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Why 50 ohm coax ? (taken from http://www./Misc/Electronics/docs/wiring/cable_impedance.html) Standard coaxial line impedance for r.f. power transmission in the this value was chosen is given in a paper presented by Bird Electronic Corp. Different impedance values are optimum for different parameters. Maximum power-carrying capability occurs at a diameter ratio of 1.65 corresponding to 30-ohms impedance. Optimum diameter ratio for voltage breakdown is 2.7 corresponding to 60-ohms impedance (incidentally, the standard impedance in many European countries). Power carrying capacity on breakdown ignores current density which is high at low impedances such as 30 ohms. Attenuation due to conductor losses alone is almost 50% higher at that impedance than at the minimum attenuation impedance of 77 ohms (diameter ratio 3.6). This ratio, however, is limited to only one half maximum power of a 30-ohm line. In the early days, microwave power was hard to come by and lines could not be taxed to capacity. Therefore low attenuation was the overriding factor leading to the selection of 77 (or 75) ohms as a standard. This resulted in hardware of certain fixed dimensions. When low-loss dielectric materials made the flexible line practical, the line dimensions remained unchanged to permit mating with existing equipment. The dielectric constant of polyethylene is 2.3. Impedance of a 77-ohm air line is reduced to 51 ohms when filled with polyethylene. Fifty-one ohms is still in use today though the standard for precision is 50 ohms. The attenuation is minimum at 77 ohms; the breakdown voltage is maximum at 60 ohms and the powercarrying capacity is maximum at 30 ohms. Another thing which might have lead to 50 ohm coax is that if you take a reasonable sized center conductor and put a insulator around that and then put a shield around that and choose all the dimensions so that they are convenient and mechanically look good, then the impedance will come out at about 50 ohms. In order to raise the impedance, the center conductor's diameter needs to be tiny with respect to the overall cable's size. And in order to lower the impedance, the thickness of the insulation between the inner conductor and the shield must be made very thin. Since almost any coax that *looks* good for mechanical reasons just happens to come out at close to 50 ohms anyway, there was a natural tendency for standardization at exactly 50 ohm. 为什么RF电路的特性阻抗大多选择50欧姆?( A lot of people ask, so here's the answer to the eternal question, "How did 50 ohms get to be the standard RF transmission line impedance?" Here are a few stories. Bird Electronics will send you a printed copy of their version if you ask for it. This from Harmon Banning of W.L. Gore & Associates, Inc. cable:There are probably lots of stories about how 50 Ohms came to be. The one I am most familiar goes like this. In the early days of microwaves - around World War II, impedances were chosen depending on the application. For maximum power handling, somewhere between 30 and 44 Ohms was used. On the other hand, lowest attenuation for an air filled line was around 93 Ohms. In those days, there were no flexible cables, at least for higher frequencies, only rigid tubes with air dielectric. Semi-rigid cable came about in the early 50's, while real microwave flex cable was approximately 10 years later.Somewhere along the way it was decided to standardize on a given impedance so that economy and convenience could be brought into the equation. In the 无限长传输线上各处的电压与电流的比值定义为传输线的特性阻抗，用Ｚ0 表示。 同轴电缆的特性阻抗的计算公式为 ： Ｚ0＝〔60/√εr〕×Log ( D/d ) [ 欧] 式中：D 为同轴电缆外导体铜网内径；d 为同轴电缆芯线外径；εr为导体间绝缘介质的相对介电常数。通常Ｚ0 = 50 欧 ，也有Ｚ0 = 75 欧的。 由公式不难看出，馈线特性阻抗只与导体直径D和d以及导体间介质的介电常数εr有关，而与馈线长短、工作频率以及馈线终端所接负载阻抗无关. 2、馈线的衰减系数 信号在馈线里传输，除有导体的电阻性损耗外，还有绝缘材料的介质损耗。这两种损耗随馈线长度的增加和工作频率的提高而增加。因此，应合理布局尽量缩短馈线长度。 单位长度产生的损耗的大小用衰减系数 β 表示，其单位为 dB / m （分贝／米），电缆技术说明书上的单位大都用 dB / 设输入到馈线的功率为Ｐ1 ，从长度为 L（m ） 的馈线输出的功率为Ｐ2 ，传输损耗TL可表示为： TL ＝ 10 ×Lg ( Ｐ1 /Ｐ2 ) ( dB ) 衰减系数为： β ＝ TL / L ( dB / m ) 例如， NOKIA 7 / 而普通的非低耗电缆，例如， SYV-9-50-1， 900MHz 时衰减系数为 β ＝ 20.1 dB / 3、匹配概念 什么叫匹配？简单地说，馈线终端所接负载阻抗ＺL 等于馈线特性阻抗Ｚ0 时，称为馈线终端是匹配连接的。匹配时，馈线上只存在传向终端负载的入射波，而没有由终端负载产生的反射波，因此，当天线作为终端负载时，匹配能保证天线取得全部信号功率。当天线阻抗为50欧时，与50欧的电缆是匹配的，而当天线阻抗为80欧时，与50欧的电缆是不匹配的。 假如天线振子直径较粗，天线输入阻抗随频率的变化较小，轻易和馈线保持匹配，这时天线的工作频率范围就较宽。反之，则较窄。在实际工作中，天线的输入阻抗还会受到四周物体的影响。为了使馈线与天线良好匹配，在架设天线时还需要通过测量，适当地调整天线的局部结构，或加装匹配装置。 4、反射损耗 前面已指出，当馈线和天线匹配时，馈线上没有反射波，只有入射波，即馈线上传输的只是向天线方向行进的波。这时，馈线上各处的电压幅度与电流幅度都相等，馈线上任意一点的阻抗都等于它的特性阻抗. 而当天线和馈线不匹配时，也就是天线阻抗不等于馈线特性阻抗时，负载就只能吸收馈线上传输的部分高频能量，而不能全部吸收，未被吸收的那部分能量将反射回去形成反射波。 5、电压驻波比 在不匹配的情况下, 馈线上同时存在入射波和反射波。在入射波和反射波相位相同的地方，电压振幅相加为最大电压振幅Ｖmax ，形成波腹；而在入射波和反射波相位相反的地方电压振幅相减为最小电压振幅Ｖmin ，形成波节。其它各点的振幅值则介于波腹与波节之间。这种合成波称为行驻波。 反射波电压和入射波电压幅度之比叫作反射系数，记为 R 反射波幅度 R ＝ ───── 入射波幅度 波腹电压与波节电压幅度之比称为驻波系数，也叫电压驻波比，记为 VSWR 波腹电压幅度Ｖmax （ＺL－Ｚ0） （1 + R） VSWR ＝ ───────────── ＝ ───── ＝ ───── 波节电压辐度Ｖmin （ＺL＋Ｚ0 ） （1 - R） 终端负载阻抗ＺL 和特性阻抗Ｚ0 越接近，反射系数 R 越小，驻波比VSWR 越接近于１，匹配也就越好。 名词注解： 馈线是配电网中的一个术语，它可以指与任意配网节点相连接的支路，可以是馈入支路，也可以是馈出支路。但因为配电网的典型拓扑是辐射型，所以大多馈线中的能量流动是单向的。但为提高供电可靠性，配网结构变化很复杂，功率的传输也并非绝对是一个方向。所以粗略地说，配电网中的支路都可称之为馈线。 对电视天线馈线（室外天线到电视机之间的连线）一般的要求是：能有效地传送天线接收的电视信号、畸变小、损耗小、抗干扰能力强，馈线与天线之间、与电视机信号输入端之间应有良好的阻抗匹配。这些要求普通导线不具备。普通导线对电视信号的高频衰减严重，抗干扰能力差，容易受到各种外来高频信号的干扰。同时，普通导线的特性阻抗不定，很难满足阻抗匹配要求，如果用普通导线作为电视机天线馈线，当天线上感应到的信号经普通导线传向电视机时，在电视机输入端因阻抗不匹配将产生反射，被反射回去的信号在普通导线与天线之间又由于阻抗不匹配而发生反射，多次反射的结果会使屏幕上图像严重重影，无法正常收看，并对电视接收天线的性能造成一定损害。常用的电视天线锅线主要有两种，一种是特性阻抗为75欧的同轴电缆馈线，另一种是特性阻抗为300欧的平地扁馈线。有的电视机没有300/75欧阻抗变换器，使用这两种馈线，一般都能满足天线、馈线、电视机信号输入端阻抗匹配的要求，获得最佳接收效果。在选拔天线馈线时，一方面要注意观察电视机天线插孔处的阻抗标记，另一方面要考虑天线本身的阻抗特性，使天线具备的特点与实际需要吻合。75欧圆馈线与300欧扁平馈线两者比较，前者因有金属屏蔽层，抗干扰能力好，传输损耗小，在需要馈线较长的情况下比较适用，但价线较贵，不易配接；后者价值虽便宜，但由于无屏蔽作用，抗干扰能力不如前者，容易拾取杂波干扰，影响接收质量，且传输损耗大些，在不需要很长馈线的情况下，可考虑选用。 |
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