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Transmission medium

A transmission medium is a method or material substance which can propagate waves or energy. For example for over the air transmission of electromagnetic (EM) waves the transmission medium is the property of EM waves to propagate without a material medium.

A transmission medium can be classified as a:

• Linear medium, if different waves at any particular point in the medium can be superposed;
• Bounded medium, if it is finite in extent, otherwise unbounded medium;
• Uniform medium, if its physical properties are unchanged at different points;
• Isotropic medium, if its physical properties are the same in different directions.

Electromagnetic radiation can be transmitted through an optical media, such as optical fiber, or through twisted pair wires, coaxial cable, or dielectric-slab waveguides. It may also pass through any physical material which is transparent to the specific wavelength, such as water, air, glass, or concrete. Electromagnetic waves do not require a physical transmission medium unlike mechanical waves, and so can travel through the "vacuum" of free space. Regions of the insulative vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions. Historically, various aether theories were used in science and thought to be necessary to explain the transmission medium. Sound is, by definition, the vibration of matter, so it requires a physical medium for transmission, as does heat energy.

Telecommunications

For telecommunications purposes in the United States, Federal Standard 1037C, transmission media are classified as one of the following:

• Guided (or bounded) - Waves are guided along a solid medium such as a transmission line.
• Wireless (or unguided) - Transmission and reception are achieved by means of an antenna. Wireless media may carry surface waves or skywaves, either longitudinally or transversely, and are so classified.

See also

• Vacuum permittivity
• Transmission (telecommunications)
• Excitable medium
• Optical medium

Contents

• Telecommunications
• See also