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

[@tomsmith3045]

This was a great demo, especially the use of the variable resistor to show termination resistance. Very cool to see that.

[@SuperJetjockey]

I am addicted. I watched a large number of your Oscilloscope videos and learned, or re-learned something in every one of them. Great work.

[@TheKnaheesh]

Me too

[@stevejagger8602]

One of my jobs as a broadcast tv engineer was sorting out which section of camera triax cable between outside broadcast vehicle and the camera position was open or shorted on long runs where lengths of 100, 200 or 400 metres were combined to make up the desired length. A TDR would have made the identification of the bad leg so much easier, and often thought about building one. Instead I made a a simple continuity tester which could detect open and shorted conductors on each cable before installation thereby saving the long march - usually in the rain on a golf course - with a camera testing back to the OB truck length by length. Whatever the method of fault detection it still required an outdoor repair with a gas soldering iron.

[@remphoto]

I just want to thank you for your wonderful style. You've managed to clear a lot of cobwebs. And, it's always a bonus to watch tutorials with the same scope I have! Makes things so easy!

[@ElectronixRUs]

As usual, top quality video. Like I said before, this gentleman should be a textbook example of a lecturer/tutor at universities.

[@jeffreyyoung4104]

I built a TDR circuit that was published in QST magazine decades ago, and soon after, my boss bought a commercial unit for the shop! I still have the unit! And I have coax to test as well, so, yes, I still use it!

[@deweywsu]

Thank you for publishing this! What a great set of insightful tips that de-mystify the concepts around transmission line theory!

[@Luzt.]

Sir, you are a teacher equal to my all-time-favourite Richard Feynman. Thank you.

[@w2aew]

WOW! Thank you!

[@moustaphamuhammad7013]

@@w2aew you are actually like him in our minds. Thanks a lot it was a great help from you.

[@josegermanparra6064]

Many thanks! Using the technique to finding a fault in one of the coax in my house! thanks again for making the lockdown very productive!!

[@wk2w168]

Built the 74AC14 generator and tried it out. Works like a champ. Great little project. Thanks very much!

[@warywolfen]

Back in the 80s, when I worked for KUAT in Tucson, we used that method to find a dent in a length of 3 1/4" rigid line, which was several hundred feet long. But we didn't use a jury-rigged scope/pulse generator! No, it happened that the station was owned by the University of AZ. So we borrowed an honest-to-goodness time domain reflectometer from the physics department. About that time, the physics dept. also bought us a spankin' new 50 KW dummy load, so one of their grad students could use it for his PhD dissertation project.

[@cdrive5757]

How big was it and was it oil cooled ?

[@chrisr069]

The pot on the end of the cable blew my mind.. thanks for posting this video! Gosh, I want a scope now...

[@Neilrrc37]

For viewers out there looking for information on how to use your test equipment, like a Oscilloscope and a RF Generator. There's a old book published by Tab Books in 1977 by George Leon which is still available through Amazon Market Place. I bought it back in college 20 years ago. All kinds of useful test jig circuits to make measurements with a AF or RF Signal Generator, Highly recommended!!

[@petepnut]

Excellent! I have two little Boxes, one with a 470 Ohm Linear Pot with an approximate scale, the other with Switched Resistors at 50, 75, 93, 100, 150, 300 and 600 Ohms - both are so useful when dealing with known lengths of unknown Co-Ax or Twin.

[@JackZimmermann]

I keep going back to your videos for information. Your doing us all a favor. Thanks, I've learned a lot from your videos.

[@rfjgfude]

Great video! TDR in network analyzers is super expensive, I like this method simply to get an estimate. Great job!

[@FEPLabsRadio]

Man I just found this video today. I figured I could do this kind of thing, I just hadn't sat down to work it out. I'm using a Siglent 2202X-E scope, and learning the ins and out (my first scope). Outstanding video brother!

[@dennisbauer65]

Thanks w2aew, that's really made transmission lines a whole lot clearer. And as smeuse wrote Having test gear is great, knowing how to use it effectively is better! I have just brought a DSO-2090 USB PC-Oscilloscope, and will follow your tutorial step by step Thanks mate.

[@carriersignal]

Thank you for all your time in doing these very helpful videos.

[@jasonhoffman6642]

Great tutorial! I love the speed at which and the depth to which you explain things. I'm going to build one of those Schmidt TDRs just for the heck of it (the parts are all floating around the table somewhere.)

[@dave-d]

Excellent vid. Great explanation and clear demonstration. S.I. units would be preferred by most students. Thank you very much.

[@tomsmith3045]

nS and Ohms ARE SI units. It's only the feet that weren't, and that's trivial to either convert or understand.

[@PeterWMeek]

Great way to determine the impedance of an unknown bit of coax. You can also calculate the VF for an unknown bit of coax. Take a measured length and back-calculate what the VF *must* be to get the measured delay from a known length. Thumbs up for bothe the #37 and #88 videos. (this comment posted on both videos)

[@w2aew]

+Peter W. Meek I did a video on that too (measuring velocity factor), exactly as you've stated. kzbin.info/www/bejne/iqGsepmqq7aDfK8

[@ashleyperez5353]

N

[@rolandbernold8004]

What amazing practical way to determine the length and the impedance of a coax cable.

[@sdscotto]

Another excellent video! Thanks for keeping the tutorials flowing.

[@pradeeplanka6976]

A great explanation in a short time which cleared my mind from lot of misunderstanding about cable impedance. Thanks a lot. I will keep on watching your other videos.

[@TheCrazyStudent]

I just realized that my question about testing other types of cables has already been answered down below. :) Thanks again for all your vids. Keep up the good work.

[@tigersaw]

I just tried this in the real world, and it works. I found to get a more accurate measurement on smaller cables, first I connected a long length of coax of approx 20m, set cursors, then added the cable under test and measure the amount the 'step' has increased. As we tend to use metres in the UK not feet, the increase in nanoseconds just happens to be close to the number of cm of cable. i.e 100ns equates to 9.9m which is handy. caveat - assuming both cables same impedance and velocity factor or 2/3

[@RadioHamGuy]

Excellent video, it is much easier seeing it done on a video than reading about how to do it. Thanks.

[@AH-wk1id]

This video is the reason I got my Extra ham license! There is a question on the current test about adjusting impedance to get a perfect square wave. Thank you.

[@cdrive5757]

Performing TDR Using a Pulse Gen and Scope was a crowning moment in my shop. I always considered a scope to be the most important and useful instrument in any electronics shop but TDR added another dimension. It seemed like breaking through a barrier of sorts. Or rather having a revelation of just how damn indispensable a scope is for any true ElectronHead! When seeing a reflected wave for the first time I damn near came in my pants! I was having a discussion with a generation Z (EE) that commented in one of the many "Don't Blow Up Your Scope" videos. He lamented that the scope's GND topology was a pain in the ass. He complained that they're not a Differential Input. He further lamented that using the algebraic ADD function of a multi-trace scope eats up available inputs. I told him that this generation is spoiled and they've never known a life without cell phones. A public telephone is something seen in old movies. I have no such complaints about the input topology of a modern scope. Think about it... A 100 Mhz scope can accurately measure

[@w2aew]

I've even used this technique to measure speaker wire - see this video: kzbin.info/www/bejne/sGe4e4Onhr6thMU

[@cdrive5757]

@@stevejagger8602 I totally concur.😉

[@ROBRENZ]

Excellent! That is exactly the type of presentation that burns a valuable light bulb moment into my brain. Thanks, Keep them coming!

[@czarekg]

Excellent tutorial. Your videos are the most resourceful. Thank you for sharing your professional knowledge and experience. Immediately subscribed to your channel to continue to learn from your fantastic lectures.

[@mefedler]

Thanks Alan for your video. I used this technique to determine the velocity factor of some coax I used for a phasing harness for stacking two loop antennas. I referenced a link to your video in the description section of one of my videos: 2 Meter Halo Antenna Part 12 -- "Phasing Harness understanding & calculations" An on screen comment is also show at 1:13 in the video. 73, Mike N6TWW

[@BrendaEM]

Thank you. The impedence-matching pot was a nice touch.

[@ElecTechie]

Hi Al .. Mentioned you in my video .. referenced this video .. My video was like an supplemental to yours. I was just showing that the energy coming back down the coax was truly reflected energy and a way to display that on the o'scope. Was just something to do .. No "reflection" intended LOL Thanks for all the nice videos

[@OctavMandru]

I keep coming back to your videos; thanks for the education you give us. Please Alan, use metric 😀

[@46bovine]

Metric, smetric! I like stating things like velocity, km/hr? No, how about fl/fn? Furlongs per fortnight not kilometers per hour!

[@Debraj1978]

Thanks for the enlightening video. It helps immensely. Very practical and lucid.

[@Afrotechmods]

Genius!!

[@Whopperjaw.]

Where have you been?

[@jimmysyar889]

Heyyy another one to the list

[@manya555]

I tried a similar setup to yours and I used a 2V p-p square wave at 1.43MHz (700nS) with 50% duty cycle based on the settings on your scope screen (video 4:30). My 50 Ohm RG58 coax cable measured about 7 inches shorter than the calculation which is pretty good. I found that if you measure the single pulse when the end of the cable is shorted (video 9:30) it is possible to get an estimate of the cable length as well, not the same figures though.

[@acestudioscouk-Ace-G0ACE]

I'm just learning these things as a returning ham after a 30yr lay off. So this was very useful, thank-you.

[@smeuse]

Keep these videos coming! Having test gear is great, knowing how to use it effectively is better!

[@doubleHLabs]

is the cable velocity factor normally listed on cable datasheet?

[@w2aew]

It usually is, for RF cables at least.

[@manya555]

I also noticed that the cable between the generator and the scope needs to be the same impendance as the cable under test (50 Ohm) as well to get a clean signal. I repeated the setup using 75 Ohm cable and it worked out similarly. Need to learn more about the internal terminating 50Ohm / 1MOhm (High Z) settings on the scope and generator.

[@typedef_]

I once heard an analogy somewhere regarding reflections in transmission lines. It had to do with the fact that the signal was traveling through a medium with certain properties and then it hits a medium with different properties. It's like when you look out the window sometimes and you can see what's outside but at the same time you kind of see your reflection in the glass. Some of the light coming from inside the house is traveling through air and all of a sudden it hits the glass (different medium) and it doesn't like that so it bounces back. The same can be said about sound waves when an echo occurs. It all boils down to the fact that signals of whatever kind don't like changes in properties of the medium. Another example could be a prism which divides the spectrum of light (that is a refraction actually but this also applies to signals in transmission lines i.e. they are attenuated by a factor related to the refraction angle I think).

[@wk2w]

Fantastic video. Perfect mix of theory and practical application. Thanks very much.

[@jonnyb1963]

VERY NICE EXPLANATION!!!! I was searching around trying to find a good VISUAL aid for reflected power and impedance for my little guy to understand... I cant wait to demo this for him! (a young ham to be) 73! Ke8cu

[@mitchellspanheimer1803]

If you're trying to measure the location of a short, where exactly should you put the cursors? I want to be as accurate as possible.

[@w2aew]

At the mid-point of each edge.

[@fjs1111]

Wow, never thought of using an o-scope for this. so cool

[@mevmevmev]

Thanks for this video. I have made a spreadsheet to calculate length based on time and velocity factor, and velocity factor based on time and lenght. Is there a formula to calcualate the cables characteristic impedance based on these measurements?

[@microflite]

Great video!

[@helipilot727]

Fantastic video, probably the most I've learned on youtube in 10 minutes.

[@manya555]

Nice video, in textbooks the waveforms look different, doesn't show the doubling. What modification needs to be made to the setup in order to get waveforms similar to Time-domain_reflectometer on wikipedia

[@TheCrazyStudent]

Awesome video! I had never heard of this technique before, and didn't even know it was possible. The calculations went by a bit too fast for me, but I'll watch it again and see if I can catch if better. A question: This method should be possible not only with coax, but with other cables as well, like stereo cables (RCA) for example, correct?

[@julietjohnson8049]

I have a question about a TDR. If one end of a cable is connected to a TDR and the other of the cable is left open, you will see the point where the incident and reflected pulses first add together. If you continue to scroll in time on the TDR past that point, you never see the reflected pulse by itself. Why is that? It is almost like the incident pulse has an infinite pulse width. (Please let me know if I gave enough detail in my question.)

[@saras9420]

Hi sorry for this basic question but it seems your pulse has a period of 700 ns aka 1.4 MHz at 0:41 but then why do you say the pulse is 100 kHz? It is keeping me from understanding the meaning of your demonstration.

[@w2aew]

Sorry - I misspoke about the frequency of the pulse.

[@w2aew]

@UCEyZw6FUEW3b_rqZ5Yf24dQ Hello Sara - This is pretty close. The pulse generator creates a 4V pulse internally which is applied to the 50ohm internal resistor which is in series with the transmission line. The transmission line "looks" like a 50 ohm resistor for voltage *changes*. Thus, the 4V is divided in half, where 2V appears across internal 50 ohm resistor, and 2V across the line. This 2V leading edge of the pulse propagates down the transmission line (and is seen by the scope as it passes by). When this 2V edge of the pulse hits the end of the transmission line (which is open), the pulse reflects the full 2V back in the other direction (superimposed on the 2V already on the line), so you get 4V total at the open end. The reflection (like a wave hitting the edge of a pool and reversing direction) propagates back towards the generator. When it passes the scope on its way back, you see the increase from 2V to 4V at the scope. When the reflection makes it back to the generator, it is terminated by the 50 ohm resistor - so now the 50 ohm resistor has 4V on each side of it -which means there is 0V across the 50 ohm resistor - thus, no current flowing. This makes sense because after the pulse edge makes the round trip, the end of the transmission line is open, thus no current flows. This video on transmission lines and reflections might help you: kzbin.info/www/bejne/nZDNqZtmhsqSfLs

[@saras9420]

@@w2aew Thank you so much!

[@zachreyhelmberger894]

I had a rabbit bite through the outer shield of my Wilson cellular antenna in a few places (RG58 cable) but the inner conductor is fine. Is there a simple way to test the cable to see if repairing the bitten portion is justifiable? I don't want to damage the cell phone with any trash coming back from the punctured parts of the shield.

[@Nikkuuu69]

Thanks for the video! I do have a question. I often see coax rated at 50 Ohm impedance, but isn't electrical impedance dependent on the frequency? How can the impedance be the same between a 100Khz and 100Mhz sine-wave for example?

[@w2aew]

Because the impedance is effectively determined by the ratio of the distributed series inductance and distributed shunt capacitance. While the individual reactances of these quantities is frequency dependent, their ratio is independent of frequency. Of course, the wavelength at 100kHz is so long, that the effect of coax impedance isn't really important unless the line is very long.

[@gvbarve]

*****

[@mavamQ]

To add to that, and I just realized this, so I hope I'm correct, it keeps the 50 ohm characteristic over frequency, because as the frequency increases the Xc of the cable capacitance decreases and the Xl of the inductance increases. This keeps the impedance of the cable at 50 ohms.

[@azzy314159]

Very nice video. Better than guys with toys fumbling with the knobs. Could use an 8112 pulse gen with a very narrow, fast rise time too? Makes for a TDR setup.

[@manya555]

So I assume you mean it needs a short pulse (low duty cycle) such that the pulse width around 10nS (wikipedia scope). I will try that using a PIC Microcontroller with PWM.

[@erflb]

I'm amazed with ths explanation! 👏🏻👏🏻👏🏻👏🏻👏🏻👏🏻👏🏻

[@Skipperj]

I just traded a battery charger for a BK Precision 2120.Thanks for the video(S) all of them.You are a good teacher and I'll check out all of the them.I work in a diesel repair shop,and I'm hoping to start testing the electronic injector systems.Do you have anything along those lines? Thanks,Skip J.

[@electronicatutorial]

So what are the differences between this test and what a TDR can do?

[@electronicatutorial]

What are the main differences between this test and what a fancy TDR can do?

[@icanacoustics-noisevibrati624]

Beautifully explained. Well done and thank you.

[@electronicatutorial]

Hi, is it possible to do something like this but with fiber optics lines? so what kind of set up could be used instead of using an OTDR?

[@saintfather7757]

I have a question. Does it effect to quality of transmission if you roll you coax cable. Is it ok if I leave 20-30feet and roll it when I go from the transmitter to antenna(150feet)?

[@w2aew]

The only affect would be the added attenuation from the additional length (coiling doesn't matter). The additional attenuation/loss will be fairly small in most HF applications.

[@saintfather7757]

w2aew thank you

[@VoidHalo]

Eugene Khutoryanski has a good animation on his channel that gives a good hand in gaining an intuitive understanding of transmission lines.

[@CodfishCatfish]

I got a bit confused heellp you specified %66 11.8 /1.66 = 7.108. Why is the figure 7.79 inches per ns Thanks for any reply Spence

[@J4e8a16n]

:o) Is it possible to adapt a 75 Ohms cable to a 50Ohms oscilloscope output without distortion? With the TDR I found Iwas using different resistance cables. JP

[@fifaham]

@2:50 Parasetic LC distributed circuit

[@Orcinus24x5]

Awesome video! I learned two more practical scope/function generator applications today! :D

[@ashrefd]

hi. i still have more problems understanding the impedance. really i will have to measure the impedance in earthing system in order to improve electrode performance.. will you advice me or give an example regarding that?? kind regards

[@spectralcodec]

Thank you so much for this, I see my scopes in a whole new light now!

[@andrefigueroa4448]

Nice video. Really helped me a lot!

[@opablo_gm]

brillant.... thanks... I will do these tricks here with a few coaxes for the sake of learning and trying to completelly understand the concept of impedance in a coax... For some reason It's quite a complex topic for my mind to understand... :/

[@OneCoolDude08]

Is the graticule really that ugly, or did the camera add in that bright red box around the perimeter of the screen due to lighting?

[@kingsonIAH]

What is the reason at 09:20 the base level is lower than 0V?

[@w2aew]

It's not really lower than 0V. I have the scope coupling set to AC, so the DC content is removed.

[@kingsonIAH]

@@w2aew Oh, I mean more that the base level, in other words the low part of the pulse signal, is lower than the base level when the impedance are matching.

[@w2aew]

@@kingsonIAH It only _appears_ that the base level is lower when unmatched. This is due to the AC coupling on the scope. If I had used DC coupling, you'd see that the base level does not change between matched and unmatched case.

[@ElPasoTubeAmps]

Very nice demonstration. Thanks.

[@fabts4]

Does it work only with a coax or could you apply it to other conductors as well?

[@w2aew]

+fabts4 It has to be a pair a pair of conductors that has a fixed physical relationship (coax, twisted pair, etc.) so that there is a constant RF impedance along the length.

[@fabts4]

+w2aew alright, thanks.

[@eigenvector7035]

Nonmetric speed of light looks very interesting.

[@miguelm203]

Nah... Metric rules 😁

[@OuterValence]

In the lumped parameter model, I find it useful to show a symmetric model with distributed inductance in the ground. This way the students don't fixate on the concept of ground as a magical black hole where things always return to.

[@galalon2417]

Fascinating. Inspiring. Insightful. Thank you for this video.

[@mikechristie3131]

Thank you for the Oscilloscope videos. Is there a way to use an Oscilloscope to look at my audio coming into my Ham FM repeater. I'd like to improve the output of the audio coming out of the repeater.

[@w2aew]

You can look at the audio, but you won't really be able to tell much about its quality (unless it is severely distorted). The problem may be with the deviation settings in the transmitter, which you won't be able to see on a scope.

[@uiticus]

Excellent video! But for a shorted coax end how do you measure where the length of the short is. Is it from negative peak to negative peak of the inverted pulses?

[@w2aew]

At 9:29 in the video, you see what it looks like when there is a short. Basically, the width of the visible pulse is equal to twice the propagation delay to the short.

[@uiticus]

@@w2aew Is "the width of the visible pulse" that you mentioned, The positive rising pulse that we see on the scope, its pulse width that measures approximately 100Ns? And if it is, then you divide 100Ns by 2 to calculate the length to where the short is?

[@w2aew]

@@uiticus Yes. At that point in the video, you can see me adjusting the load at the end of the coax - all the way to zero (short).

[@uiticus]

@@w2aew Thank you! I appreciate it.

[@Scuba_Bro]

Excellent explanation! I made my interns watch this one 👍🏻

[@waynegram8907]

what's the difference between twist wire shielded cable compared to Coax Cable? Because the cable's capacitance is LESS using twist wire shield of the same diameter size as the coax cable will have MORE cable capacitance. I'm not sure why Twisted wire plus the shield will have LESS capacitance compared to using Coax cable.

[@KeenanTims]

Awesome video. I guess this is poor man's time domain reflectometry? How constant is VF in a practical sense? Is it useful that you can measure that this way as well? The imperial units though *shudder*

[@kolabuzlu]

Hi. I've tried everything described in this video, but waveform does not change when I insert a coax cable to the T adapter. Where am I missing? I use low cost oscilloscope and low cost signal generator. I always see a square waveform. Input impedance says 1MOhm on the oscilloscope and it is not changeable. The signal I see is perfect. What is the problem? Please help. Best regards.

[@w2aew]

The problem is either that the rise time of your pulse generator is too slow, or the bandwidth of your scope is too low, or both. With a slow generator and a slow scope, this technique will only work to test very long cables.

[@billwilliams6338]

Thanks. The lower the AC power cord capacitance means the capacitance from the shielding braided wire to the White ( Neutral ) and Black ( Hot ) wires? or the capacitance value of the wire insulation that you strip off to see the strained wire?

[@niknam.]

thank you for this simple impedance measuring method!!!

[@billwilliams6338]

Thanks, Since you get the amp currents how do you calculate the wire gauge? Is there a Chart that tells you the wire gauge for the current amps?

[@Kc8qdq]

Can you do this test with one of the less expensive scopes ?

[@w2aew]

Yes, but only for longer lengths of coax. You need a fast risetime (on the signal generator and the scope) to resolve shorter distances.

[@Mr.Leeroy]

Thank you! What would be the simplest but appropriate for the test 100kHz circuit in case I am unable to use a proper function generator right now? Edit: I guess from next video that would be "#88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter"

[@w2aew]

Or even simpler... kzbin.info/www/bejne/sGe4e4Onhr6thMU assuming you have a digital scope...

[@Shawn_White]

Thanks I never knew why you need to terminate BNC connections before watching this video makes sense now.

[@billwilliams6338]

Do you know what material they use for the outer jacket to be water resistant, because AC cord says its water resistant but the white wire, black wire, green wire sleeves jackets look the same but the outer jacket has some material to protect it from water

[@xiatian7566]

but before doing this experiment, how do you setup the impedance match network at the pulse generator if what you want is to measure the character impedance of the coax cable?

[@w2aew]

The impedance match at the generator really isn't necessary. There won't be a reflection from the far end when the potentiometer is adjusted to match the line impedance - regardless of the source impedance.

[@rfn944]

I think this is just what I need to find a high resistance in a 200 ft buried arcnet coax cable. The cable has two center conductors. One is copper and the other is silver in color with a braided shield of course. The shield and silver conductor read close to zero ohms end to end, but the copper one has 100 ohms of resistance. Would like to find where the high resistance is. I believe the cable impedance will be close to 100 ohms as that is the value of the termination resistor. Would I see a spike at the point of failure, or what else could I expect to be there? Thanks

[@w2aew]

rfn944 If it is an abrupt failure point (not a distributed region of higher resistance), then you should see a reflection from it.

[@doubleHLabs]

What value is the pot you're using here? I'm guessing if you're dealing with 50 and 75ohm cables, you'd only need a 100ohm pot.

[@w2aew]

it was a 100 or 200 ohm pot.

[@metallitech]

Any book recommendations for understanding reflection etc in the cables? It's a bit beyond me.

[@mikeobrien7724]

Hi! I found this through googling for other things. Let me sum up my issue and maybe you can help... I have a cable pair that can be from 2000 ft. to 4500 ft. long that carries 75v DC. This cable can get a short or open at any point. is it possible to use something like this to determine distance from measuring point to said open/short. Also, how would I determine that? I figure factors such as total number of connectors, cable length, etc... all play a part. I was looking into a TDR (time domain reflector) but I wasn't sure if it would work for this specific application yet.

[@w2aew]

If there is a consistent distance/coupling between the conductor pair you're testing, then a TDR could work. If the coupling is random (i.e. - not twisted pairs, random positions inside the cable, etc.), then the impedance won't be sufficient to give you a good result.

[@mikeobrien7724]

@@w2aew So it's a two stranded wire pair that runs around that whole distance. They're not necessarily a twisted pair, but they're stuck together. Forgetting the gauge atm but not sure if that matters.

[@w2aew]

@@mikeobrien7724 That should give you a consistent enough impedance (probably between 100-150ohms) that a TDR should work.