Is the vibration direction and wave propagation direction similar to the rhythm of the Humen Bridge?

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Is the vibration direction and wave propagation direction similar to the rhythm of the Humen Bridge? [Copy link]

First of all, what is a mode? A mode is the solution of Maxwell's equations without an excitation source.

T is the abbreviation of transverse, which means "transverse". In the model, it specifically refers to the "direction perpendicular to the transmission direction". For example, if the transmission direction of the electromagnetic wave in the waveguide is the z direction, then the transverse direction is the x, y direction in the rectangular coordinate system; or the \rho, \phi direction in the cylindrical coordinate system.

The TE mode means "all electric field components are perpendicular to the transmission direction", that is, "there is no electric field component in the transmission direction"; the same is true for the TM mode. The TEM mode means "both the electric field and the magnetic field components are perpendicular to the transmission direction".

TEM waves are transverse waves. HxE and k are perpendicular to each other. There is no component in other directions, but some have H waves or E waves in the wave propagation direction k, which produces the so-called TE waves or TM waves.

Electromagnetic waves that propagate along a certain path (such as a waveguide) are called guided electromagnetic waves. According to Maxwell's equations, guided electromagnetic waves generally have E and H components in the propagation direction.

Classification of light propagation forms: According to whether there is an electric field component or a magnetic field component in the propagation direction, it can be divided into the following three categories. Any light can be represented by the composite form of these three waves.

1. TEM wave : There is no electric field and magnetic field component in the propagation direction, which is called transverse electromagnetic wave. If the propagation direction of the laser in the resonant cavity is in the z direction, then the electric field and magnetic field of the laser will have no z-direction component! The actual laser mode is a quasi-TEM mode, that is, the existence of Ez and Hz components is allowed, but they must be << transverse components, because a larger Ez means that the wave vector direction deviates greatly from the optical axis and is easy to overflow the cavity, so the loss is large and it is difficult to form oscillation.

2. TE wave (i.e. s wave) : There is a magnetic field component but no electric field component in the propagation direction, which is called transverse electric wave. In a planar optical waveguide (closed cavity structure), the electromagnetic field components are Ey, Hx, Hz, and the propagation direction is the z direction.

3. TM wave (i.e. p wave) : There is an electric field component but no magnetic field component in the propagation direction, which is called transverse magnetic wave. In a planar optical waveguide (closed cavity structure), the electromagnetic field components are Hy, Ex, Ez, and the propagation direction is the z direction.

The three can be remembered like this: transverse electromagnetic waves are when both electricity and magnetism are horizontal, transverse electric waves have only the electric field horizontal, and transverse magnetic waves have only the magnetic field horizontal.

The so-called horizontal means that it is perpendicular to the direction vector k of electromagnetic wave propagation. We can imagine that a single cluster of light is a straight water pipe, and the light on the cross section of the water pipe is perpendicular to the direction of water flow. This is what the so-called horizontal means.

There are more detailed descriptions of TEM, TE, and TM modes in courses such as microwave engineering and electromagnetic field theory. Here, there is a question: Aren’t the electric field and magnetic field and the propagation direction orthogonal to each other? Then why are there TE/TM waves that are not perpendicular to each other?

The reason is that when propagating in the medium, especially after refraction, refraction occurs. The reason is that the medium, due to the existence of non-orthogonal components, may actually be the cause of dielectric loss!

The most basic situations of the vibration direction of the wave can be divided into two types:

Figure: The vibration direction is parallel to the propagation direction of the wave. This wave is called a longitudinal wave.

Figure: The vibration direction is perpendicular to the propagation direction of the wave. This wave is called a transverse wave.

Vibration has linear superposition, so the wave also satisfies linear superposition. Therefore, any wave with any other direction relationship can be obtained by linear superposition of these two waves.

The complexity of dynamic propagation depends first on the distribution of the propagation medium.

If the propagation medium is one-dimensional, then the propagation direction of the wave can only be on one line.

If the propagation medium is multidimensional, the wave will propagate in any direction from the wave source. The direction of wave propagation is called wave ray (abbreviated as wave line).

Since the propagation of waves in a medium is a time process, some temporal characteristics of the wave distribution in space must be considered.

First, the concept of phase. Since wave is a periodic phenomenon, the position of the oscillator in a cycle at a certain moment is a very important physical feature, and this position is called phase. Secondly, a geometric surface composed of points with the same phase in the spatial distribution of waves is called a wavefront (abbreviated as wavefront). The concept of wavefront is very critical when using wave images to analyze wave phenomena.

The shape of the wavefront is closely related to the distribution properties of the wave source and the medium. The two simplest cases are: in an isotropic medium, the wavefront of a wave emitted from a point source is a cluster of concentric spheres, called spherical waves; the wavefront of a wave emitted from a plane source is a cluster of parallel planes, called plane waves.

In waveguide theory, a waveguide that can transmit TEM mode must have a cross-sectional structure that can support the existence of a stable electrostatic field. Therefore, a single-conductor hollow metal waveguide cannot transmit TEM mode.

Waveguide classification: Usually, waveguide refers specifically to hollow metal waveguide tubes and surface wave waveguides of various shapes. The former completely confines the transmitted electromagnetic waves within the metal tube, also known as a closed waveguide ; the latter confines the guided electromagnetic waves around the waveguide structure, also known as an open waveguide .

The characteristic of surface wave waveguide is the existence of electromagnetic field outside the boundary. Its propagation mode is surface wave. In the millimeter wave and submillimeter wave bands, the loss is increased and the manufacturing is difficult because the size of the metal waveguide is too small. At this time, the use of surface wave waveguide, in addition to having good transmission properties, the main advantages are simple structure, easy manufacturing, and can have the planar structure required by integrated circuits. The main forms of surface wave waveguide are: dielectric line, dielectric mirror line, H-waveguide and mirror concave waveguide.

Field structures of different modes: There may be an infinite number of electromagnetic field structures or distributions in a waveguide. Each electromagnetic field distribution is called a wave type (mode). Each wave type has a corresponding cutoff wavelength and a different phase velocity. A hollow waveguide with a uniform cross section is called a uniform waveguide. The wave types of electromagnetic waves in a uniform waveguide can be divided into two categories: electric waves (TM mode) and magnetic waves (TE mode).

Rectangular waveguide: Rectangular waveguide is a hollow metal tube filled with air with a rectangular cross section. It is the most widely used microwave transmission line in practice. Standard rectangular waveguide data

Standard rectangular waveguide specification comparison table