NanoVNA measuring velocity factor and cable length

[@KjartanAndersen]

A tip. Never use auto focus when you film an instructional video where you routinely is touching the equipment or in any way put something in the line of sight of the object. Better to lock the focus on the object. Or the table in your video. When measuring DTF or VF it would be best to not mix cables like you do. Your adapter cable has a VF of 66 % while your CUT has a VF of 78 %. Since your adapter cable is short, this isn't a big error, but none the less an error.

[@wb7ond]

Great video, one of the best explanations on what that "phase saw-tooth" is really describing. Sent here from NanoVNA io group posting. Thanks for making that video. I am anxious to try this out... Regards wb7ond...

[@wildbill1]

Excellent video thanks for posting. Been trying to understand how to measure velocity factor of some 75ohm coax. Your video helps a lot

[@Miller7747]

If you use manual focus, and set the distance equal to distance from camera to NanoVNA you will get perfect focus every time, regardless of what you do with your hands.

[@turiluis]

exelent explanation! thank you to take you time for this!

[@DrunkPacifist]

Second time I use this video to compare cables length, very useful

[@WibisonoCahyadi]

Thank you very much for share the knowledge. Very clear explanation.. 👍🙏

[@k8osradioscience545]

Well Done! Many Thanks!!

[@jbx907]

cool video, i was thinking of directly using the nanovna to make phasing harness for array antennas where i need multiple quarter wave 75ohm of a specific freq for a phase harness of stacked col-linear antennas, 1/4 3/4 etc.

[@TalpaDK]

As this is an education video wouldn't it have been nice to have described that the calculation was and included the units? Something like "wavelength = speed_of_light / the frequency" or 299792458 m/s / 61MHz here. And for common usage you can just divide 300 by the frequency in MHz.

[@atkore]

Thanks for your clear explanation. Manufacturer's side: A part of calculations could be automaticaly displayed by entering our own variables (Without computer). Maybe an update. 73.

[@kavindugilshan]

So it means antenna element should be 78% shorter ??

[@georgeveropoulos2489]

This method work only for long cable ?

[@izzzzzz6]

Could anyone help me with how i can do this for an open wire or balanced feedline. I would like to measure the velocity factor and impedance of the home made feedline or ladder line. I would also like to be able to then compensate / calibrate for the balanced feedline length on the vna so i can hoist it up 100ft to the antenna feedpoint and then read the parameters of the antenna back at the tuner end with the VNA. From what i understand i will have to make a 1:1 unbal and a 1:1 balun to be able to calibrate the vna with the open wire feedline attached and then be able to look at the antenna with the vna attached at the transceiver end of the balanced or open wire feedline. Problems could include a difference in velocity factor of any pigtails used to get to and from the 1:1 unbal and 1:1 balun. For example the pigtails could have a velocity factor of 66% and the open wire might have a velocity factor of 88%. The mission is to build a balanced HF antenna, probably inverted v or doublet, then measure it's parameters at height (100ft high feedpoint). After this, design and build a ladder line as best matched in impedance to the antenna as possible and then finally to be able to measure the impedance back at the tuner in order to choose the best ratio balun for the tuner in order to go back to 50 ohms. Just trying to get a vna to do all these things and in the correct manner and order does not seem easy to me. lol One solution i thought of was to hoist the vna up to the feedpoint to either connect back to me, either via powerful bluetooth, with a complicated array of usb extensions or by observing it with a camera such as a gopro. or to first use coax and calibrate the vna at the end of the coax, then use coax to observe the antennas parameters in order to find out what parameters my ladder line or balanced feedline should be to best match that antenna. finally to build the ladder line and then to measure it back at the tuner to help choose the correct balun. I have some 300 ohm ribbon feedline which i could initially run up to the feedpoint but it's all starting to sound complicated just so i can observe the antennas parameters at height and then build the ideal balanced feedline as every open wire feedline has it's own impedance depending on spacing and wire diameter. Ideally i want my feedline to have the same impedance as the antenna. So step one will be finding out the antennas parameters at height, step two decide on feedline impedance and try to build it in order to obtain an impedance matched to the antennas impedance once built. Step 3 will be to measure total impedance of the system at the tuner end and step 4 will be selecting the best ratio balun for the tuner to convert back to 50 ohm unbalanced. If anyone has any ideas on this i would much appreciate any input. Thanks.

[@Rlaay]

How do I apply this procedure to measure the VF in a vertical antenna? Do I have to take the length L of the antenna or 2L?

[@RomanKuechler]

Strange. I get a VF of about 0.79 with a cable considered to have 0.66? (It's a RG174 coax cable). Therefore the lenght calclation is wrong too. Anyway; it's a very interesting video! And I would like to understand it deeper. For example, why are the 61 MHz (171 MHz - 110 MhZ) you calculate with an essential number to calculate with? I have understand, that every 61 MHz the phase makes a full 360° turn, but I don't understand what a 360° turn in the phase during a frequency sweep has to do with the wave lenght? Would be glad you could explain the background to this calculation. Thanks!

[@pixonian]

I just picked some arbitrary frequency values where the phase shifts full 360 degrees, but can be any two. Not a very precise method, but shows what's happening. Consider the cable above if the frequency changes from 0 to 61Mhz. We start with a very long waive that gets shorter and shorter, and at 61MHz the entire one period will fit in the cable. The length of that one period at 61Mhz is 300/61=4.9m. At that length the phase shift makes the full 360 degrees, this is just an indication that the entire wave fits nicely inside the cable.

[@RomanKuechler]

@@pixonian After a long time, I got stuck on your very good video again because I wanted to determine the velocity factor (VF) of the two short adapter coax cables that come with the NanoVNA. I noticed that you include the 10 cm length of the short adapter cable in your calculations in your video? This doesn't work for me and leads to an incorrect result. Isn't it the case that after you have carried out the *_full_* calibration for S21 measurements (i.e. open, short, load, isolation, through), the short coax needed for this is calculated out by the NanoVNA, means _after_ the calibration it is "zeroed-out"? From then on, the measurement plane of zero *_includes_* the short adapter coax, right? If I do it like I say, the calculation works and I get a velocity factor of 73% for the two short adapter coax cables. Am I wrong?

[@e63acd52]

I keep ending up with only half the value that I'm expecting for the velocity factor. Do I have to consider twice the physical length of the cable, due to reflection? Doing a measurement on a S11 port on a VNA

[@pixonian]

If you use the TDR (time domain reflectometry) with a cable connected to CH0, the software automatically takes into account the "round trip", so no need to change the readings. What are you getting/expecting?

[@nikttaki9068]

I have a identical problem, after meauserments i have only 0,4 or 0.41 score, not 0,7 or more.

[@nikttaki9068]

I have such an example and I am following your description strictly. I want to determine the shortening factor of a 75 ohm coaxial cable. I have measured the exact length of 7.835 m of cable, which I connect to a calibrated NanoVNA 2v4 and read the results at 0 degrees phase. I get: 136MHz and 120,800 MHz, these are the places where the wave passes through phase zero. Difference: 15.2 MHz. I divide 300 / 15.2 = 19.736m. I now divide 7.835 / 19.736 = 0.3969, and the manufacturer says about 78%, which is twice as much.

[@zeljkoadzic1692]

Can you make a new video with new frimware from DiSlord 1.3.25 with new features

[@best3140]

Can we measured single core cable velocity?

[@arconeagain]

Doesn't the pigtail affect the VF measurement?

[@vidasvv]

YES, and so do the connectors and adapters but if the coax is long the result is barely noticeable. Just the opposite if your measuring SHORT cable. When using TDR method you will be able to see all connectors as little bumps on the horizontal axis. Have fun, 73 N8AUM

[@muppetpaster]

The longer the cable , the smaller the effect (relative)

[@Lareo1964]

to autofocus or not to is the question

[@sanmeethundal3397]

what is the significance of "300"?

[@SteveWrightNZ]

Repeat your tests at 1GHz and watch what happens to the VF. Prepare to be surprised..

[@jonathancotner7040]

Would you use that at 1Ghz? No.

[@muppetpaster]

10:29 the size of an apparatus says NOTHING about its precision.....

[@jean-francoisavon62]

the focus makes this video unbearable. better too listen to the audio only.

[@The-End]

sorry... no focus, no video