08-08-2017, 09:07 PM
16-08-2017, 12:42 PM
In communications and electronic engineering, a transmission line is a specialized cable or other structure designed to conduct radio frequency alternating current, ie currents with a sufficiently high frequency to take into account its wave nature. Transmission lines are used for such purposes as the connection of radio transmitters and receivers with their antennas, the distribution of cable television signals, trunklines between telephone switching centers, computer network connections and data buses Of high speed computer.
This article covers the transmission line of two conductors, such as parallel line (ladder line), coaxial cable, band line and microstrip. Some sources also refer to waveguide, dielectric waveguide, and even fiber optic as a transmission line, however these lines require different analytical techniques and therefore are not covered by this article; See Guide of waves (electromagnetism).
Ordinary electrical cables are sufficient to carry low-frequency AC (AC), such as mains power, which reverses direction 100 to 120 times per second, and audio signals. However, they can not be used to carry currents in the radio frequency range, above about 30 kHz, because the energy tends to radiate from the cable as radio waves, causing power losses. Radiofrequency currents also tend to be reflected from discontinuities in the cable, such as connectors and seams, and run the cable toward the source. These reflections act as bottlenecks, preventing the power of the signal from reaching the destination. The transmission lines use a specialized construction and an impedance adaptation to transport electromagnetic signals with minimal reflections and power losses. The distinguishing feature of most transmission lines is that they have uniform transverse dimensions along their length, giving them a uniform impedance, called the characteristic impedance, to prevent reflections. Transmission line types include parallel line (ladder line, twisted pair), coaxial cable, and flat transmission lines such as band line and microstrip. The higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the wavelength of the waves. Transmission lines are required when the length of the cable is greater than a significant fraction of the wavelength of the transmitted frequency.
At microwave and higher frequencies, power losses in transmission lines become excessive, and waveguides are used, which function as "pipes" to confine and guide electromagnetic waves. Some sources define waveguides as a type of transmission line; However, this article will not include them. At even higher frequencies, in the terahertz, infrared and visible intervals, waveguides become detrimental, and optical methods (such as lenses and mirrors) are used to guide electromagnetic waves.
The theory of sound wave propagation is very similar mathematically to that of electromagnetic waves, so transmission line theory techniques are also used to construct structures to conduct acoustic waves; And these are called acoustic transmission lines.