Microwave transmission
1. Transmission of microwaves and their distribution in space
The transmission of microwave energy can no longer use the form and concept of ordinary wire circuit, but the concept of "electromagnetic field" in UHF should be used to explain. Microwave is an ultra-high frequency electromagnetic wave. Electromagnetic waves are transmitted in the form of alternating electric and magnetic fields, that is, transmitted by mutual conversion of electrical energy and magnetic energy. The following describes in detail how the microwave is transmitted.
1.1 Microwave transmission
Can microwaves be transmitted in the same way as our electromagnetic waves in low-frequency and high-frequency circuits? Practice has proved that it is completely ok. In electromagnetic wave transmission technology, different forms of conduction should be used for electromagnetic waves of different wavelengths. In the meter wave band, energy can be conducted with two wires. When the frequency is increased to the decimeter wave band, the two wires are no longer suitable because the electromagnetic field radiates into the space along the two wires, causing a large power loss. At this time, a coaxial line is usually used. In this way, the electromagnetic field is shielded between the inner and outer conductors. When the frequency continues to increase to the centimeter band, the current along the inner conductor surface of the coaxial line will cause a large power loss, and the heat loss of the medium supporting the inner conductor cannot be ignored. At the same time, as the wavelength decreases, the microwave power that the coaxial line allows to conduct decreases as the wavelength decreases. Therefore, in the centimeter band, it is generally not necessary to transmit a large power to the coaxial line. At this time, the coaxial line is replaced by a waveguide. The waveguide is a metal tube with a circular or rectangular cross section. Electromagnetic waves are transmitted in the waveguide. If the waveguide size and inner surface finish meet the quality requirements, the power loss is small, so the waveguide becomes the ideal for the high-power transmission of the centimeter band. The circuit is up. Waveguides are widely used in microwave heating and drying equipment. The microwave energy generated in the magnetron enters the microwave heater (microwave chamber) through the waveguide. The waveguide is made of a reflective microwave material (such as the one described above). Theoretically, the waveguide can be perfectly matched to directly transmit the microwave to the microwave cavity. Moreover, in a microwave device, sometimes the waveguide itself is a heater.
1.2 Field distribution and wave pattern of microwave
In order to visually describe the electromagnetic field of microwaves, the physics predecessors imagined a very visual and intuitive power line and magnetic lines of force. The form of the microwave electromagnetic field distribution is the structure form of the power line and the magnetic line. In the transmission and operation, the microwave can be divided into many determined waveforms according to this structure, also called "mode".
In the waveguide, the electric field is distributed on the cross section of the rectangular waveguide, and the longitudinal component of the electric field is zero. The corresponding waveform of this electric field is called a transverse electric wave and is referred to as a TE wave. Similarly, the corresponding waveform of the magnetic field with only the transverse component is called the transverse magnetic wave, which is recorded as the TM wave.
In addition, according to the number of large values ​​of zui on the broad and narrow sides of the waveguide, the microwave can be divided into many waveforms, which are recorded as TEmn waves or TMmn waves, and the angles m and n are 2. 3. . . . Equal to an integer.
Among the many waveforms transmitted by a rectangular waveguide, the zui simple, zui useful waveform is the TE10 wave. By plotting the TE10 wave field structure and the electric field intensity distribution curve, it can be seen that a large value of zui appears in the broad side electric field intensity of the waveguide, that is, m=1. On the narrow side of the waveguide, the electric field strength component is zero, that is, n=0, so it is called TE10 wave, and so on.
1.3 Distribution of high frequency current on the waveguide
The microwave is transmitted in the waveguide, and a high-frequency alternating high-frequency current is induced on the inner wall surface of the waveguide, and the magnitude and flow direction thereof change with the strength and direction of the magnetic field.
Take the TE10 wave as an example:
If we open a slot of appropriate width along the centerline of the wide side of the waveguide (a/2), or open a small slit at the narrow side of the waveguide, it will not affect the distribution of the microwave field, and the leakage of microwave power is also small. . In the microwave heating equipment, this distribution characteristic of the microwave field is often used to perform the following work: A. The wall surface current path is cut by the slot to cause the energy in the waveguide to radiate outward to achieve the coupling or excitation function of the microwave energy. B. The method mentioned in the common A causes the heated object to be processed through the slot. At this time, the slot opening is located at a strong position of the electric field zui, so that the processed article can obtain a larger heating power. C slits on narrow sides can be used for ventilation. In order to facilitate the processing of the slotted waveguide, the waveguide can also be made into two pieces and then combined. Suitable slot locations are based on electromagnetic wave radiation theory. In addition, this distribution characteristic suggests that we should pay attention to the rigor of interconnecting devices in the design of microwave systems, and form the necessary surface current path on the device to prevent and reduce microwave leakage.
1.4 Microwave transmission conditions
For a waveguide of a certain size, electromagnetic waves and arbitrary waveforms of arbitrary wavelengths can be propagated. The electromagnetic wave propagates in the waveguide to meet certain conditions, which can be expressed by the following inequality: λ < λc. That is, the transmitted microwave wavelength λ must be smaller than the critical wavelength λc of the waveguide, and waves larger than the critical wavelength cannot propagate within the waveguide. This is why the waveguide is only suitable for the microwave band and not for the high frequency or low frequency. Because at high frequencies, the waveguide size is so large that it loses its practical significance.
The critical wavelengths of TEmn and TMmn waves can be calculated by the following formula: λc<[(m/a)2+(n/b)2]?
Where a-----the width of the wide side of the waveguide
B---- Waveguide narrow side inner wall size
It can be seen from the above formula that the larger the value of the waveform index m and n, the shorter the critical wavelength. Among the many waveforms that may exist in the waveguide, the critical wavelength zoi of the TE10 wave is long, so we call the TE10 wave the zui low-frequency waveform, the corresponding mode is called the main mode or the fundamental mode, and the other waveforms are called high-order waveforms (or High-order mode).
The commonly used TE10 wave m=1, n=0, the critical wavelength is λc(TE10)λ=2a, and the critical wavelength λ≤a of the higher order mode;
Therefore, the condition for guaranteeing TE10 wave transmission in the waveguide is a<λ<2a. For certain microwave heating systems to be TE10 waves, the above inequality has a certain significance for selecting the waveguide cross-section size. For example, the inner wall of the waveguide has a wide side dimension of a=24.8 cm and a critical wavelength of 49.6 cm. This waveguide cannot transmit any wave having a wavelength greater than 49.6 cm. For a wave with a working frequency of 915 MHz (λ = 32.2 cm), the waveguide can realize the transmission of a single mode (ie, TE10 wave), and the high-order mode can be excited in the waveguide when the wavelength is less than 24.8 cm. It can be seen that the waveguide also has the function of "filter", which can only transmit microwaves smaller than the cutoff wavelength. With this feature, we can design microwave devices that are both "open" and safe.
When the microwave is transmitted in the waveguide, the wavelength will increase. We refer to the actual wavelength in the waveguide as the waveguide wavelength, and we have the following formula:
Λg=λ0/1-[λ0/λc]2
When working on TE10 waves
Λg=λ0/1-[λ0/2a]2
1.5 standing wave
In an ideal microwave transmission system, electromagnetic waves are transmitted in only one direction without causing reflections, and such waves are called traveling waves. However, in the actual transmission line, there are always factors such as waveguide bending, uneven processing size, poor connection, and incomplete load matching, which all cause reflected waves of the same frequency and opposite directions. The electromagnetic waves reflected and incident in the transmission line cancel each other out at certain positions due to the phase relationship, and a microwave wave pattern which is periodically distributed over an integer length and whose position is fixed is usually called a "standing wave".
In microwave transmission systems, the concept of standing wave ratio is often used to indicate the matching state of transmission.
φ=EMAX/Emin
Where φ standing wave ratio;
E electric field strength
The standing wave ratio is equal to the ratio of the small electric field strength of zui big and zui. In practice, this magnitude is reflected by the measured voltage standing wave ratio. φ is large, that is, the reflection is large, indicating that the microwave source and the load are poorly matched. In the case of high power, if the standing wave ratio is too large, sparking breakdown will occur at the strong electric field zui. When performing cold measurement of microwave equipment, it is required to have as small a standing wave ratio as possible to avoid excessive power and affect the normal operation of the microwave tube. The ideal matching state is φ=1, but it is actually unachievable. The general load φ?1.1 is considered to be a good match.
When the standing wave ratio is 1.5, the transmission power is reduced by 4%: when the standing wave ratio is 2, the transmission power is reduced by 11%; when the standing wave ratio is 3, the transmission power is reduced by 25%; when the standing wave ratio is 4, when the standing wave ratio is 4 The transmission power is reduced by 36%. This makes the microwave power supply work abnormally, the output power and frequency are unstable, and it is easy to generate a situation in which the fuse is burned out. Therefore, the standing wave ratio of the microwave heater is better between 1.1 and 3.
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