Metasurface microwave imaging system

Abstract

The application relates to a metasurface microwave imaging system. The system comprises a host computer, an FPGA chip, a radio frequency transceiver unit, a transmitting antenna, an adjustable metasurface, a receiving antenna unit, the host computer is connected with the FPGA chip, the FPGA chip is connected with the radio frequency transceiver unit, the radio frequency transceiver unit is respectively connected with the transmitting antenna and the receiving antenna unit, the FPGA chip is connected with the adjustable metasurface through an expansion interface circuit, the expansion interface circuit comprises a binary decoder, and the host computer comprises an image reconstruction module. The metasurface microwave imaging system improves the synchronization, stability and real-time performance of code switching, radio frequency transmission and data processing, realizes high-speed independent control of the FPGA chip on a large-scale adjustable metasurface, and improves the imaging speed, imaging quality and stability.

Description

Technical Field

[0001] This invention relates to the field of imaging technology, and in particular to a metasurface microwave imaging system. Background Technology

[0002] Microwaves refer to electromagnetic waves with wavelengths between 1 millimeter and 1 meter and corresponding frequencies between 300 MHz and 300 GHz. They possess excellent penetration capabilities, scattering characteristics, and high security, thus finding wide application in fields such as communications, radar, and imaging. Microwave imaging utilizes the reflection and scattering of microwaves by a target to obtain its internal or surface structure. It offers advantages such as non-contact and penetrability, making it crucial for military reconnaissance, biomedical detection, and civilian equipment.

[0003] Existing microwave imaging systems typically employ methods such as mechanical scanning, focal plane imaging, synthetic aperture imaging, phased array imaging, or digital multibeam imaging. These systems generally suffer from problems such as complex structure, large size, slow acquisition speed, limited resolution, complex signal processing, and insufficient real-time performance. Furthermore, mechanical scanning devices also have drawbacks such as high failure rate and difficulty in meeting the requirements of high-speed imaging.

[0004] With the development of metasurface technology, by integrating a large number of subwavelength units on ultrathin structures, the amplitude and phase of electromagnetic waves can be flexibly controlled to form complex radiation fields suitable for microwave imaging. This metasurface-based microwave imaging system simplifies the hardware structure, improves imaging resolution, supports broadband or multi-band operation, and can achieve programmability and reconfigurability through digital coding, while also being relatively inexpensive. Nevertheless, existing metasurface microwave imaging systems suffer from several drawbacks: low imaging speed and stability; unsatisfactory imaging quality; high requirements for control I / O (Input / Output) resources for large-scale metasurface units, making efficient driving difficult using FPGA chips or other control devices; and the use of network analyzers for signal transmission and reception, resulting in bulky equipment that is difficult to integrate. Summary of the Invention

[0005] Based on this, the purpose of the present invention is to provide a metasurface microwave imaging system, which has the advantages of being lightweight, having a small system size, and having a high degree of system integration.

[0006] This invention provides a metasurface microwave imaging system, including a host computer, an FPGA chip, a radio frequency transceiver unit, a transmitting antenna, an tunable metasurface, and a receiving antenna unit. The host computer is connected to the FPGA chip, the FPGA chip is connected to the radio frequency transceiver unit, the radio frequency transceiver unit is connected to the transmitting antenna and the receiving antenna unit respectively, the FPGA chip is connected to the tunable metasurface through an expansion interface circuit, the expansion interface circuit includes a binary decoder, and the host computer includes an image reconstruction module.

[0007] The metasurface microwave imaging system of this invention significantly reduces FPGA chip control I / O resources through a binary decoder, achieving efficient FPGA chip driving. Simultaneously, due to the high parallel processing capability of the FPGA chip, its use improves the synchronization, stability, and real-time performance of code switching, RF transmission, and reception, thereby enhancing imaging speed and stability. Furthermore, it can achieve high-speed independent control of the tunable metasurface, even completing radiation mode switching at microsecond rates, providing a high temporal resolution measurement matrix for target object imaging, thus improving imaging speed and quality.

[0008] Further, the radio frequency transceiver unit includes a first frequency synthesizer, a second frequency synthesizer, a first mixer, a second mixer, a first power divider, and a second power divider; wherein, the first frequency synthesizer is connected to the first power divider, the first power divider is connected to the first mixer, the first mixer is connected to the second power divider, the second frequency synthesizer is connected to the second power divider, and the second power divider is connected to the second mixer; the first frequency synthesizer generates an electromagnetic wave signal acting on the tunable metasurface, and the second mixer receives a signal received from the receiving antenna unit; the first mixer outputs a mixed reference signal, and the second mixer outputs a mixed received signal, the reference signal and the received signal being transmitted to the FPGA chip via an analog-to-digital converter.

[0009] Furthermore, the FPGA chip includes an information processing module, which acquires the amplitude, phase, real part, and imaginary part of the electromagnetic wave based on the received signal and the reference signal, and the image reconstruction module of the host computer acquires an image of the target object based on the real part and the imaginary part.

[0010] Furthermore, the FPGA chip includes a signal processing module, which is configured to: set a sliding window on the acquired received signal and extract the maximum and minimum values ​​of the data within the sliding window, according to the formula (max... The amplitude of the received signal is calculated by (min) / 2. At the same time, the received signal and the reference signal are converted into two corresponding square wave signals after zero detection. Then, the two square wave signals are XORed and the duty cycle of the XOR signal is calculated to obtain the phase of the received signal relative to the reference signal.

[0011] Furthermore, the signal processing module acquires the amplitudes of multiple received signals and calculates the average of the multiple amplitudes, then uses the average as the amplitude of the electromagnetic wave.

[0012] Furthermore, the receiving antenna unit includes multiple receiving antennas, and a control switch connected to both the receiving antennas and the FPGA chip is provided between them.

[0013] Furthermore, the image reconstruction module employs the following imaging model:

[0014]

[0015]

[0016] Where g is the complex data vector of electromagnetic waves. Let be the complex data vector of all electromagnetic wave signals received by the i-th receiving antenna. Each element in the vector represents electromagnetic wave data in complex form, and its vector dimension is the number of amplitudes encoded by the metasurface × 1. m is the number of receiving antennas in receiving antenna element 7; H is the observation matrix; f is the image of the target object to be imaged that needs to be inverted; n is the noise.

[0017] Furthermore, the image reconstruction module uses a compressed sensing algorithm to invert the image of the target object.

[0018] Furthermore, the receiving antenna is a Vivaldi antenna.

[0019] Furthermore, the control switch is a radio frequency switch.

[0020] To better understand and implement this invention, the following detailed description is provided in conjunction with the accompanying drawings. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the metasurface microwave imaging system architecture of the present invention; Figure 2 This is a schematic diagram of the expansion interface circuit structure according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the structure of a radio frequency transceiver unit according to an embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention more readily understood by those skilled in the art, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] refer to Figure 1 , Figure 1 A schematic diagram of the metasurface microwave imaging system architecture of the present invention is shown. The system includes a control unit 1, a radio frequency transceiver unit 2, an expansion interface circuit 3, a transmitting antenna 4, an adjustable metasurface 5, an imaging region 6, a receiving antenna unit 7, and a control switch 8.

[0024] The control unit 1 includes a host computer 1-1 and an FPGA (Field Programmable Gate Array) chip 1-2, wherein the host computer 1-1 transmits data signals to the FPGA chip 1-2.

[0025] The host computer 1-1 can be a computer or other device capable of outputting control signals, receiving feedback signals, and processing data. It is connected to the FPGA chip 1-2 and includes a metasurface coding design module, an RF transceiver control module, an RF switch switching module, an image reconstruction module, and a storage module. The metasurface coding design module is used to set the metasurface coding. The RF transceiver control module is used to set the electromagnetic wave signals and control the RF transceiver unit 2 via the FPGA chip 1-2 to transmit and receive electromagnetic wave signals from the receiving antenna unit 7. The RF switch switching module is used to set the switching information of the control switch 8. The image reconstruction module is used to image the target object. The storage module is used to save the data. Through the host computer, users can conveniently manage and control the system, ensuring its efficient operation.

[0026] The FPGA chip 1-2 includes a serial port module, an encoding generation module, a transmission frequency control module, an RF switch control module, and a signal processing module. The serial port module is used for bidirectional information transmission with the host computer 1-1. To achieve rapid switching of metasurface encoding, the serial port module receives metasurface encoding data signals from the host computer 1-1 and transmits these signals to the expansion interface circuit 3 via the encoding generation module in an address-data signal stream manner. This enables real-time encoding switching of the independently adjustable devices on the adjustable metasurface 4. To transmit electromagnetic waves, the serial port module receives electromagnetic wave data signals from the RF transceiver control module of the host computer 1-1 and controls the RF transceiver unit 2 via the transmission frequency control module to cause the transmitting antenna 4 to transmit electromagnetic waves of a specific frequency. To achieve signal reception by the receiving antenna in the receiving antenna unit 7 and transmission of the signal to the radio frequency transceiver unit 2, the radio frequency switch control module controls the switching of the radio frequency switch 6 according to the control signal transmitted by the radio frequency switch switching module of the host computer 1-1. At the same time, the FPGA chip 1-2 controls the radio frequency transceiver unit 2 to receive electromagnetic wave signals from the receiving antenna unit 7. The signal processing module processes the data received and transmitted by the receiving antenna unit 7 to obtain the amplitude, phase, real part and imaginary part data of the electromagnetic wave, and transmits the processed data to the host computer 1-1 through the serial port module.

[0027] refer to Figure 2The figure illustrates a schematic diagram of an expansion interface circuit according to an embodiment of the present invention. In this invention, the expansion interface circuit 3 is connected to the FPGA chip 1-2 and includes a binary decoder, specifically a 3-8 decoder, a 4-16 decoder, a 5-32 decoder, or a 6-64 decoder, to achieve independent control of multiple independently adjustable devices on the adjustable metasurface 4. This method can significantly reduce the demand on FPGA chip I / O resources. Specifically, the address signal and data signal transmitted from the encoding generation module of the FPGA chip 1-2 control the adjustable metasurface 4 to achieve corresponding metasurface encoding; the address signal uses a binary signal to sequentially select the output port of the binary decoder, and then outputs the corresponding metasurface encoding signal containing the data signal to control the switching state of the independently adjustable devices on the adjustable metasurface 4. The independently adjustable devices include PIN diodes or varactor diodes. In another embodiment, a latch connected to a binary decoder is also included. In this embodiment, the aforementioned metasurface encoding signal containing the data signal is input to a latch selected by the binary decoder. Each latch stores and outputs the corresponding metasurface encoding signal to control the switching state of the independent adjustable device of the adjustable metasurface 4. The adjustable metasurface 4 is a 1-bit adjustable metasurface.

[0028] In one specific embodiment, four 3-to-8 decoders and latches are cascaded, including eight address signals, each corresponding to a 32-bit data signal. Each time an address signal is transmitted to the expansion interface circuit 3, the 32-bit data signal is transmitted simultaneously. Each time a metasurface encoding signal is transmitted, the address signal is switched eight times, transmitting a total of 256 bits, corresponding to the on / off data of the 256 independent adjustable devices on the adjustable metasurface 4. That is, by combining the four cascaded 3-to-8 decoders and latches, independent control of 256 independent adjustable devices can be achieved. This method only requires 44 IO resources, compared to 256 IO resources required in the prior art. The IO resource saving rate in this embodiment is as high as 82.8%, which not only effectively improves the system resource utilization rate, but also ensures the fast response and precise control of the metasurface unit through the FPGA chips 1-2.

[0029] refer to Figure 3The figure shows a schematic diagram of a radio frequency transceiver unit structure according to an embodiment of the present invention. The radio frequency transceiver unit 2 is used for transmitting and receiving electromagnetic wave signals, and includes a first frequency synthesizer 2-1, a second frequency synthesizer 2-2, a first mixer 2-3, a second mixer 2-4, a first power divider 2-5, and a second power divider 2-6; wherein, the first frequency synthesizer 2-1 is connected to the first power divider 2-5, the first power divider 2-5 is connected to the first mixer 2-3, the first mixer 2-3 is connected to the second power divider 2-6, the second frequency synthesizer 2-2 is connected to the second power divider 2-6, and the second power divider 2-6 is connected to the second mixer 2-4; the first frequency synthesizer 2-1 is used to form the imaging system signal... An electromagnetic wave signal with a specified frequency (e.g., 6.4 GHz) is evenly distributed to two channels by a first power divider 2-5. One channel's signal is connected to the transmitting antenna 4 and transmitted to the tunable metasurface 5. The other channel's signal enters the RF port of the first mixer 2-3. The RF port of the second mixer receives the signal from the receiving antenna in the receiving antenna unit 7. The mixed signal generated by the second frequency synthesizer 2-2 (e.g., 6.397 GHz) is evenly distributed into two signals by a second power divider 2-6 and then transmitted to the LO ports of the first mixer 2-3 and the second mixer 2-4, respectively. Based on the principle of down-conversion in the mixer... IF =f RF -f LO The first mixer 2-3 outputs a low-frequency reference signal after mixing (e.g., if the RF input is 6.4GHz and the LO input is 6.397GHz, then the IF output is 3MHz). At the same time, the second mixer 2-4 outputs a low-frequency received signal after mixing from its IF port. The reference signal and the received signal are converted from analog to digital by an analog-to-digital converter (ADC) and then transmitted to the FPGA chip 1-2 for data processing.

[0030] In this invention, the radio frequency (RF) signal transceiver unit 2 is used for transmitting electromagnetic wave signals and for receiving and processing the signals from each antenna in the receiving antenna unit 7 into data that can be processed by the FPGA chips 1-2. This eliminates the system's reliance on large experimental equipment such as network analyzers. Instead, through the integrated design of frequency synthesis, power division, mixing, and analog-to-digital conversion modules, it achieves high-stability transmission and high signal-to-noise ratio reception of microwave signals. Furthermore, the RF signal transceiver unit described in this invention has a compact structure, low loss, and can be directly embedded into the imaging system, making the entire imaging system lightweight and significantly reducing its size and weight. This facilitates the miniaturization, integration, mobility, and engineering application of microwave imaging equipment.

[0031] The transmitting antenna 4 applies electromagnetic waves to the tunable metasurface 5. At this time, the scattered field projected onto the imaging region 6 by each metasurface encoding illuminates the target object. The electromagnetic wave signal reflected from the target object is received by the receiving antenna unit 7. The receiving antenna unit 7 includes multiple receiving antennas, preferably Vivaldi antennas, to increase data acquisition and ensure comprehensive signal reception, thereby improving the imaging quality and resolution of the imaging system. In this invention, multiple receiving antennas are spatially distributed to achieve multi-point sampling of the electromagnetic field. This not only expands the receiving aperture of the imaging system but also increases the sampling dimension because multiple receiving antennas can receive different data. This increases the area and resolution of the inversion imaging surface, resulting in higher resolution, lower noise, and higher stability imaging results during reconstruction, thus improving imaging quality.

[0032] To achieve rapid switching of the radio frequency switch, the control switch 8 adopts a radio frequency switch, and the radio frequency switch includes a power module based on the TPS5430 chip to provide the positive and negative 5V voltage required to drive the radio frequency switch, thereby ensuring the stability and reliability of the imaging system at high frequencies and ensuring that the signals from multiple receiving antennas can be effectively and independently transmitted to the radio frequency transceiver unit for processing.

[0033] The imaging system of the present invention uses multiple receiving antennas and combines them with radio frequency switches to achieve electronic scanning reception. This allows the multi-channel signals of the receiving antenna unit 7 to quickly acquire echo data from multiple spatial angles without the need for mechanical scanning reception. Compared with the traditional mechanical scanning reception method, the present invention has the advantages of no vibration, no wear, and high speed. Furthermore, by increasing the sampling dimension through multiple receiving antennas, the imaging quality is improved.

[0034] This invention receives the received signal and reference signal transmitted from the RF transceiver unit 2 via FPGA chip 1-2, and obtains the electromagnetic wave amplitude, phase, real part, and imaginary part corresponding to the received signal through the signal processing module. Specifically, for amplitude, the signal processing module of FPGA chip 1-2 sets a sliding window on the received signal. This sliding window can cover at least one electromagnetic wave cycle of the received signal. The data within the sliding window is buffered, and the maximum and minimum values ​​within the data within the sliding window are extracted according to the formula (max... The amplitude of the received signal is calculated by subtracting (min) / 2. Further, the sliding window can be moved multiple times, and the amplitude can be calculated using the same method to obtain the average of multiple amplitudes. This average is then used as the amplitude of the electromagnetic wave. The data within adjacent sliding windows do not overlap. For the phase, the received signal and the reference signal are first converted into two corresponding square wave signals after zero-crossing detection by the zero-crossing detection module. Then, an XOR operation is performed on the two square wave signals, and the duty cycle of the XOR signal is calculated to obtain the phase information of the received signal relative to the reference signal. Afterwards, the real and imaginary parts of the received signal are calculated based on the calculated amplitude and phase, using the following formula:

[0035] Where Real represents the real part, Imag represents the imaginary part, and A represents the amplitude. Indicates phase.

[0036] The system of the present invention can acquire the complex information of the electromagnetic field corresponding to each frame of metasurface encoding, namely the aforementioned real and imaginary part information, providing a high-precision data foundation for subsequent imaging reconstruction algorithms.

[0037] Furthermore, in this invention, after a receiving antenna receives a signal and the RF transceiver unit 2 receives and performs down-conversion processing on the signal, the signal is transmitted to the FPGA chip after analog-to-digital conversion by the analog-to-digital converter (ADC), and then the control switch 7 switches to the next receiving antenna for signal acquisition.

[0038] In this invention, after the FPGA chip 1-2 calculates the electromagnetic wave amplitude, phase, real part and imaginary part data of the received signal, these data are transmitted to the host computer 1-1 for imaging the target object to be imaged. The image reconstruction module in the host computer 1-1 adopts the following imaging model and preferably uses the compressed sensing algorithm to invert the image of the target object to be imaged.

[0039]

[0040]

[0041] Where g is the complex data vector of electromagnetic waves. Let be the complex data vector of all electromagnetic wave signals received by the i-th receiving antenna. Each element in the vector represents electromagnetic wave data in complex form, with the real and imaginary parts described above. Its vector dimension is the number of amplitudes encoded by the metasurface × 1. m is the number of receiving antennas in receiving antenna unit 7; H is the observation matrix, which is the scattering field of each metasurface code on imaging region 6 when there is no target object to be imaged; f is the image of the target object to be imaged that needs to be inverted; n is noise, such as environmental noise, noise from the devices in radio frequency transceiver unit 2, etc.

[0042] In this invention, an FPGA chip is used to calculate the real part, imaginary part, amplitude, and phase of the received signal in real time, and the resulting structured data is uploaded to the host computer. This method greatly reduces the data processing burden on the host computer and improves the imaging speed. This invention employs a compressed sensing imaging algorithm, which can achieve high-quality imaging with fewer measurements, further improving system efficiency and significantly enhancing the usability and engineering value of microwave imaging technology.

[0043] The workflow of the metasurface microwave imaging system of the present invention is as follows: First, without a target object to be imaged, the scattering field distribution generated by each metasurface encoding in the imaging region 6 is measured and used as the observation matrix H in the imaging model. Then, the target object to be imaged is placed within the imaging region 6. The host computer 1-1 sequentially sends metasurface encoding commands to the FPGA 1-2 to control the adjustable metasurface 4. Simultaneously, the system switches the control switch 7 according to a set sequence, connecting each receiving antenna in the receiving antenna unit 6 sequentially to the radio frequency transceiver unit 2, thereby achieving independent signal acquisition for each receiving antenna channel. The signals received by the receiving antennas are mixed and converted from analog to digital by the radio frequency transceiver unit 2. The FPGA chip 1-2 then calculates the amplitude, phase, real and imaginary parts of the converted data. The calculated data is then transmitted to the host computer 1-1. The host computer 1-1 uses the data transmitted by the FPGA chip 1-2 and, based on the aforementioned imaging model, uses a compressed sensing algorithm to inversely solve the problem, ultimately reconstructing the microwave imaging result of the target object.

[0044] Compared to existing technologies, the metasurface microwave imaging system of this invention significantly reduces FPGA chip control I / O resources through a binary decoder, achieving efficient FPGA chip driving. Simultaneously, the FPGA chip's highly parallel processing capabilities improve the synchronization, stability, and real-time performance of encoding switching, RF transmission and reception, and data processing, thus enhancing imaging speed and stability. Furthermore, it enables high-speed independent control of the tunable metasurface, even achieving radiation mode switching at microsecond rates, providing a high-temporal-resolution measurement matrix H for target object imaging, further improving imaging speed and quality. Moreover, RF transmission and reception, metasurface encoding switching, and data processing are all implemented within the same system, avoiding the timing instability and structural redundancy issues associated with multi-instrument collaboration in traditional systems, significantly improving overall system reliability. The use of multiple receiving antennas increases the sampling dimension, improving imaging quality. By utilizing an FPGA chip to perform real-time calculations of the real, imaginary, amplitude, and phase of the received signal and generating structured data that is then uploaded to the host computer, the processing burden on the host computer is greatly reduced, and the imaging speed is improved. Furthermore, a compressed sensing imaging algorithm is adopted to enable high-quality imaging with fewer measurements, further enhancing the system's efficiency.

[0045] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and the present invention also intends to include these modifications and variations.

Claims

1. A metasurface microwave imaging system, characterized in that, The system includes a host computer, an FPGA chip, a radio frequency transceiver unit, a transmitting antenna, an adjustable metasurface, and a receiving antenna unit. The host computer is connected to the FPGA chip, the FPGA chip is connected to the radio frequency transceiver unit, the radio frequency transceiver unit is connected to the transmitting antenna and the receiving antenna unit, respectively, the FPGA chip is connected to the adjustable metasurface through an expansion interface circuit, the expansion interface circuit includes a binary decoder, and the host computer includes an image reconstruction module.

2. The imaging system according to claim 1, characterized in that, The radio frequency transceiver unit includes a first frequency synthesizer, a second frequency synthesizer, a first mixer, a second mixer, a first power divider, and a second power divider; wherein, the first frequency synthesizer is connected to the first power divider, the first power divider is connected to the first mixer, the first mixer is connected to the second power divider, the second frequency synthesizer is connected to the second power divider, and the second power divider is connected to the second mixer; the first frequency synthesizer generates an electromagnetic wave signal acting on the tunable metasurface, and the second mixer receives a signal received from the receiving antenna unit; the first mixer outputs a mixed reference signal, and the second mixer outputs a mixed received signal, and the reference signal and the received signal are transmitted to the FPGA chip through an analog-to-digital converter.

3. The imaging system according to claim 2, characterized in that, The FPGA chip obtains the amplitude, phase, real part, and imaginary part of the electromagnetic wave based on the received signal and the reference signal, and the image reconstruction module of the host computer obtains the image of the target object based on the real part and the imaginary part.

4. The imaging system according to claim 3, characterized in that, The FPGA chip includes a signal processing module, which is configured to: set a sliding window on the acquired received signal and extract the maximum and minimum values ​​of the data within the sliding window, according to the formula (max... The amplitude of the received signal is calculated by (min) / 2. At the same time, the received signal and the reference signal are converted into two corresponding square wave signals after zero detection. Then, the two square wave signals are XORed and the duty cycle of the XOR signal is calculated to obtain the phase information of the received signal relative to the reference signal.

5. The imaging system according to claim 4, characterized in that, The signal processing module acquires the amplitudes of multiple received signals, calculates the average of the multiple amplitudes, and uses the average as the amplitude of the electromagnetic wave.

6. The imaging system according to claim 5, characterized in that, The receiving antenna unit includes multiple receiving antennas, and a control switch connected to both the receiving antennas and the FPGA chip is also provided.

7. The imaging system according to claim 6, characterized in that, The image reconstruction module uses the following imaging model: Where g is the complex data vector of electromagnetic waves. Let be the complex data vector of all electromagnetic wave signals received by the i-th receiving antenna. Each element in the vector represents electromagnetic wave data in complex form, and its vector dimension is the number of amplitudes encoded by the metasurface × 1. m is the number of receiving antennas in receiving antenna element 7; H is the observation matrix; f is the image of the target object to be imaged that needs to be inverted; n is the noise.

8. The imaging system according to claim 7, characterized in that, The image reconstruction module uses a compressed sensing algorithm to invert the image of the target object.

9. The imaging system according to any one of claims 6 to 8, characterized in that, The receiving antenna is a Vivaldi antenna.

10. The imaging system according to any one of claims 6 to 9, characterized in that, The control switch is a radio frequency switch.

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