NanoVNA cable measurements

Having a VNA opens up so many possibilities for RF measurements. Typical measurements are antenna matching, RF filters, impedance matching and cable loss. But it an do more. You can use it even to measure the coaxial cable length – a task that is normally performed with time domain reflectometer (TRD), an instrument I have some knowledge about as I have designed/built few such circuits (Time Domain Reflectometer pulse generator, TDR kit, Avalanche pulse generator and Potato chip to TDR circuit).

Accurately measuring cable length with NanoVNA article will go through some mathematics on deriving TDR response with VNA data. Remember that the VNA does its measurements in the frequency domain. If we transform the frequency domain data into the time domain, we should see the time domain nature of our measurement (with certain limitations though). By using the magnitude and phase of the signal measured throughout the frequency sweep, we can compute the distance from where reflection occurred.

NanoVNA To Test The Loss & Length Of Coax Cables by Jim W6LG YouTube Elmer for Ham Radio Basics

The NanoVNA does many things. Two of which are testing coax cables for loss and length. Jim W6LG shows how to do both in this first video about the NanoVNA. In a subsequent video, Jim will show how the NanoVNA can be used to check antennas for resonance, impedance and reactance. The software that Jim uses is NanoVNA Saver. That software is free and easy to use.
Here is the link to the software : https://github.com/mihtjel/nanovna-saver

#316: Use NanoVNA to measure coax length – BONUS Transmission Lines and Smith Charts, SWR and more

nanoVNA – Coaxial Cable Measurement Methods (Characteristic Impedance and Cable Loss) – VE6WGM

1. Calibrating the nanoVNA
2. The Smith Chart – Briefly (Begins at 8:24)
3. Measuring the characteristic impedance of coax cable. (Begins at 11:46)
4. Measuring coax cable loss. (Begins at 22:34) *Correction: At 23:34 I stated that the nanoVNA has a directional coupler. I have since discovered that this is not accurate information. The nanoVNA actually uses a bridge to make it’s measurements. My apologies. For more information on how this works, please see this excellent video here: https://youtu.be/yGKWBpgN8PU

#502 NANOVNA Demystified

#37: Use a scope to measure the length and impedance of coax

Cable Basics; Transmission, Reflection, Impedance Matching, TDR

Reflected waves on a cable

34 Comments

  1. Tomi Engdahl says:

    nanoVNA – Coaxial Cable Measurement Methods (Characteristic Impedance and Cable Loss) – VE6WGM
    https://www.youtube.com/watch?v=G66_iqOu-Bs

    Reply
  2. Tomi Engdahl says:

    1EZ53September2004-Subject to change J. SimonProduct: Vector Network Analyzer ZVBMeasuring Balanced Components with Vector Network Analyzer ZVB
    https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_application/application_notes/1ez53/1EZ53_0E.pdf

    Balanced RF components are advantageous compared to traditional single-ended components, since they cause less EMI, and are less susceptible to EMI. This application note describes the fundamental concepts of differential and common mode signals and of mixed-mode parameters, which are essential for balanced components. Techniques for the measurement of mixed-mode parameters are presented.

    Reply
  3. Tomi Engdahl says:

    nanoVNA – measuring cable velocity factor – demonstration – open wire line
    https://owenduffy.net/blog/?p=19070

    Reply
  4. Tomi Engdahl says:

    nanoVNA-H – measure ferrite transformer – Noelec balun
    https://owenduffy.net/blog/?p=17897

    Reply
  5. Tomi Engdahl says:

    #316​: Use NanoVNA to measure coax length – BONUS Transmission Lines and Smith Charts, SWR and more
    https://www.youtube.com/watch?v=9thbTC8-JtA

    Reply
  6. Tomi Engdahl says:

    https://groups.io/g/nanovna-users/topic/tdr_testing_of_twisted_pair/78699889

    Has anyone in the group used a nanovna to run TDR tests on twisted-pair cables to measure length of cable or distance to a fault? Although not a coaxial cable, most references list a characteristic impedance of about 100 ohms and velocity factor at about 0.65. Does anyone have tips for using a nanovna calibrated for 50 ohms to measure such twisted pair cable?

    For CAT5/5e certification we produced plots (on each of the four wire pairs) of attenuation, RL, NEXT, FEXT as well as measurements of delay/m, capacitance/m, resistance, length, and impedance. The Lantek had a differential driver sourced through a RL bridge.

    I was contemplating using a BNC-to-SMA adapter, along with a BNC-to-Banana Jack adapter to connect an individual pair. Obviously not ideal, but if “calibrated” to a known length of Cat-5e or Cat-6 cable, it might give a ballpark TDR measurement.

    I hadn’t thought of using a 50:100 ohm balun as the interface. Would one of the CCTV video baluns possibly provide a better match to the nanovna? Here’s one example of what one can get on the web for relatively cheap money:
    https://www.cctvcamerapros.com/CCTV-Balun-p/vb-2vs.htm

    Since most of these top out at about 8 to 10 MHz, you’d have to limit the test frequency to something like 5 MHz.

    Don’t bother, try clip leads or banana jacks and see what happens.
    Sure, you get a bump from the mismatch at the beginning of the cable, but what you want is the length of the cable and whether there’s any damage, and that will reflect, regardless of the impedance of the cable.

    Make two and you can put a 100-120 ohm termination on the other end.

    On another aspect of TSP: TP by itself is nominally 90-100 ohms
    characteristic impedance (somewhat dependent on the dielectric, dielectric
    thickness, and twist tightness). This is without shield, just plain ol’
    TP. Now add the shield. In an EMC course I once took from Howard Johnson,
    himself, one of his demos clearly showed that 80+ % of the fields of each
    conductor of the TSP close onto the shield. That leaves only 20% of the
    induced fields to close between the TP conductors, as intended and
    modelled. So, what’s the deal with 90 to 100 ohms characteristic impedance
    between the conductors of the TSP?

    Also, from practice, after some 10-lamnda (my own estimate) in TSP, most of
    the energy becomes differential mode between the TP conductors and the
    shield – much like coaxial cable, which is considered a common mode cable
    (NOT TP or open wire feeders in the case of antennas). However, the shield
    embodied in the TSP of today is no where near of the integrity of the
    shield on a good grade of 50 (or 75) ohm coaxial cable. So, the stuff
    leaks like a sieve.

    Reply
  7. Tomi Engdahl says:

    https://nanorfe.com/forum/UPDATE-to-DM-LOSS-MEASEREMENT.html

    . say you have a coax choke … then connect the
    shields to the hot ends of your test jig …

    if you have a two wire (twisted pair) then connect only ONE of the two
    to your test jig (does not matter if the “white” or “black” wire used
    cause they should be identical)

    for trough loss you connect both wires on input and output of your balun

    for common mode surpression only connect ONE on input and output
    (normally shield used if you have a coax choke)

    a good choke not only has good match on input and output and low trough
    loss … a good choke also has a high isolation (common mode
    surpression) .

    Reply
  8. Tomi Engdahl says:

    NanoVNA – Testing the Common Mode Attenuation of a DG0SA 1:1 Current Balun by VE6WGM
    https://www.youtube.com/watch?v=_vvCBarRaPY

    NanoVNA and FT240-43 1:1 50 Ohm Current Balun
    https://www.youtube.com/watch?v=LiuDs9-N6-w

    Reply
  9. Tomi Engdahl says:

    Calculate the Common Mode Rejection Ratio (CMRR) of a Balun
    https://github.com/NanoVNA-Saver/nanovna-saver/issues/189

    Reply
  10. Tomi Engdahl says:

    #316​: Use NanoVNA to measure coax length – BONUS Transmission Lines and Smith Charts, SWR and more
    https://www.youtube.com/watch?v=9thbTC8-JtA&feature=youtu.be

    Reply
  11. Tomi Engdahl says:

    #95: Three Methods to Measure Impedance with the NanoVNA
    https://www.youtube.com/watch?v=1UbEz73FGCU

    Are you aware that there is more than one method to measure impedance with a VNA? You might find this video interesting.

    Reply
  12. Tomi Engdahl says:

    https://en.wikipedia.org/wiki/Characteristic_impedance

    A surge of energy on a finite transmission line will see an impedance of Z o {\displaystyle Z_{\text{o}}} {\displaystyle Z_{\text{o}}} prior to any reflections returning; hence surge impedance is an alternative name for characteristic impedance.

    The analysis of lossless lines provides an accurate approximation for real transmission lines that simplifies the mathematics considered in modeling transmission lines. A lossless line is defined as a transmission line that has no line resistance and no dielectric loss.

    Z = sqrt ( L / C)

    In particular, Z o {\displaystyle Z_{\text{o}}} {\displaystyle Z_{\text{o}}} does not depend any more upon the frequency. The above expression is wholly real, since the imaginary term j has canceled out, implying that Z o {\displaystyle Z_{\text{o}}} {\displaystyle Z_{\text{o}}} is purely resistive. For a lossless line terminated in Z o {\displaystyle Z_{\text{o}}} {\displaystyle Z_{\text{o}}}, there is no loss of current across the line, and so the voltage remains the same along the line. The lossless line model is a useful approximation for many practical cases, such as low-loss transmission lines and transmission lines with high frequency. For both of these cases, R and G are much smaller than ωL and ωC, respectively, and can thus be ignored.

    Reply
  13. Tomi Engdahl says:

    Cable Impedance Calculator
    https://www.omnicalculator.com/other/cable-impedance

    Coaxial Cable Impedance Calculator

    Pasternack’s Coaxial Cable Impedance Calculator allows you to enter the Outer Diameter Dielectric width, Inner conductor Diameter width and either the Dielectric Constant or Velocity of Propagation (VoP) values in order to calculate the impedance of the coax.
    https://www.pasternack.com/t-calculator-coax-cutoff.aspx

    Reply
  14. Tomi Engdahl says:

    Twisted-Pair Impedance Calculator
    A tool designed to calculate the characteristic impedance of a twisted-pair cable
    https://www.apogeeweb.net/tools/twisted-pair-impedance-calculator.html

    Reply
  15. Tomi Engdahl says:

    #564​ NANOVNA Coax Loss Measurement
    https://youtu.be/mU71rGUKlBI

    Reply
  16. Tomi Engdahl says:

    https://m.facebook.com/groups/368777730463838/permalink/740026686672272/
    Hello,

    I almost never post because I try to manage myself, but I don’t know how to achieve a measure and my search on the net does not give me much explicit results.

    How can we measure losses in a system already installed (coaxial cable, connectors, CMC, filters, etc.) and where it is not possible to access both ends with nanoVna?

    Thank you in advance for your help

    ANSWERS:

    One testing strategy is to have RF noise generator on one end and spectrum analyzer on other end. You get idea of amplitude response and attenuation.

    You can measure the return loss. You have the remote end of the cable either open or short circuit, which reflects back all the RF. Short circuit is preferred as a short is more definite than an open circuit at RF. You then measure the amount of signal coming back. If the return loss is say 14dB, then the cable loss is 7dB i.e. the signal has travelled there and back.
    Hope this helps

    Reply
  17. Tomi Engdahl says:

    If you’re testing a 75 ohm cable, nanoVNA uses a 50 ohm bridge, so allow for two 0.177 dB mismatch losses at the VNA in addition to the cable loss. Not a big error in a long cable run.

    Reply
  18. Tomi Engdahl says:

    #564​ NANOVNA Coax Loss Measurement
    https://www.youtube.com/watch?v=mU71rGUKlBI

    #583​ NANOVNA Not all 50ohm loads are 50ohms
    https://www.youtube.com/watch?v=BQEXgl2xBwI

    Reply
  19. Tomi Engdahl says:

    #316: Use NanoVNA to measure coax length – BONUS Transmission Lines and Smith Charts, SWR and more
    https://www.youtube.com/watch?v=9thbTC8-JtA&feature=youtu.be

    Reply
  20. Tomi Engdahl says:

    https://www.hamradioforum.net/threads/9227-Normalizing-impedance-measurements

    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?

    Default

    Just use a 75 ohm dummy load when calibrating.

    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?

    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.

    Reply
  21. Tomi Engdahl says:

    Impedance Matching Basics: Smith Charts
    Sept. 7, 2021
    This article offers an introduction to the Smith chart and how it’s used to make transmission-line calculations and fundamental impedance-matching circuits.
    https://www.mwrf.com/technologies/systems/article/21174601/electronic-design-impedance-matching-basics-smith-charts?utm_source=RF%20MWRF%20Today&utm_medium=email&utm_campaign=CPS210910077&o_eid=7211D2691390C9R&rdx.ident%5Bpull%5D=omeda%7C7211D2691390C9R&oly_enc_id=7211D2691390C9R

    What you’ll learn:

    Plot complex impedances on a Smith chart.
    Determine SWR from the Smith chart.
    Determine the impedance of a load at the end of a transmission line.
    Identify impedance-matching component values from the Smith chart.

    Reply
  22. Tomi Engdahl says:

    The NanoVNA and Some of Its Many Uses in the Ham Shack

    https://m.youtube.com/watch?v=FLL2haqvyfM

    Reply
  23. Tomi Engdahl says:

    Impedance Matching Basics: Smith Charts
    Sept. 7, 2021
    This article offers an introduction to the Smith chart and how it’s used to make transmission-line calculations and fundamental impedance-matching circuits
    https://www.mwrf.com/technologies/systems/article/21174601/electronic-design-impedance-matching-basics-smith-charts?utm_source=RF%20MWRF%20Today&utm_medium=email&utm_campaign=CPS210910077&o_eid=7211D2691390C9R&rdx.ident%5Bpull%5D=omeda%7C7211D2691390C9R&oly_enc_id=7211D2691390C9R

    What you’ll learn:

    Plot complex impedances on a Smith chart.
    Determine SWR from the Smith chart.
    Determine the impedance of a load at the end of a transmission line.
    Identify impedance-matching component values from the Smith chart.

    Reply
  24. Tomi Engdahl says:

    Practical Use of the NanoVNA Time-Domain Reflectometer
    https://www.youtube.com/watch?v=rEFOgp2RYu0

    In this video I use the TDR (low-pass step function) transform feature of the NanoVNA V2 to verify that I have an outside antenna problem on my roof, and not a coax cable problem located inside. This serves as a field-expedient way to identify the location of an antenna system problem that can be easily done with any NanoVNA.

    0:00 Preface
    1:22 Background Info and TDR Setup
    4:52 SOL 1-Port Calibration
    5:54 TDR Distance to Fault Measurements
    9:20 Pre-Maintenance Return Loss Checking
    11:07 Post-Maintenance Results Summary
    12:50 End

    Reply
  25. Lynn says:

    Nano VNA is one of the most interesting items I’ve ever purchased. I’m amazed how much that little instrument can do. One of the best purchases I’ve ever made. I bought it off a website a month or so ago as an alternative to my SWR Bridge. And now I’m finding a bunch of other stuff it’s able to do. Who ever invented the thing deserves a Nobel Prize in Electronics if there is such a thing. If not … give him or her the Nobel for Physics!

    Reply
  26. Lynn says:

    Nano VNA is one of the most interesting items I’ve ever purchased. I’m amazed how much that little instrument can do. One of the best purchases I’ve ever made. I bought it off a website a month or so ago as an alternative to my SWR Bridge. And now I’m finding a bunch of other stuff it’s able to do. Who ever invented the thing deserves a Nobel Prize in Electronics if there is such a thing. If not … give him or her the Nobel for Physics!

    Reply

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