The microwave is an electromagnetic wave having a wavelength of 1 m to 1 mm, which has both electromagnetic wave properties and ordinary radio waves and light waves. The microwave has the following characteristics with respect to the longer wavelength electromagnetic wave: 1 the device for space radiation is easy to manufacture; 2 the obstacles are easy to reflect; 3 the diffraction ability is poor; 4 the transmission characteristics are good, and the smoke and dust are transmitted during the transmission. The influence of strong light and the like is small; 5 the absorption of microwave by the medium is proportional to the dielectric constant of the medium, and the absorption of microwave by water is the strongest.
Microwave oscillators and microwave antennas are an important part of microwave sensors. A microwave oscillator is a device that generates microwaves. Since the microwave is very short and the frequency is very high (300 MHz to 300 GHz), the oscillation circuit is required to have a very small inductance and capacitance, so the microwave oscillator cannot be constituted by an ordinary transistor. The devices that make up the microwave oscillator are klystrons, magnetrons or some solid components. Small-body microwave oscillators can also be used with body effect tubes.
The oscillating signal generated by the microwave oscillator needs to be transmitted by a waveguide (a coaxial line having a wavelength of 10 cm or more) and transmitted through the antenna. In order to have uniform directionality of the emitted microwaves, the antenna should have a special configuration and shape. Commonly used antennas include a horn antenna and a parabolic antenna.
The microwave emitted by the transmitting antenna will be absorbed or reflected when it hits the measured object, causing the power to change. If the receiving antenna is used to receive the microwave reflected by the object or reflected by the object to be measured, and converted into an electrical signal, and then processed by the measuring circuit, microwave detection is realized. According to this principle, microwave sensors can be classified into two types: reflective and occlusion.
1, reflective sensorSuch a sensor expresses parameters such as the position and thickness of the object to be measured by detecting the microwave power reflected from the object or passing through the interval.
2, occlusion sensorThe sensor determines the presence or absence of the position of the object to be tested or the object to be tested between the transmitting antenna and the receiving antenna by detecting the magnitude of the microwave power received by the receiving antenna. Figure 11-5 shows a schematic diagram of the microwave liquid level, which forms a certain angle between the transmitting antenna and the receiving antenna with a distance of s. The microwave of wavelength λ is reflected from the measured liquid surface and enters the receiving antenna. The power received by the receiving antenna will vary with the level of the liquid level being measured. The power received by the receiving antenna Pr can be expressed as
When the transmission power, the wavelength, and the gain are both constant, the height d of the measured liquid level can be obtained as long as the received power Pr is measured.
Figure 11-6 is a schematic diagram of a microwave switch type level gauge. When the measured object level is low, the microwave beam emitted by the transmitting antenna is all received by the receiving antenna, and the normal working signal is sent after the amplifier and the comparator. When the measured level rises to the height of the antenna, the microwave beam is partially absorbed, partially reflected, and the power received by the receiving antenna is correspondingly weakened. The amplifier and the comparator can give the measured level higher than the setting. The signal of the level.
When the measured level is lower than the set level, the power received by the receiving antenna is
When the measured level rises to the height of the antenna, the power received by the receiving antenna is
Where η is the coefficient determined by the shape, material properties, electromagnetic properties and height of the measured object.
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