Waveguide

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A waveguide (English: wave guide ) is an inhomogeneous medium , the one by its physical nature shaft so bundles that it therein as a traveling wave is guided.

Example of a waveguide (waveguide)

Waveguides for electromagnetic waves

Cable shape

A waveguide can consist ( symmetrically ) of a ribbon line made of two strands or rigid wires, sometimes also in a common shield, or asymmetrically from a coaxial cable . These cables are suitable for the transmission of high frequencies in the VHF ( ultra-short wave ) and UHF ( decimeter wave ) range. Conductor cross-section and insulating material influence the attenuation per length. As a rule, the thinner the cable, the higher the attenuation. Kinks and bruises, which change the cable geometry, cause reflections and / or radiation.

Application: antenna cable for radio transmitters and receivers, transmission of high-frequency power up to the VHF range; Usually up to 1 GHz, for some applications up to 10 GHz depending on the application. Waveguides are used for even higher frequencies .

Single-wire waveguide

An electromagnetic wave can propagate even on a single wire. However, no damping or conductive materials may be located in an area of ​​approx. 1/2 wavelength around the cable. The line must also not have any kinks. This very low-loss type of transmission is therefore limited to a few applications: In the past, remote places were supplied with such a line running through the entire settlement and television reception was possible with low transmission power - the participants only had to bring their antennas close to this line. For this purpose, there was ideally a powerful receiving system on a suitable elevated location. From this the actual reception area was then supplied with a Goubau line over several kilometers. The feed and also the reflection-free termination of such a line takes place with a coaxial exponential funnel , which, however, can also consist of several rods in a skeleton construction.

Tube shape

Waveguides are metal tubes without an inner conductor with a rectangular, circular or elliptical cross-section. They are used for the highest frequencies and powers. The internal cross-section must have certain minimum dimensions that depend on the largest wavelength to be transmitted. Metallic objects in it cause a mismatch, which can even damage the inner walls through flashovers. Metallic waveguides can be fed by a coaxial cable, the inner conductor of which protrudes into the waveguide. The waves are "trapped" in the metal hollow body and spread along the conductor from the coupling to the exit with little loss and without any influence from or to the outside. However, there are also dielectric "waveguides" in which the wave is guided like in an optical fiber cable.

Metallic waveguides are used in the SHF frequency range for high power. A magnetron , which often already has a waveguide flange, is therefore usually responsible for the supply .

The width of a rectangular waveguide must be greater than λ / 2 so that electromagnetic waves can propagate in it over greater distances. In the case of a circular cross-section, the general rule is that the circumference must be greater than the wavelength λ. This also limits the usability of coaxial cables at very high frequencies, because unwanted waveguide modes can then occur. Most common use: radar , microwave oven .

Stripline

Strip lines for impedance and delay matching

Waveguides can also be designed as planar (flat) strip lines ( English: strip line, microstrip line ), for example as microstrip or coplanar conductors. Thin metal films are applied to non-conductive (dielectric) materials. Such lines can be found, for example, on circuit boards for the highest frequencies (e.g. in satellite receivers ( LNBs )) and are used in strip-line antennas (also known as panel antennas or flat panel antennas ) and some helical antennas .

More waveguides

optics

Optical waveguides (LWL) or fiber optic cables are used for data transmission, as fiber lasers , for flexible transmission of laser radiation or for lighting or decoration purposes. In addition, there are waveguides in substrate materials in integrated optics , which z. B. can be generated by doping.

sound

Waveguide effect in the ocean

In the ocean, the minimum sound velocity, which occurs in subpolar regions at a depth of approx. 200 m, in subtropical regions at a depth of approx. 1200 m, functions as a waveguide. This is z. B. used for acoustic data transmission (see also SOFAR channel ). In physical oceanography , this is used to measure temperature profiles or currents (see acoustic tomography ).

Also in the ocean, the equator is a waveguide along which equatorial Kelvin waves propagate. This plays e.g. B. plays an important role in the El Niño phenomenon.

stethoscope

With stethoscopes and playground telephones , sound waves are guided in pipes and hoses.

Waveguide effect in the atmosphere

Overreaches of ultra-short waves caused by inversion layers are known in the atmosphere . The radio waves propagate in these like in a waveguide.

In a broader sense, the back and forth reflection of medium and short waves between the earth's surface and the ionosphere can also be understood as a waveguide. Such radio waves travel around the earth in this way. In the area between the earth's surface and the ionospheric D-layer (less than 90 km altitude), low-frequency waves (less than 30 kHz) are guided like in a waveguide ( ionospheric waveguide ).

literature

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