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另一种解释
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, A- Z, q( v4 i; ^选择50Ω基准阻抗是对空气同轴电缆的最小损耗和最大功率容量尺寸之间折中的结果。对于空气介质同轴电缆，为使损耗最小，要求外导体内径与内导体外径的最佳比值为3.6，对应于阻抗Zo为77Ω。虽然从损耗的角度看，这时的性能最佳，但这一尺寸比不能提供出现介质击穿前允许的最大峰值功率容量。最大功率容量时，外导体内径与内径导体之比应为1.65，对应的阻抗Zo为30Ω。77Ω和30Ω的几何平均近似为50Ω，50≈（77*30）^0.5。这样，50Ω标准即是同轴电缆最小损耗和最大功率容量之间的折中值。 至于选取75Ω,就是损耗考虑损耗了。- X8 k; ^" d6 V
' q- C- m9 N6 c K* ^9 f/ HTransmission line geometry and dielectric material determine its characteristic impedance.
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& S' m1 O9 ^! ^: o0 f# a1 \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)
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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.
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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.
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% i) c" _$ @$ x) C1 O75 ohm is best for receiving,- Z/ r) c. }/ I6 N
23 ohm is best for transmitting,
' z `# S% |" V6 _so 50 ohm is a compromise for mobile applications which require both receiving and transmitting.2 K( c; U4 b! p) m0 p
there are also 300 ohm |
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发表于 2010-4-7 17:05
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发表于 2010-7-16 17:30
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