Microwave is one kind of electromagnetic wave which frequency range from 0.3GHz to 3000GHz and its wavelength between 0.1 mm to 1.0 m. It is an abbreviation to one limited wave band in radio waves and is a general designation to decimeter wave, centimeter wave, millimeter wave and sub-millimeter wave. The microwave frequency is higher than that of radio wave, so it is commonly called UHF electromagnetic wave, meanwhile, as one kind of electromagnetic wave, microwave also has wave-particle duality and its basic nature usually presents three characteristics which are penetration, reflection and absorption. As to the glass, plastic and porcelain, microwave can pass through them without being absorbed. For water and food, they can absorb microwave to make themselves heat, however, for metal stuff, they commonly reflect microwave.

Microwave Nature
Looking from the view of electronics and physics, microwave’s electromagnetic spectrum has some important features different from other wave bands. Its basic nature usually presents three characteristics which are penetration, reflection and absorption. As to the glass, plastic and porcelain, microwave can pass through them without being absorbed. As to water and food, they can absorb microwave to make themselves heat, however, for metal stuff, they commonly reflect microwave.

Penetrability
As the microwave has longer wavelength than other electromagnetic waves such as infrared ray and far infrared ray which are used for radiant heating, it has better penetrability. When microwave penetrate media, due to the interaction between microwave energy and media and with the frequency of 2450 MHz, it causes the molecules to shock 2.4 billion and fifty million times per second and the friction between molecules will further cause temperature increase and then simultaneously makes heating up both internal and external of media to form heat body state. This method greatly reduces heating time compared to conventional heating. And when it appears negative correlation between dielectric loss factor and medium temperature the heating will be uniform from inside to outside.

Selective Heating
The capacity of absorbing microwave for materials is mainly determined by dielectric loss factor. The matter with big dielectric loss factor has big microwave absorption capacity; on the contrary, the matter with small dielectric loss factor has small absorption capacity. Due to the differences of loss factor in each material, microwave heating often appears selective heating characteristics. Different matters have different heating effect. The water molecule belongs to polar molecule and it has large permittivity and dielectric loss factor, so it has strong absorption capacity to microwave. However, for protein and carbohydrates, as they relatively have small permittivity, they have less microwave absorption capacity than water molecules. So for food, how much water content they have will greatly affect microwave heating result.

Small Thermal Inertia
Microwave could instantaneously heat dielectric materials and increase the materials temperature rapidly, on the other hand, the output power of microwave can be adjusted and the medium temperature can be free changed without remaining afterheat. That greatly meets the need of automatic control and continuous production.

Inertia
Microwave’s wavelength is very short and its size is much smaller than general object in plant (such as aircraft, ships, cars, building etc.); or in the same order of magnitude, microwave characteristics are similar to geometrical optics —-namely the so-called light-like. When we use microwave to work, we could use small circuit elements and design compact system and make small and high-benefit antenna system with features of narrow beam and strong directivity. This system can accept weak signal reflected by various objects from the ground or space and confirm position and distance and then analysis target feature.
Both the microwave’ wavelength and some object size such as wireless equipment used in laboratory have same order of magnitude, so the characteristics of microwave are similar to sonic wave, namely so-called sound-like. For example, microwave waveguide is similar to megaphone in acoustics; both horn antenna and slot antenna are similar to horn and flute and microwave cavity is similar to acoustic resonance chamber.

Molecule Bond
Due to the high microwave frequencies, under condition of relatively band width, the frequency band can be used is very wide, it can reach up to hundreds or even thousands MHz. As for this point, low-frequency radio waves can not be compared. That means that microwave has big Information capacity, so modern multi-channel communication systems including satellite communications systems are all working in the microwave band. Further, the microwave signal also can provide phase information, polarization information, Doppler frequency information. This is very important in the some applications such as target detection and remote sensing target analysis.

Microwave Generation
Microwave commonly can be got through 50Hz AC or DC passing special device. The devices that can produce microwave are many kinds, which are mainly divided into two categories: semiconductor devices and electric vacuum devices. Electric vacuum devices are commonly called tube that can achieve energy conversion by means of electrons movement in vacuum.
In the all electron vacuum devices, the ones that can produce high-power microwave energy include magnetron, multi-cavity klystron, microwave three or four diodes and traveling wave tubes and so on. In the field of microwave heating, especially in industrial applications, we mainly use magnetron and klystron.