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A transmission medium (plural transmission media) is a material substance (solid, liquid or gas) which can propagate energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound.

The absence of a material medium (the vacuum of empty space) can also be thought of as a transmission medium for electromagnetic waves such as light and radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media through which they pass, for instance by absorption or by reflection or refraction at the interfaces between media.

The term transmission medium can also refer to the technical device which employs the material substance to transmit or guide the waves. Thus an optical fiber or a copper cable can be referred to as a transmission 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.

Coaxial Cable, one example of a transmission medium

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. Sound is, by definition, the vibration of matter, so it requires a physical medium for transmission, as does other kinds of mechanical waves and heat energy. Historically, various aether theories were used in science and thought to be necessary to explain the transmission medium. However, it is now known that electromagnetic waves do not require a physical transmission medium, 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.

Many transmission media are used as a communications channel.

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.

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