Here's another method that's based on the engineering textbook transmission line equation and use of an impedance analyzer to make an impedance measurement.
You can look up the transmission line equation on Wikipedia: https://en.wikipedia.org/wiki/Transmission_line. Scroll down the page to the paragraph with the title "Input impedance of lossless transmission line". This equation gives the impedance Zin looking into a transmission line in terms of the line's characteristic impedance (Zo), the load impedance (ZL) at the far end, and the electrical length of the transmission line. If electrical length of the line is 1/8-wavelength (lambda/8), you terminate the line in a short circuit (ZL=0), and you solve for Zin, you find: Zin=jZo or Zo=-jZin. Everything drops out of the transmission line equation except for the characteristic impedance Zo that you are trying to find! (Homework assignment for you mathematicians: Terminate the line in an open circuit (ZL=infinity) and solve for Zin in terms of Zo for the same 1/8-wave line). Now you need to set up a measurement with your analyzer that duplicates these conditions. Terminating the line in a short circuit is the easy part. Next you have to determine the frequency where the line is 1/8-wave long. How do you do this? First use your impedance analyzer to find the frequency where the line is a 1/4-wave long (if the line is terminated in an open circuit at the far end, the input impedance goes to zero at the lowest frequency where the line is a 1/4-wave long). Divide that frequency by 2 and you have the frequency where the line is 1/8-wave long. Now tune your analyzer to that frequency and measure the input impedance Zin to the line. In general Zin=R+jX. For an ideal line, that impedance is purely imaginary, i.e. R=0 and X is some finite number. If your analyzer gives you some small R value that is not zero, ignore it and work with the just the X value. You now have Zo=-X as derived above. You're done. 73, John W1FV -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of Martin Sent: Wednesday, January 24, 2018 6:28 AM To: topBand List Subject: Topband: How to determine impedance Topbanders, for the construction of the beverage ( a 2 wire reversible beverage, that is) from my post a few days ago i wanted to know how to measure or determine the impedance of the open wire. I stumbled across a very simple method some of of you might know , some might not. Anyway, here is the method. It works for coaxial cable or open wire. You need no fancy measurement gear- rule, pocket calculator and LC meter will do. For really long runs you might need the help of a tape rule or even google maps. Measure the length of the unknown coaxial cable or open wire (DUT Device under test) in meters m. With both ends open, measure capacitance C of DUT, divide it by it's length ( C' = C/m) . Shorten one end, measure inductance of DUT, divide it by length ( L' = L/m) . Now calculate Z= 1000 * squareroot(L'/C') Voila. It may also help to determine if and where your cable or 2 wire beverage has an intermittance when you already know it's impedance it had before the failure. Transform the formula accordingly. I think it should also help to determine the impedance of the beverage ( single wire or 2 wire) measured against ground(?). -- Ohne CW ist es nur CB.. 73, Martin DM4iM _________________ Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
