QUOTE:
引用第10楼jknothing于2007-12-06 09:03发表的“”:
下载不了哦.楼主检查一下哦.
Transmission line geometry and dielectric material determine its characteristic impedance.
When people design Transmission line, they optimize geometrical parameters for different purpose (early days they only have air as the dielectric material). 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). Optimum diameter ratio for minimum attenuation is 3.6 corresponding to 77-ohm (which is why 75-ohm is also widely available)
In the early days, 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. This is where the 50-ohm coming from.
Another thing, 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. So there was a natural tendency for standardization at exactly 50 ohms.
电磁兼容(EMC)小小家下载讨论中心 -> 电磁兼容(EMC)资料-下载 -> 为什么是50欧姆？（资料下载）
