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Thread: Normalizing impedance measurements

  1. #1

    Default Normalizing impedance measurements

    The nanoVNA-F arrived and I am starting to chip away at my list of projects with it. I ran into a problem.

    I wanted to use my abundance of 75ohm coax for my SDR for FM broadcast reception so the first thing I did was make an L-match to convert 50ohm to 75ohm (as the SDR is 50ohm). That was an absolute breeze with the analyzer! Heres where i am a bit unsure. I would like to take accurate impedance measurements at the end of the 75ohm coax, but the nanoVNA has a fixed system impedance of 50 ohms. I am thinking the proper way to do that is to make a 75ohm calibration load, connect the VNA to the L-match, the 75ohm coax to the L-match, then the 75ohm load to the coax end and perform the OSL as usual. Am I correct in assuming, once that is done, I would take any impedance readings on the VNA, un-normalize from 50ohm then normalize those values to 75ohm? That should give me the correct readings at the end of the 75ohm coax, correct? If not, how can I do this on an analyzer fixed to 50ohm?

    Edit. Someone on youtube pointed out to me that you can calibrate the center of a smith chart to whatever you want, and i understand that. But without pc software, the unit itself will call a 75ohm load 50 if i use it in a calibration at the end of the cable. I dont want to use pc software where i can simply tell it that 75 is the center and this analyzer doesnt have that option in the field.
    Last edited by brandon lind; Sun 26th Jan 2020 at 16:15.

  2. #2

    Default

    Just use a 75 ohm dummy load when calibrating.

  3. #3

    Default

    Thank you for the response. Unfortunately, that suggestion does not work. I asked this question elsewhere and had responses from people who have used VNAs their entire life completely miss the fundamental concept of my question. Let me clarify:

    1- Simply calibrating with a 75 ohm load does not work (for this analyzer) as the analyzer I have then tells me that ANY connected load is 50ohm after calibration. It don't just put it in the middle of the smith chart and call it 75ohm, it puts it in the middle and calls it 50ohm, NOT CORRECT. I tried this with a 75ohm resistor directly connected to the analyzer and the analyzer was convinced (after calibration) that it was 50ohm. Had I calibrated to 75ohm with the analyzer expecting to see 75ohm during the calibration, all would be fine, but this is not possible for this particular device in the field ~ there's no option to tell my nano to expect 75ohm during calibration.

    2- I understand that all I really need to do (to tune an antenna to 75 ohm coax) is leave the device calibrated to 50ohm, connect up that 75ohm coax to the analyzer, and stop focusing on the center of the chart as if its some magic spot. If I tune my antenna so that the coax input reads 75ohm (even though that's not in the middle of the chart, silly me for wanting the end result to land there), that my antenna will be matched to the 75ohm coax. But, sometimes I like to know more than that, sometimes I want to know what the antenna impedance is at the end of this 75ohm cable. Thus:

    3- I need a means of calibrating out the effects of the 75ohm coax while the analyzer is calibrated to 50ohm, therefore:

    4- My question was whether or not manually re-normalizing the complex impedance values AS SEEN THROUGH A 50-75ohm MATCHING NETWORK would tell me the real value on the other side of the coax assuming the calibration was done as "analyzer---50-75ohm match---75ohm coax---75ohm load". Obviously, that 75ohm load through the coax and matching network will read 50ohm, which is what the calibration is expecting. But, what happens when the load has reactance and the match sees something its not designed for?

    Example: There is now a 75ohm dummy load out there and my analyzer says its 50 at the port (perfectly correct at this point due to the matching network). I can take that reading of 50, divide it by 50, and multiply it by 75. I just re-normalized my reading to a 75ohm system and my result, based on a little math, reads what is really at the end of my coax. Now, if I take off that dummy load and connect the 75ohm coax to an antenna with reactance and repeat said math, will the resulting complex impedance be what is really at the end of the coax? Essentially, I am asking if the proportionality of the matching network holds true when the load it sees varies?
    Last edited by brandon lind; Fri 31st Jan 2020 at 15:26.

  4. #4

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    My question is now moot.

    Turns out plotting the transformative effects of any impedance coax on a smith chart by hand is easier and faster than I originally realized. My fault for ending my smith chart education at inductor and capacitor effects. I never thought to teach myself how transmission line and toroid transformers move on the chart. Never been so excited to feel this dumb lol! Appreciating the old ways once again!

  5. #5

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    I tried the 75 ohm load on my Nano and saw what you are talking about. My large VNA will normalize on any impedance you feed it it, but the Nano won't. Like you, I've overlooked the Smith Chart too long I guess. Oh, I also use 75 ohm feedline on HF.

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