|
In telecommunications and electrical engineering, electrical length (or phase length) refers to the length of an electrical conductor in terms of the phase shift introduced by transmission over that conductor〔Ron Schmitt, Electromagnetics explained (resource ): a handbook for wireless/RF, EMC, and high-speed electronics. (8 )〕 at some frequency. ==Usage of the term== Depending on the specific usage, the term "electrical length" is used rather than simple physical length to incorporate one or more of the following three concepts: *When one is concerned with the number of wavelengths, or phase, involved in a wave's transit over a segment of transmission line especially, one may simply specify that electrical length, while specification of a physical length, frequency, or velocity factor is omitted. The electrical length is then typically expressed as ''N'' wavelengths or as the phase φ expressed in degrees or radians. Thus in a microstrip design one might specify a shorted stub of 60° phase length, which will correspond to different physical lengths when applied to different frequencies. Or one might consider a 2-meter section of coax which has an electrical length of one quarter wavelength (90°) at 25 MHz and ask what its electrical length becomes when the circuit is operated at a different frequency. *Due to the velocity factor of a particular transmission line, for instance, the transit time of a signal in a certain length of cable is equal to the transit time over a ''longer'' distance when travelling at the speed of light. So a pulse sent down a 2-meter section of coax (whose velocity factor is 2/3) would arrive at the end of the coax at the same time that the same pulse arrives at the end of a bare wire of length 3 meters (over which it propagates at the speed of light), and one might refer to the 2 meter section of coax as having an electrical length of 3 meters, or an electrical length of 1/2 wavelength at 50 MHz (since a 50 MHz radio wave has a wavelength of 6 meters). *Since resonant antennas are usually specified in terms of the electrical length of their conductors (such as the ''half wave'' dipole), the attainment of such an electrical length is loosely equated with electrical resonance, that is, a purely resistive impedance at the antenna's input, as is usually desired. An antenna that has been made slightly too long, for instance, will present an inductive reactance, which can be corrected by physically shortening the antenna. Based on this understanding, a common jargon in the antenna trade refers to the achievement of resonance (cancellation of reactance) at the antenna terminals as ''electrically shortening'' that too-long antenna (or ''electrically lengthening'' a too-short antenna) when an electrical matching network (or antenna tuner) has performed that task without ''physically'' altering the antenna's length. Although a very inexact use of terminology, this usage is widespread, especially as applied to the use of a loading coil at the bottom of a short monopole (a vertical, or whip antenna) to "electrically lengthen" it and achieve electrical resonance as seen through the loading coil. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「electrical length」の詳細全文を読む スポンサード リンク
|