nanoVNA - Coaxial Cable Characteristic Impedance Measurement Methods - Testing 75 Ohm Coax - VE6WGM
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- Опубликовано: 16 июл 2024
- 1. Testing the methods described in the previous video to measure coax cable characteristic impedance using the nanoVNA... this time on 75 ohm coax cable.
2. How to measure coax cable length and impedance using time domain reflectometry. (Begins at 4:56)
Reference:
Reflections of signals on conducting lines & phase change: en.m.wikipedia.org/wiki/Refle...
Mistake: At 0:28 in this video I noticed that I pointed to the 180° S11 phase point displayed on the nanoVNA and was talking about staying away from the resonant point. Well, for whatever reason, every so often I seem to mix up the S11 phase graph and the Reactance (X) graphs... and I'm sure that this is not the only place in my videos that I 've done this, LOL! So, if that was the reactance graph it would be correct. My apologies. I will be watching for this mistake more closely in future videos... 
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This has to be the BEST demonstration and explanation that I've ever seen !!! I wish back in the stone age I had teachers that explained things like you have in this video.
73 N8AUM
Thank you. I’m glad you found it helpful :-)
Nice work. A very good job of applying basic physics to the problem of identifying coax cable impedances.
In my tool box I have a set of 600 Ohm, 300 Ohm, (just in case. It's usually flat, but I've also seen a tubular version that might fool someone,) 95 Ohm, 75 Ohm, 50 Ohm, and 25 Ohm carbon composition (almost no inductance,) resistors with stranded leads, heat-shrink, and labeled with their values, with alligator clips on the ends that I can attach to the far end of the coax cable once I have it set up as you have. Knowing the coax is almost certain to be one of those characteristic impedances I only need those values. I start with the 50 Ohm, then try 75 Ohms if necessary. They usually do the trick in seconds, but I've occasionally had to try the other values. 600 Ohms sometimes pops up for audio work.
I found it handy to have them prepped in advance for when I'm building something and go through my random cable scrap box looking for a particular impedance cable, and short pieces seldom have the cable ID on that particular piece. That way I don't have to take the time to set up and tweak a pot or trimpot, especially as pots are always inductive to one degree or another anyway. I thought of building a small, battery-powered pulser with switchable output impedance as part of a cable impedance test kit, but I never did. I'm lazy. :)
Nice! Thanks for sharing :-)
Very Good presentation and the comparison of using the NanoVNA and TDR really was good.
Thank you
There is another advantage of measuring the capacitance and inductance near the 90 degree points. There the impedance is roughly similar to the internet impedance of the vna which makes the measurement more accurate. If you go further away then both measurements get more reflection which costs you accuracy. Distinguishing between a lot of reflection and a bit more it's difficult while the difference in impedance to cause that effect is quite significant
I agree.
Thanks Gregg, another great presentation...
Thanks Gregg, back in the 70's I used my Tek 547 and it's built-in calibrator to adjust the pot. Note that your rg11 can also be quickly measured on the Smith chart with a 50 ohm term. That will make a circle around the 75 ohm point on the real axis.
73 wa7krn
Thank you for that. I will have to experiment and try it out :-)
@@ve6wo Some interesting points about the Smith chart you may already know:
- If you sweep any xmission line with the vna you will get a spiral circle that actually spirals down converging to the Zo of the line. If the line is long and/or lossy enough it doesn't matter if it is terminated and will converge to effectively a dummy load at the Zo of the line. This is why the SWR gets better on lossy feedlines.
- You can use the nanovna to readout the Zo by adjusting the frequency for the cursor to the 90 deg point (top or bottom of the circle) and noting the resistive value.
- The circles represent half-waves of line length with a quarter-wave at 180 deg. A quarter-wave is known as an impedance inverter at this frequency. You will see a half circle starting at the center of the chart (assuming a 50 ohm term on the line) and crossing touching the real axis at the line's input Z (real valued) at that frequency. The Zo is the geometric mean (sq root of product) of those two values for a lossless line. If you know the Zo of the line you can compute the input Z as Zo^2/RL.
73 wa7krn
@@danbaker5474 very nice. Thank you for sharing :-)
Thanks for sharing. Very informative
This is very good, thank you.
I watched another video about measuring coax length but he didn't explain how he reached his figure and it didn't make sense to me. Even his reply to me was that knowing the time and velocity factor enables me to calculate length. It still didn't make sense and still didn't work.
I now know because of your video it's, c x t x Vf.
G4GHB.
electronics math is vry fascinating interesting and therapeutic and vry helpfull in everyday amateur work
....tyvm for ur posts...73s
This was nothing short of amazing! Very interesting and relaxing to watch. Amazingly informative. Definitely makes me want a signal generator and O'scope again. 73 KN4EFP
Thank you.
Thanks a lot for your share !
Very useful
how can we get 1/4 and 1/2 wave length of a coax for a coax, for perfect phasing harness and baluns
Brilliant! AC7DN
Hi Gregg...great vid. I’m looking for a portable scope like the one you are using. Did you look at the JDS2022a before going with the 2023. The 2nd analog channel seems more useful, although a built in generator is handy. Just wondering what you would buy if you were buying one today...thx and 73’s from VE7 land
I wanted (and need for work) the signal generator and scope in one, so if I was to buy one again, it would be the same one :-)
Hi:
Does this method also work for ladder line or twin lead?
I have not tried it, sorry.
Perhaps another operator can take what’s been shown in this video and apply it to measuring non-50 ohm transmission lines, and then post a link to their video?
hi do you have the step in a pdf file ?
I do not. Sorry.
You could be a Mr. Carlson voice double! We’re you two separated at birth?
Hahaha! This is quite possible. Doppelgänger!
Nice video explanation。73 BD7NJB
Just a minor point. You say Ns but write Us. You do use the correct exponent in crunching the numbers.
Yes, I realized that mistake when I was editing the video. Didn’t think it significant enough to re-shoot, so just left it alone :-)