从哈工大微波网站看到的，原文是e文如下：
the history of 50 w
a lot of people ask, so here's the answer to the eternal question, "how did 50 w 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. able: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 us, 50 ohms was chosen as a compromise. there was a group known as jan, which stood for joint army and navy who took on these matters. they later became desc, for defense electronic supply center, where the mil specs evolved. europe chose 60 ohms. in reality, in the us, since most of the "tubes" were actually existing materials consisting of standard rods and water pipes, 51.5 ohms was quite common. it was amazing to see and use adapter/converters to go from 50 to 51.5 ohms. eventually, 50 won out, and special tubing was created (or maybe the plumbers allowed their pipes to change dimension slightly).further along, the europeans were forced to change because of the influence of companies such as hewlett-packard which dominated the world scene. 75 ohms is the telecommunications standard, because in a dielectric filled line, somewhere around 77 ohms gives the lowest loss. (cable tv) 93 ohms is still used for short runs such as the connection between computers and their monitors because of low capacitance per foot which would reduce the loading on circuits and allow longer cable runs.volume 9 of the mit rad lab series has some greater details of this for those interested. it has been reprinted by artech house and is available.
下面文字是我译的，e文太差，凑活看！
50欧阻抗的历史
很多人问这个问题，更深入一点的提问是“为什么50欧姆成为了射频传输线的阻抗标准？”这里有个小故事如果你就此问题，咨询鸟牌电子公司，他们会给你一个他们的版本。这份资料来自于harmon banning of w.l. gore & associates, inc.电缆：关于50欧姆的来历可能有很多故事。这是最多流传的一个。
在微波应用的初期----二次世界大战期间，阻抗的选择完全依赖于使用的需要.对于大功率的处理，30欧姆和44欧姆常被使用。另一方面，最低损耗的空气填充线的阻抗是93欧姆。在那些岁月里，对于很少用的更高频率，没有易弯曲的软电缆，仅仅是填充空气介质的刚性导管。半刚性电缆诞生于50年代早期，真正的微波软电缆出现是大约10年以后了。随着技术的进步，需要给出阻抗标准，以便在经济性和方便性上取得平衡。在美国，50欧姆是一个折中的选择。为联合陆军和海军解决这些问题，一个名为jan的组织成立了。就是后来的desc，由mil特别发展的。欧洲选择了60欧姆。事实上，在美国最多使用的导管是由现有的标尺竿和水管连接成的，51.5欧姆是十分常见的。看到和用到50欧姆到51.5欧姆的适配器/转换器，感觉很奇怪的。最终50欧姆胜出了，并且特别的导管被制造出来（也可能是装修工人略微改变了他们管子的直径）。不久以后，在象hewlett-packard这样在业界占统治地位的公司的影响下，欧洲人被迫改变了。
75欧姆是远程通讯的标准，由于是介质填充线，在77欧姆获得最低的损耗。
93欧姆一直用于短接续，如连接计算机主机和监视器，其低电容的特点，减少了电路的负载，并允许更长的接续。
感兴趣的读者mit rad lab series的第9卷，有更详细的描述。
关于译文外的话：
我是美国百通电缆的代理，对电缆接触相对多些，同轴缆除了50欧、75欧以外，93欧也常有人用，还有32欧的产品，但还没有人向我询问过。
电缆的特征阻抗与内外导体的直径，和绝缘的介电常数有直接的关系，实心pe（聚乙烯），高效物理发泡，氮气发泡，空气绝缘等其内外导体的直径，电容，电感，波速，延迟，衰减都不同，但有一定的内在关系。附件是一款93欧空气绝缘的电缆的技术资料，有兴趣的可以看看，一个型号的电缆应该有多少必要的参数，可不是一个“-5‘之类的说法。
