What Is a Waveguide?

Waveguide (WAVEGUIDE), a structure used to direct electromagnetic waves.

Generally, waveguides refer to hollow metal waveguides of various shapes and

In 1893 JJ Thomson first proposed the concept of a waveguide. In 1894 OJ Lodge first proved the waveguide experimentally. In 1897, Rod Rayleigh first completed the mathematical analysis of the propagation mode in a hollow metal cylindrical waveguide. (McLachan, 1947.)

The propagation of electromagnetic waves in a waveguide is limited and reflected by the inner wall of the waveguide. The conductivity of the waveguide wall is very high (usually made of copper, aluminum and other metals, and sometimes the inner wall is plated with silver or gold). Generally, the waveguide wall can be assumed to be an ideal conductor. The electromagnetic field distribution in the waveguide can be combined by the Maxwell equations and the waveguide boundary Conditions to solve. TEM waves cannot be transmitted in the waveguide, and there is a serious dispersion phenomenon in the propagation of electromagnetic waves in the waveguide. There may be an infinite variety of electromagnetic field structures or distributions in a waveguide. The distribution of each electromagnetic field is called a mode (mode), and each mode has a corresponding cut-off wavelength and a different phase velocity. Hollow waveguides with uniform cross sections are called homogeneous waveguides. The wave patterns of electromagnetic waves in homogeneous waveguides can be divided into two categories: electric waves (TE mode) and magnetic waves (TM mode). ^[2]

From an application perspective, there are four points to describe the characteristic parameters of the waveguide:

Dispersion characteristic

Dispersion characteristics represent the longitudinal propagation constant of the waveguide

With frequency

Relationship

Curve representation on the plane.

Characteristic impedance

Characteristic impedance Z and propagation constant

related

TE:

TM:

rectangle

There can be an infinite number of TMmn modes in a rectangular waveguide. The mode index m, n represents the maximum number of standing waves of the electromagnetic field along the broad side a and narrow side b of the waveguide, m, n = 1, 2, ... The simplest It is TM11 mode. Similarly, there can be an infinite number of TEmn modules, m, n = 0, 1, 2, ... but not zero at the same time. The lowest mode in a rectangular waveguide is the TE10 mode, and its longest cut-off wavelength is C = 2a. Therefore, it is possible to achieve single-mode transmission in the waveguide. The TE10 mode is also called the main mode in a rectangular waveguide and is the most important mode in a rectangular waveguide. In practice, rectangular waveguides work in TE10 mode.

circle

Circular waveguides can also have an infinite number of TMmn and TEmn modes, where m and n represent the number of changes in the field along the circumference and radial direction, respectively. There are only TM0n, TMmn (m, n = 1, 2, ...), TE0n and TEmn (m, n = 1, 2, ...) modes in a circular waveguide. The main wave with the longest cut-off wavelength in a circular waveguide is the TE11 mode, and its cut-off wavelength c = 3.41a (a is the waveguide radius). Commonly used modes are TM01 and TE01.