Microwave six-port measuring device for surface acoustic wave sensor

Abstract

The invention relates to a microwave six-port measuring device for a surface acoustic wave sensor, and belongs to the technical field of sensor measurement. The system comprises an antenna, a radio frequency transmitting link, a radio frequency receiving link, a radio frequency switch module, a man-machine interaction module and a main control module. The antenna is connected with the radio frequency switch module and is used for transmitting and receiving signals. The radio frequency emission link generates a radio frequency signal, and the radio frequency signal is transmitted to the antenna through the radio frequency switch module to be emitted outwards. The radio frequency receiving link collects echo signals and radio frequency signals of the transmitting link through a radio frequency switch module, and interference frequency mixing is carried out. And the main control module receives the signal processed by the radio frequency receiving link and analyzes the signal to obtain a measurement result of the surface acoustic wave sensor. And the man-machine interaction module receives and displays the measurement result transmitted by the main control module, and provides parameters for setting the surface acoustic wave sensor to calibrate the surface acoustic wave sensor. Passive wireless surface acoustic wave sensor measurement is achieved through microwave six ports, precision is high, and the device is simple in structure.

Description

Technical Field

[0001] This invention belongs to the field of sensor measurement technology and relates to a microwave six-port measurement device for surface acoustic wave sensors. Background Technology

[0002] Surface acoustic wave (SAW) sensors, as highly sensitive micro-acoustic devices, reflect changes in external physical parameters through shifts in their resonant frequencies and have been widely used in wireless detection of various physical quantities such as temperature, pressure, strain, and humidity. These sensors possess significant advantages, including a wide frequency response range, high sensitivity, and stable operation in harsh environments, playing a crucial role in industrial process monitoring, structural health monitoring, aerospace sensing systems, and IoT sensing nodes. With the development of wireless sensor networks and embedded detection systems, higher demands are placed on the miniaturization, low power consumption, integration, and real-time performance of sensor detection devices, necessitating the optimization and upgrading of traditional detection methods.

[0003] Currently, the mainstream detection methods for surface acoustic wave (SAW) sensors mainly rely on measurement systems based on vector network analyzers or superheterodyne receiver architectures. While vector network analyzers offer high measurement accuracy and comprehensive functionality, they suffer from drawbacks such as large size, high cost, and high power consumption, making them difficult to widely apply in field environments requiring portability or long-term deployment. This is particularly true in multi-point distributed sensing and embedded integration scenarios, where such instruments are severely limited by their inability to achieve system miniaturization and energy efficiency optimization. Although superheterodyne receiver solutions achieve some degree of device miniaturization, their system architecture still includes multiple complex modules such as mixers, local oscillator signal sources, intermediate frequency filtering, and demodulation circuits. This not only increases system complexity and design difficulty but also introduces additional noise and signal distortion factors, affecting overall measurement accuracy and stability. Furthermore, these receivers typically have extremely high requirements for component consistency and temperature stability, further increasing manufacturing costs and debugging difficulty.

[0004] On the other hand, in dynamic measurement tasks that require real-time extraction of sensor resonant frequency changes, existing methods mostly rely on frequency estimation algorithms based on Fast Fourier Transform. Although such algorithms are versatile in frequency domain analysis, they are limited by frequency resolution and computational complexity, making it difficult to achieve high-precision, low-latency frequency tracking on resource-constrained embedded platforms. This is especially true in practical applications where the sensor echo signal has a low signal-to-noise ratio or multipath interference, where their dynamic response speed and measurement real-time performance often fail to meet the requirements of high-speed, high-precision sensing.

[0005] Furthermore, traditional detection systems also suffer from shortcomings in integration and multi-functional coordination. For example, most existing solutions fail to effectively integrate signal transmission, reception, frequency demodulation, and data communication functions into a single system, resulting in numerous modules, complex interconnections, and uneven power consumption distribution, thus limiting their adoption in low-power wireless sensing nodes. Particularly in terms of adaptability to multiple sensor types and frequencies, existing instruments often lack flexible and configurable hardware architectures, making it difficult to achieve rapid adaptation and accurate measurement of different surface acoustic wave sensors across a wide frequency range.

[0006] In summary, existing surface acoustic wave (SAW) sensor detection technologies still face challenges such as complex system structures, high costs, difficulty in achieving high integration and low power consumption designs, and insufficient real-time frequency demodulation capabilities. Therefore, there is an urgent need to develop a novel detection architecture that can significantly simplify the system structure, reduce manufacturing costs, enhance integration and practicality, and improve the ability to process and communicate sensor signals in real time, while ensuring measurement accuracy. This will meet the widespread demand of modern wireless sensor networks for high-performance, miniaturized sensing terminals. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a microwave six-port measurement device for surface acoustic wave (SAW) sensors, so as to solve the problems that SAW sensor detection relies on time-domain signal-frequency-domain signal conversion to extract frequency information, resulting in complex systems, high costs, and low integration.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A microwave six-port measurement device for surface acoustic wave sensors includes an antenna, an RF transmission link, an RF reception link, an RF switch module, a human-machine interface module, and a main control module.

[0009] The antenna is connected to the radio frequency switch module and is used to transmit radio frequency signals and receive echo signals fed back by the surface acoustic wave sensor. The antenna can be an omnidirectional multi-frequency antenna. The radio frequency transmission link generates radio frequency signals under the control of the main control module and transmits them to the antenna for outward transmission through the radio frequency switch module. The radio frequency receiving link acquires the echo signal received by the antenna and the radio frequency signal generated by the radio frequency transmitting link through the radio frequency switch module, and performs interference mixing of different phases on the echo signal and the radio frequency signal. The main control module receives the signal processed by the radio frequency receiving link, calculates the instantaneous frequency of the echo signal, and then obtains the measurement result of the surface acoustic wave sensor based on the instantaneous frequency of the echo signal. The human-machine interface module receives and displays the measurement results transmitted by the main control module, and also provides parameters for setting the surface acoustic wave sensor and calibrating the surface acoustic wave sensor.

[0010] Furthermore, the radio frequency transmission link includes a radio frequency signal source, a filtering module, and a radio frequency power amplifier module connected in sequence; wherein, the radio frequency signal source generates radio frequency signals according to the control of the control module, and the radio frequency power amplifier module transmits the radio frequency signals to the antenna through the radio frequency switch module.

[0011] Furthermore, the RF signal source outputs different RF signals depending on the operating mode; when measuring a surface acoustic wave sensor, the frequency of the RF signal source output signal is adapted to the surface acoustic wave sensor being measured; when it is necessary to broadcast the measured data, the frequency of the RF signal source output signal is 433 MHz.

[0012] Furthermore, the RF receiving link includes a filtering module, a low-noise amplifier module, an attenuation module, a six-port network module, and a power detector module; one end of the filtering module is connected to the RF switch module, and the other end is connected to the low-noise amplifier module; the low-noise amplifier module is connected to the first port of the six-port network module; the attenuation module is connected to the second port of the six-port network module; the input end of the power detector module is connected to the third to sixth ports of the six-port network module, and the output end is connected to the main control module; wherein, the power detector module converts the output signals of the third to sixth ports of the six-port network module into power values.

[0013] Furthermore, the six-port network module includes an equal-ratio power divider and a first 90° bridge to a third 90° bridge; one end of the equal-ratio power divider serves as the first port of the six-port network module, and the other end of the equal-ratio power divider is connected to the input terminals of the first 90° bridge and the second 90° bridge, respectively. The output terminals of the first 90° bridge serve as the third and fourth ports of the six-port network module, and the output terminals of the second 90° bridge serve as the fifth and sixth ports of the six-port network module. The input terminal of the third 90° bridge serves as the second port of the six-port network module, and the output terminal of the third 90° bridge is connected to the input terminals of the first and second 90° bridges, respectively.

[0014] Furthermore, the control module converts the output power value based on the power detector module. , , , To calculate the instantaneous power of the echo signal from the surface acoustic wave sensor, firstly, based on... , , and structure Quantity:

[0015]

[0016] In the formula, This represents the portion of the surface acoustic wave sensor's echo signal that is in phase with the reference carrier. This represents the portion of the surface acoustic wave sensor echo signal that is orthogonal (90° phase difference) to the reference carrier. Together, they form a complex baseband signal, carrying all amplitude and phase information; thus, the instantaneous phase is obtained. :

[0017] The instantaneous frequency of the sensor echo signal is:

[0018] In the formula, For frequency difference, For phase difference, This refers to the frequency of the output signal from the radio frequency transmission link.

[0019] Furthermore, the main control module includes a power management unit, a microcontroller, and a memory; the power management unit is connected to each module and provides power; the microcontroller is used to execute the program stored in the memory to complete the surface acoustic wave sensor measurement and broadcast the measured data; the memory stores the program and surface acoustic wave sensor calibration data.

[0020] The beneficial effects of this invention are as follows: (1) This invention uses a passive six-port network as the core and uses the power detection principle to replace the complex mixer, local oscillator and intermediate frequency demodulation circuit in the traditional superheterodyne receiver. This effectively reduces the application of active devices, reduces the overall complexity and hardware cost of the system, and avoids the noise and error introduced by multi-stage frequency conversion and complex analog circuits, thereby improving the reliability and stability of the system.

[0021] (2) The vector demodulation mechanism based on the six-port network can directly construct the I / Q quadrature components through the four power detection values ​​and quickly calculate the instantaneous frequency of the echo signal. It overcomes the computational delay and resolution limitations caused by the traditional reliance on fast Fourier transform for frequency domain analysis, and realizes the direct and rapid extraction of sensor resonant frequency changes. It is particularly suitable for monitoring rapidly changing physical quantities and can enhance the real-time response and measurement capabilities of the measuring device in dynamic environments.

[0022] (3) This invention highly integrates functional modules such as radio frequency transmission, signal reception, six-port demodulation, main control processing, and wireless communication into one unit. Through the main control module and a controllable radio frequency switch, it realizes flexible switching between excitation signals and measurement channels, and can adapt to surface acoustic wave sensors with different frequency band characteristics. In addition, through embedded parameter setting, sensor calibration, and data broadcasting functions, this invention makes the measuring device a fully functional, independently operating wireless sensing node, which greatly expands its practical value and application prospects in distributed sensor networks and embedded monitoring scenarios.

[0023] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is a schematic diagram of the overall structure of a microwave six-port measurement device for a surface acoustic wave sensor provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of a microwave six-port network structure; Figure 3 A schematic diagram illustrating the working principle of microwave six-port network signal phase conversion; Figure 4 This is a flowchart illustrating the working logic of a microwave six-port measurement device. Detailed Implementation

[0025] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0026] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0027] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0028] Please see Figure 1 One embodiment of the present invention provides a microwave six-port measurement device for a surface acoustic wave sensor. The device includes an antenna, an RF transmission link, an RF reception link, an RF switch module, a human-machine interaction module, and a main control module.

[0029] The radio frequency (RF) switch module includes a first RF switch module and a second RF switch module. Both the first and second RF switch modules are three-port non-reflective RF switches, controlled by the main control unit, used to switch the signal transmission direction. The first RF switch module is connected to both the second RF switch module and the RF receiving link; the second RF switch module is connected to both the RF transmitting link and the RF receiving link.

[0030] The antenna is an omnidirectional multi-frequency antenna, and it is connected to the first radio frequency switch module.

[0031] The aforementioned radio frequency (RF) transmission link includes an RF signal source, a filtering module, and an RF power amplifier module, controlled by a main control unit to generate a sufficiently powerful RF excitation signal. The RF signal source generates the RF signal under the control of the main control module; the output of the RF signal source is connected to the input of the filtering module; the output of the filtering module is connected to the input of the RF power amplifier module. The RF power amplifier module outputs the RF excitation signal to a second RF switching module.

[0032] The RF receiving link includes: a low-noise amplifier module, a filter module, a six-port network module, a power detection module, and an attenuation module. The input of the filter module is connected to the first RF switch module, and its output is connected to the input of the low-noise amplifier module. The output of the low-noise amplifier module is connected to the first port of the six-port network module. The input of the attenuation module is connected to the second RF switch module, and its output is connected to the second port of the six-port network module. The third, fourth, fifth, and sixth ports of the six-port network module are connected to the first, second, third, and fourth power detector inputs of the power detector module, respectively. The outputs of the first, second, third, and fourth power detectors of the power detector module are connected to the analog-to-digital converter (ADC) pins of the main control module. The four power detectors convert power into four output power values ​​from the six-port network module, denoted as follows: , , , Thus constructed Components obtained Then, the instantaneous frequency of the surface wave sensor echo signal is further determined.

[0033] The human-computer interaction module includes a touch-enabled TFT-LCD, which is connected to the main control module and is used to set the parameters of the surface acoustic wave sensor, calibrate the surface acoustic wave sensor, and output measurement results.

[0034] The main control module includes a power management unit, a microcontroller, and a memory. The power management unit is connected to the switching module, power amplifier module, attenuation module, and RF signal source for power supply. The microcontroller executes the pre-stored program in the memory to complete sensor data measurements and broadcast the measured data. The memory stores the program and calibration data of the surface acoustic wave sensor. The broadcast measurement data can be modulated onto the RF signal using binary frequency shift keying (2FSK), and the modulated signal includes a preamble, data packets, and a checksum.

[0035] When the RF signal source outputs an RF signal, the frequency of the RF signal is variable depending on the operating mode. Specifically, when measuring data from a surface acoustic wave (SAW) sensor, the frequency of the output RF signal is adapted to the SAW sensor being measured. When broadcasting measurement data, the output signal frequency is 433 MHz, which can be generated by a phase-locked loop (PLL) or a voltage-controlled oscillator (VOC).

[0036] The filtering module is a bandpass filter, whose bandpass frequency band matches and is replaceable with the bandpass frequency band of the surface acoustic wave sensor being measured.

[0037] The frequency band of the radio frequency power amplifier module includes the frequency band of the surface acoustic wave sensor being measured.

[0038] The frequency band of the low-noise amplification module includes the frequency band of the measured surface acoustic wave sensor.

[0039] like Figure 2 As shown, the six-port network module consists of an equal-ratio power divider and three 90° bridges. The first port serves as the input terminal, receiving external input signals and distributing them to the output ports. The output ports of the power divider are connected to the input terminals of the first and second 90° bridges. These two 90° bridges decompose the input signal into two signals with a 90° phase difference, resulting in signals with a fixed phase relationship output at the third, fourth, fifth, and sixth ports. One port of the third 90° bridge serves as the second port of the six-port network module for the local oscillator signal input, while the other port is grounded for impedance matching and a reference.

[0040] pass Figure 2 The six-port network module shown performs interferometric mixing of the input echo signal and the signal output from the RF transmit link with different phases, with the relative phase set to 0. , , It can realize the multiplexing and phase difference transformation of input signals to obtain multiple mutually orthogonal or correlated output signals, and is suitable for application scenarios such as multi-port detection and vector signal measurement.

[0041] like Figure 3 The diagram illustrates the working principle of signal phase transformation in a six-port network module. It can be seen that after the first and second ports receive the input signal, the signal undergoes a step-by-step phase shift through multiple phase shift units. The phase shift angle of each phase shift unit is... The specific process of phase transformation is as follows: The signal input at the first port is passed through a... After the phase shift unit, the signal is added to the signal from the second port and output to the third port; The signal input at the first port passes through two... After being processed sequentially by the phase shift unit, the signal is added to the signal from the second port and output to the sixth port; The signal input at the first port passes through three... After being processed sequentially by the phase shift unit, the signal is added to the signal from the second port and output to the fourth port; The signal input at the first port is added directly to the signal input at the second port without phase shifting, and then output to the fifth port.

[0042] Through the step-by-step phase shift unit Phase shifting can generate multiple signal outputs with phase differences and synthesize them with the reference input signal, thereby realizing multi-port signal phase transformation.

[0043] like Figure 4 As shown, another embodiment of the present invention provides a detection method for a microwave six-port measurement device for a surface acoustic wave sensor, the process of which includes the following steps: 1. Based on the device characteristics of the surface acoustic wave sensor, the surface acoustic wave sensor is calibrated through a human-machine interface to obtain the relationship between the echo signal frequency of the surface acoustic wave sensor and the physical characteristics it represents (such as the relationship between the echo signal frequency and temperature, the relationship between the echo signal frequency and strain, which can be linear or nonlinear). The result is stored in the memory of the main control module through a lookup table.

[0044] 2. Set parameters through the human-computer interaction interface so that the subsequent detection process can be carried out according to the predetermined conditions.

[0045] Specifically, the parameters of the surface acoustic wave sensor are set to adjust the frequency of the radio frequency excitation signal output by the radio frequency signal source, so as to ensure that the surface acoustic wave sensor can be excited.

[0046] 3. According to the set parameters, the microcontroller controls the RF switch module to connect the RF transmission link to the antenna, and controls the RF transmission link to output an RF excitation signal with a time of t1, which is then radiated as an electromagnetic wave through the antenna.

[0047] 4. After time t1, the microcontroller controls the RF switch module to connect the RF transmit link and the RF receive link, and controls the RF receive link to connect the antenna. This allows the RF excitation signal and the surface acoustic wave sensor echo signal to simultaneously enter the six-port network module.

[0048] When receiving the echo signal from the surface acoustic wave sensor, it is necessary to determine whether a valid echo signal has been received: if no valid echo signal is received, the excitation and reception operations are resumed until a valid echo signal is detected; if a valid echo signal is received, the subsequent processing steps are initiated.

[0049] 5. The microcontroller acquires the data output by the power detection module, calculates the instantaneous frequency of the surface acoustic wave sensor echo signal, and further obtains the physical quantity value characterized by the surface acoustic wave sensor based on the instantaneous frequency of the echo signal.

[0050] The calculation process is as follows: Power values ​​obtained from conversion by four power detectors , , , ,structure Quantity:

[0051]

[0052] Obtain the instantaneous phase :

[0053] The instantaneous frequency of the sensor echo signal is:

[0054] in, For frequency difference, For phase difference, The frequency of the output signal from the radio frequency signal source.

[0055] 6. The microcontroller determines whether the measurement data needs to be broadcast: If broadcasting is not required, the measured value is directly transmitted to the human-machine interface and the measurement result is displayed on the interface; if broadcasting is required, the measured value is first processed by 2FSK modulation to obtain the modulated wireless data signal. Then, the microcontroller controls the RF switch module to connect with the RF transmission link and antenna to broadcast the data wirelessly, while displaying the measurement data on the human-machine interface.

[0056] 7. Complete the detection process and output the measurement results of the physical quantities of the monitored object.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A microwave six-port measurement device for surface acoustic wave sensors, characterized in that, This includes an antenna, an RF transmission link, an RF reception link, an RF switch module, a human-machine interface module, and a main control module; The antenna is connected to the radio frequency switch module to transmit radio frequency signals and receive echo signals fed back by the surface acoustic wave sensor. The radio frequency transmission link generates radio frequency signals under the control of the main control module and transmits them to the antenna for outward transmission through the radio frequency switch module. The radio frequency receiving link acquires the echo signal received by the antenna and the radio frequency signal generated by the radio frequency transmitting link through the radio frequency switch module, and performs interference mixing of different phases on the echo signal and the radio frequency signal. The main control module receives the signal processed by the radio frequency receiving link, calculates the instantaneous frequency of the echo signal, and then obtains the measurement result of the surface acoustic wave sensor based on the instantaneous frequency of the echo signal. The human-machine interface module receives and displays the measurement results transmitted by the main control module, and also provides parameters for setting the surface acoustic wave sensor and calibrating the surface acoustic wave sensor.

2. The microwave six-port measuring device according to claim 1, characterized in that, The radio frequency (RF) transmission link includes an RF signal source, a filtering module, and an RF power amplifier module connected in sequence. The RF signal source generates RF signals under the control of the control module, and the RF power amplifier module transmits the RF signals to the antenna through an RF switch module.

3. The microwave six-port measuring device according to claim 2, characterized in that, The radio frequency (RF) signal source outputs different RF signals depending on the operating mode; when measuring a surface acoustic wave (SAW) sensor, the frequency of the RF signal source output signal is adapted to the SAW sensor being measured; when it is necessary to broadcast the measured data, the frequency of the RF signal source output signal is 433MHz.

4. The microwave six-port measuring device according to claim 1, characterized in that, The RF receiving link includes a filtering module, a low-noise amplifier module, an attenuation module, a six-port network module, and a power detector module. One end of the filtering module is connected to the RF switch module, and the other end is connected to the low-noise amplifier module. The low-noise amplifier module is connected to the first port of the six-port network module. The attenuation module is connected to the second port of the six-port network module. The input end of the power detector module is connected to the third to sixth ports of the six-port network module, and the output end is connected to the main control module. The power detector module converts the output signals from the third to sixth ports of the six-port network module into power values.

5. The microwave six-port measuring device according to claim 4, characterized in that, The six-port network module includes an equal-ratio power divider and a first 90° bridge to a third 90° bridge. One end of the equal-ratio power divider serves as the first port of the six-port network module, and the other end of the equal-ratio power divider is connected to the input terminals of the first 90° bridge and the second 90° bridge, respectively. The output terminals of the first 90° bridge serve as the third and fourth ports of the six-port network module, and the output terminals of the second 90° bridge serve as the fifth and sixth ports of the six-port network module. The input terminal of the third 90° bridge serves as the second port of the six-port network module, and the output terminal of the third 90° bridge is connected to the input terminals of the first and second 90° bridges, respectively.

6. The microwave six-port measuring device according to claim 5, characterized in that, The control module converts the output power value based on the power detector module. , , , To calculate the instantaneous power of the echo signal from the surface acoustic wave sensor, firstly, based on... , , and structure Quantity: In the formula, This represents the portion of the surface acoustic wave sensor's echo signal that is in phase with the reference carrier. This represents the portion of the surface acoustic wave sensor's echo signal orthogonal to the reference carrier; together they form a complex baseband signal, carrying all amplitude and phase information; thus, the instantaneous phase is obtained. : The instantaneous frequency of the sensor echo signal is: In the formula, For frequency difference, For phase difference, This refers to the frequency of the output signal from the radio frequency transmission link.

7. The microwave six-port measuring device according to claim 1, characterized in that, The main control module includes a power management unit, a microcontroller, and a memory; the power management unit is connected to each module and provides power; the microcontroller is used to execute the program stored in the memory, complete the surface acoustic wave sensor measurement, and broadcast the measured data; the memory stores the program and the surface acoustic wave sensor calibration data.

8. The microwave six-port measuring device according to claim 1, characterized in that, The antenna is an omnidirectional multi-frequency antenna.

Images