如何测量同轴电缆的阻抗

> {quote：title = paulh写道：} {quote}>你好这是我使用的方法。
您可以通过了解给定频率下电缆的电感和电容来计算电缆的阻抗。
>>按照您感兴趣的频率在vna上进行S11单端口校准。
>>将一小段同轴电缆连接到vna上的校准端口。
我通常在输出端口上使用一个sma连接器，然后使用“连接器”，“打开。短路，负载”>>取一小段同轴电缆，长度为几英寸并将其连接到带电的端口。
>>在同轴电缆的另一端打开时，读取vna电缆的电容值并记下它。
>>>缩短电缆的另一端并读取电缆的感应值，并记下这一点。
>>如果你测试半刚性电缆之类的东西，你的电容值将在20pf范围内，你的电感值将在低nH范围内。
>>使用以下公式以您使用的测试频率计算ZO。
>> zo = SQRT（L / C）谢谢保罗。
我喜欢这种方法的简单性，但很明显，如果电缆长于四分之一波，则开路将是电感而不是电容，短路将是电容性的而不是电感性的。
等式Zo = sqrt（L / C）中的L和C的值当然是分布值，单位为亨利每米法拉每米。
戴夫



     以下为原文

  > {quote:title=paulh wrote:}{quote}
> Hello here is a method I use. You can calculate the impedance of the cable by knowing both the inductance and capacitance of the cable at a given frequency.
> 
> Do a S11 single port cal on the vna at your frequency of interest.
> 
> Connect a small piece of coax cable to the calibrated port on the vna. I usualy use a sma connector on the output port and cal with that in place, "Open. Short, Load"
> 
> Take a small piece of coax, a few inches in length and connect it to the caibrated port.
> 
> With the other end of coax left open read off the vna the capacitive value of the cable and make a note of it.
> 
> Short the other end of the cable and read off the inductive value of the cable, make a note of that too.
> 
> If you test something like semi-rigid cable your capacitive value will be in the 20pf range and your inductive value will be in the low nH range.
> 
> use the following formula to calculate ZO at the test frequency you used.
> 
> zo = SQRT ( L/C )

Thank you Paul. 

I like the simplicity of the method, but clearly if the cable is longer than a quarter wave, then an open will be inductive and not capacitive and a short will be capacitive and not inductive. The values of L and C in the equation Zo=sqrt(L/C) are of course distributed values, with units of Henrys per metre Farads per metre. 

Dave

嗨Dave，你说的是同轴电缆的长度是正确的，我打赌取决于长度可以绕史密斯图表几次，环路循环eh :)同轴电缆虽然高端模拟器有点有趣
如果将其配置为4：1或9：1巴伦，则会出现模拟问题。
谢谢保罗



     以下为原文

  Hi Dave, you are right about the length of the coax, I bet depending on the length one could take a trip around the smith chart a few times, loop the loop eh :)

Coax cable is kind of interesting though even the high end simulators have problems simulating it in particular if it is configured as a 4:1 or 9:1 balun.

Thanks Paul

> {quote：title = paulh写道：} {quote}>有趣的天线戴夫。
是的，这是一个有趣的天线。
我曾经为一家制造类似天线的公司工作。
这是一个非常好的公司，因为所有规格都是发明的！
发明了增益和辐射模式。
测量了SWR，但是如果它超出规格，它通常会被出售。
他们销售的设计中有各种电感器，没有关于如何计算值的文档。
这一切都被切断了，我想，并且由于没有人关心性能，如果90％的功率喷射到天空或变成热量并不重要！
在那里工作的时候，我接受了HFSS的试验，并试图了解基本的同轴共线天线。
有很多关于它们的文章，但找到很多有用的信息似乎很难。
如果增益为9 dBi或9 dBd，我设置链接的设计甚至没有说明，但它实际上约为7 dBi，并且假定无损耗的电介质和导体。
>另外还有另一种测量同轴电缆或双绞线等阻抗的方法，如果您可以使用示波器和函数发生器，那么“长长度或线轴”就很简单了。这次没有VNA
周围:) >>你做的是沿着电缆拍摄频率为几Mhz的方波。
使用BNC T连接器将电缆连接到一侧，将信号发生器连接到另一侧，并将其连接到示波器。
将电缆留在远端。
>>>调整示波器的时基，直到您可以看到方波中的振铃和步进。
由于反射波回到发电机，它看起来不像是一个干净的方波。
>>现在在电缆的远端连接一个可变的锅并调整锅，直到你的方波看起来干净。
>>>断开锅并测量其阻力。
测量结果将是电缆的阻抗或非常接近它。
>>谢谢保罗。
我懂了。
我想当线路的阻抗终止时，没有反射，因此没有方波的扭曲。
好主意。
您应该写一篇论文“100种方法来测量同轴电缆的阻抗”。
戴夫



     以下为原文

  > {quote:title=paulh wrote:}{quote}
> Interesting Antenna Dave. 

Yes, it is an interesting antenna. I used to work for a company that made antennas similar to them. It was a pretty alful company to work for, as all specifications were invented! Gain and radiation pattern were both invented. SWR was measured, but if it was out of spec, it was usually sold anyway. The design they sold had various inductors in them, with no documentation about how to calculate the values. It was all cut and try I think, and since nobody cared about the performance, it did not matter if 90% of the power was squirted into the sky or turned into heat! 

Whilst working there, I got a trial of HFSS and set about trying to understand the basic coaxial colinear antenna. There is a lot written about them, but finding much useful information appeared to be hard. The author of that design I put a link to does not even state if the gain is 9 dBi or 9 dBd, but it is in fact about 7 dBi and that assumes lossless dielectrics and conductors. 

> There is another method of measuring the impedance of the coax cable or twisted pair etc that comes to mind, "Long lengths or spool" that's fairly simple to do if you have access to a scope and a function generator., No VNA this time around :) 
> 
> What you do is Shoot a Square wave of a few Mhz in frequency down the cable. Using a BNC T connector connect the cable to one side and your signal generator to the other and hook it into a scope. Leave the cable open at the far end. 
> 
> Adjust the time base on the scope until you can see the ringing and steps in the square wave. It wont look like a clean square wave on the scope because of the reflected waves coming back to the generator. 
> 
> Now on the far end of the cable hook up a variable pot and adjust the pot until your square wave looks clean. 
> 
> Disconnect the pot and measure its resistance. That measured result will be the impedance of the cable or pretty close to it. 
> 
> Thanks Paul.

I see. I guess when the line is terminated in its impedance there is no reflection, so no distorsion of the square wave. Nice idea. You should write a paper "100 ways to measure the impedance of a coaxial cable". 

Dave