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电长度

秦力电缆提醒您:

在传输线中常用一个称为电长度的参数(单位:MHZ)来衡量电缆的电气性能。工厂生

产电缆时,因为制造工艺的关系,使得每一批的电缆的电气指标都存在着差别,比如同是一段物理长度一样的两条电缆,对同一个高频信号来说它反映的电性能就不一样,因此就引入了一个电长度的概念。它反映了在一段单位物理长度内,电缆对某一频率信号所表现出来的特性。在制作发射天线的馈电系统中,此项参数尤为重要。例如在我发表的“双层十字型发射天线的配接"一文中的各分馈电缆,在物理长度一样但电长度不一

样的情况下,分馈线的实际阻抗就会产生偏移且会引起附加相移,使得整个天线系统难

以做到很好的配接。

那么如何去检测一段电缆的电长度呢?具体方法是这样的,例如发射天线工作的中心频率为F,其对应的波长为λ,截取一根物理长度为λ/2的电缆将它的终端短路,使它对信号形成全反射,用扫频仪进行测试,调节扫频仪输出的中心频率使扫频仪屏幕上产生一个下陷的波形(如图L所示),这个下陷波就是电缆的反射波形。从长线理论中我们知道,终端短路的传输线对于某一频率信号来说,离终端λ/2处,它的反射波电

流幅值最大,所以此时图中的A点(即波峰处所对应的频率)就是这根电缆的电长度。如A点处的频标所指示的频率等于F,就说明此电缆的电气性能达标,如不等F,则说

明电缆的电气性能存在着差异,如用此电缆作天线系统的分馈线时,就必须要对其(物

理长度)进行修正。

对于特性相同的电缆来说,当它的物理长度相同时,它们的电长度也相同;当它的特性不等时,电缆的物理长度相同而它们的电长度不相同,所以我们可以用电长度这个

指标来衡量电缆性能的一致性。

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electrical length

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electrical length:

1. Of a transmission medium, its length expressed as a multiple or submultiple of the wavelength of a periodic electromagnetic or electrical signal propagating within the medium.

Note 1: The wavelength may be expressed in radians or in artificial UNITS of angular measure, such as degrees. Note 2 : In both coaxial CABLES and OPTICAL fibers, the velocity of propagation is approximately two-thirds that of free space. Consequently, the wavelength will be approximately two-thirds that in free space, and the electrical length, approximately 1.5 times the physical length.

2. Of a transmission medium, its physical length multiplied by the ratio of (a) the propagation time of an electrical or electromagnetic signal through the medium to (b) the propagation time of an electromagnetic wave in free

space over a distance equal to the physical length of the medium in question. Note: The electrical length of a physical medium will always be greater than its physical length. For example, in coaxial CABLES, distributed resistances, capacitances and inductances impede the propagation of the signal. In an OPTICAL fiber, interaction of the lightwave with the materials of which the fiber is made, and fiber geometry, affect the velocity of propagation of the signal.

3. Of an antenna, the effective length of an element, usually expressed in wavelengths.

Note 1: The electrical length is in general different from the physical length.

Note 2: By the addition of an appropriate reactive element (capacitive or inductive), the electrical length may be made significantly shorter or longer than the physical length.

Definition electrical Length

From :https://www.sodocs.net/doc/726596965.html,

其主要偏重于波长的概念!

electrical length: 1. Of a transmission medium, its length expressed as a multiple or submultiple of the wavelength of a periodic electromagnetic or electrical signal propagating within the medium. Note 1: The wavelength may be expressed in radians or in artificial units of angular measure, such as degrees. Note 2 : In both coaxial cables and optical fibers, the velocity of propagation is approximately two-thirds that of free space. Consequently, the wavelength will be approximately two-thirds that in free space, and the electrical length, approximately 1.5 times the physical length. 2. Of a transmission medium, its physical length multiplied by the ratio of (a) the propagation time of an electrical or electromagnetic signal through the medium to (b) the propagation time of an electromagnetic wave in free space over a distance equal to the physical length of the medium in question. Note: The electrical length of a physical medium will always be greater than its physical length. For example, in coaxial cables, distributed resistances, capacitances and inductances impede the propagation of the signal. In an optical fiber, interaction of the lightwave with the materials of which the fiber is made, and fiber geometry, affect the velocity of propagation of the signal. 3. Of an antenna, the effective length of an element, usually expressed in wavelengths. Note 1: The electrical length is in general different from the physical length. Note 2: By the addition of an appropriate reactive element (capacitive or inductive), the electrical length may be made significantly

shorter or longer than the physical length.

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