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Thread: velocity factor via vector impedance analyser

  1. #1

    Default velocity factor via vector impedance analyser

    I got the bright idea of trying to use an N1201SA portable vector impedance analyser to find the velocity factor of some 300 ohm TV ribbon. I figured just connect a random length of open circuit ribbon to the analyser, note the lowest frequency where the impedance peaked & that would correspond to half a wavelength. So I hooked up a 156mm length & noted the impedance peaked at 1094MHz. Now 1094MHz is a wavelength of 274mm, a half wavelength of 137mm. So my physical 156mm half wavelength line at 1094MHz has an electrical half wavelength of 137mm, meaning the velocity factor is 137/156=0.88. So far so good.

    Next I noticed another impedance peak at 1980 MHz. This has me worried because I expected the full wavelength peak to be at 1094 x 2 = 2188MHz. The 1980 value gives me a velocity factor of 0.97 I get the feeling I'm missing something?

  2. #2
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    I was always told open feeder had the lowest loss (at H.F. anyway).

    Try it with a load on the end, or a simple short and see what happens...

  3. #3

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    What I like to do is connect the open-ended transmission line with a 50ohm resistance in parallel (or 300 ohm in your case and assuming your analyzer can be set to a 300ohm system impedance) at the analyzer end. Then do an SWR sweep with the analyzer with a range in the expected neighborhood. Then lessen the range and do it again for a closer look at the dip. Since a half-wavelength open-ended transmission line will have an infinite impedance at the input end, and this is in parallel to the resistor, the only load the analyzer will see is the resistor. This gives you an SWR dip at the half-wavelength frequency and avoids attempting to make the measurement at the limits of the analyzers capability where accuracy is compromised.

    EDIT: It is likely that the second peak not being where you expect could be because, as frequency changes, so does the skin effect. When the skin effect changes, the current distribution in the conductor changes which in turn changes the inductance. Being inductance is one of the governing factors in the velocity factor, you should expect some variation.
    Last edited by brandon lind; Sun 19th May 2019 at 14:52.

  4. #4

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    You can also measure velocity factor with a DVM.

    At this point, a dozen people are ready to yell at me, so here’s the disclaimer: The DVM needs to have the ability to make very accurate capacitance measurements, your average Walmart DVM isn’t going to cut it here. The concept is sound and it’s fun to try, so here it goes.

    Take a capacitance measurement (F) of the open-ended cable and divide it by the number of feet in that cable. This is capacitance per foot. Multiply that by the characteristic impedance of the line. The result will be the time in seconds it takes for a signal to go down one foot of that line. Being we know it takes light 1.017ns to go a foot in free space, dividing that by the cable time gives the velocity factor.

    I just did this with my Klein Tools MM400 DVM and came to .61VF. I know this cable to be .66VF using stub resonance at HF. Not bad for a Walmart DVM 

    http://www.montana.edu/blameres/cour...L_apr96a11.pdf

  5. #5

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    Quote Originally Posted by brandon lind View Post
    This gives you an SWR dip at the half-wavelength frequency .
    Even with the analyser fixed at 50ohm output Z, terminating the 300ohm lengths of ribbon with a 47ohm resistor & looking for SWR minimums gives much more consistent results, both in terms of the frequency multiples where the dips occur for any given length & across different lengths. Much obliged.

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