The basic properties of microwave are usually characterized by penetration, reflection and absorption. For glass, plastic and porcelain, microwave almost passes through without being absorbed. For water and food, it will absorb microwave and make itself hot. For metal objects, microwave will be reflected.
From the point of view of electronics and physics, the electromagnetic spectrum of microwave has the following important characteristics different from other wavebands:
Penetrability
Microwave has longer wavelength than other electromagnetic waves used for radiation heating, such as infrared, far-infrared, etc., so it has better penetrability. When the microwave penetrates into the medium, due to the interaction between the microwave energy and the medium, the molecules of the medium will vibrate 2450 million times per second at the microwave frequency of 2450 MHz, and the molecules of the medium will rub against each other, which will cause the rise of the medium temperature, so that the internal and external parts of the medium will be heated almost simultaneously, forming a body heat source state, The heat conduction time in conventional heating is greatly shortened, and the heating inside and outside of the material is uniform when the dielectric loss factor is negatively correlated with the dielectric temperature.
Selective heating
The ability of material to absorb microwave is mainly determined by its dielectric loss factor. The material with a large dielectric loss factor has a strong ability to absorb microwave, on the contrary, the material with a small dielectric loss factor has a weak ability to absorb microwave. Due to the difference in the loss factors of various substances, microwave heating It shows the characteristics of selective heating. Different substances produce different thermal effects. Water molecule is a polar molecule with a large dielectric constant and a large dielectric loss factor. It has strong microwave absorption ability. The dielectric constant of protein, carbohydrate, etc. is relatively small, and their microwave absorption capacity is much smaller than that of water. Therefore, for food, the amount of water content has a great impact on the microwave heating effect.
Low thermal inertia
Microwave is an instantaneous heating for dielectric materials, with fast heating speed. On the other hand, the output power of microwave can be adjusted at any time, and the temperature rise of medium can be changed without inertia. There is no "waste heat" phenomenon, which is very conducive to the need of automatic control and continuous production.
Photolikeness and sonolikeness
Microwave wavelength is very short, which is much smaller than the size of general objects on the earth (such as aircraft, ships, automobile buildings, etc.), or on the same order of magnitude. The characteristics of microwave are similar to those of geometrical optics, namely the so-called photolike property. Therefore, using microwave to work can reduce the size of circuit components; Make the system more compact; It can be made into an antenna system with small size, narrow beam, strong directionality and high gain to receive weak signals reflected from various objects on the ground or in space, so as to determine the orientation and distance of objects and analyze target characteristics.
Because the microwave wavelength and the size of the object (wireless equipment in the laboratory) have the same order of magnitude, the characteristics of the microwave are similar to those of the sound wave, which is the so-called acoustic similarity. For example, microwave waveguide is similar to microphone in acoustics; Horn antenna and slot antenna are similar to acoustic horn, whistle and flute; Microwave resonators are similar to acoustic resonators.
Nonionization
The quantum energy of microwave is not large enough to change the internal structure of material molecules or destroy the bonds between molecules (except for some substances: for example, microwave can regenerate waste rubber, that is, change the molecular bonds of waste rubber through microwave). In addition, the way of physics, many resonance phenomena of molecular atomic nuclei under the periodic force of external electromagnetic field occur in the microwave range, so microwave provides an effective research means for exploring the internal structure and basic properties of matter. On the other hand, many microwave devices can be made by using this feature.
Informational
Because the microwave frequency is very high, its available frequency band is very wide, up to hundreds or even gigahertz under a small relative bandwidth. This is incomparable to low-frequency radio waves. This means that microwave information capacity is large, so modern multi-channel communication systems, including satellite communication systems, almost without exception, work in the microwave band. In addition, microwave signal can also provide phase information, polarization information and Doppler frequency information. This is very important in target detection, remote sensing target feature analysis and other applications.
