to each other along the optical axis direction, which can ensure that the extremely low frequency magnetic field communication receiver can detect the magnetic field to be measured in any direction and eliminate the measurement "dead zone".

8. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 7, characterized in that: Two extremely low frequency magnetic field communication receivers separated by a certain distance can constitute a magnetic field gradiometer. The directions of the two extremely low frequency magnetic field communication receivers do not need to be strictly aligned. The magnetic field gradient of the transmitted signal magnetic field is detected through differential calculation to offset the environmental magnetic field noise, improve the sensitivity of the extremely low frequency magnetic field communication receiver, and increase the communication distance.

9. The extremely low frequency magnetic field communication receiver based on an atomic magnetometer according to claim 8, characterized in that: Multiple extremely low frequency magnetic field communication receivers are arranged at certain intervals, wherein the header directions of each extremely low frequency magnetic field communication receiver do not need to be strictly aligned, and a receiver array can be formed. The coherent accumulation and beam synthesis methods are used to enhance the detection signal-to-noise ratio, improve the sensitivity of the extremely low frequency magnetic field communication receiver, and increase the communication distance.

Images