What Is a Waveguide?
A waveguide is a tube that passes electromagnetic waves in the vicinity of microwaves or millimeter waves.
In a broad sense, the term can be used for both sound waves and visible light and thus includes optical fibers, etc. However, it is generally used as the name of a tube for passing electromagnetic waves with wavelengths in the vicinity of microwaves and millimeter waves. It is a hollow tube structure with a square or circular cross section.
Microwaves are used in microwave ovens, etc., and are electromagnetic waves that are highly effective in warming objects. Because of their long wavelength, however, optical fibers cannot be used, and special microwave waveguides must be used to transmit the waves from the microwave source to the processing system.
Uses of Waveguides
Uses of waveguides are often for high-power microwave generators, radar antennas, and transmission of signals in the millimeter wave frequency band. For example, in the case of microwave ovens, a low-power microwave generator, such as a household microwave oven, has a magnetron installed inside the housing and can directly irradiate the microwaves.
However, at high power, the magnetron itself is destroyed by the microwaves, so the irradiation process must be carried out in a separate location. High-power microwaves can be supplied by connecting a waveguide to the processing chamber where the heating or other processing takes place.
Waveguides are also used in radar, where they serve as slot antennas for radio wave irradiation devices. As well, coaxial cables can be used for the evaluation of RF and other communication transmission, but in many cases, waveguides are used for sub THz applications in the millimeter wave band.
Principle of Waveguides
The principle of waveguide is based on the physical property that electromagnetic waves of the target frequency propagate inside the tube while forming propagation modes according to the shape of the tube. In the hollow-type structure often used in waveguides, there is no built-in conductor, so there is little internal loss, and it is possible to propagate electromagnetic waves of relatively high power.
As an example, electromagnetic waves with a frequency of 2.45 GHz often used in microwave ovens are also microwaves, and the wavelength of the electromagnetic waves in this microwave oven is 12.2 cm. To pass microwaves in a microwave oven, the diameter of the waveguide must be about the wavelength of 12.2 cm, and the longer the wavelength of the microwaves, the larger the diameter of the waveguide.
Thus, the design of the waveguide is based on the wavelength of the electromagnetic wave. Waveguides are generally rectangular in cross section, but there are also circular waveguides and ridge-shaped waveguides used for special purposes.
Other Information on Waveguides
1. Comparison With Coaxial Cable
Waveguides have often been used in wireless communication systems such as radar and horn antennas in air traffic control towers, but recently they are increasingly being used to develop millimeter wave applications in next-generation wireless communication systems such as millimeter wave radar in automobiles and 5G/beyond 5G. In the past, coaxial cables were used in the microwave band.
Compared to conventional coaxial cables in the microwave band, the following points can be cited as advantages:
- Transmission is possible with low loss, and the power resistance is excellent.
- Metal structure, stable in shape, easy to match, and less reflective.
However, in addition to advantages, the following disadvantages also exist:
- Narrow frequency bandwidth.
- Heavy and expensive.
When the frequency is microwaved, coaxial cables are used more frequently, partly because of the ease of handling cables and partly because the frequency bandwidth is lower than that of a millimeter wave. However, in the millimeter wave band, especially in the high frequency range, including D-band from 110 GHz, coaxial cables, except for some products, are inadequate in terms of characteristics, so waveguides are generally used in many cases.
2. Size of Waveguide Specifications
In the case of waveguides, it is necessary to select a waveguide size that is compatible with various application frequencies. In particular, in the case of a millimeter wave, it may not be possible to use a waveguide with a different flange standard. For your information, “WR-xxx” for waveguide size means “square waveguide,” and the number xxx indicates the inner width of the waveguide in one-hundredths of an inch.