Electromagnetic Videos - 2023-05-07
We investigate a 75 Ohm TV cable to see where the 75 Ohm measurement comes from. We also look at reflections in the cable and why you cant just connect more than one device like a TV to a signal source without loosing signal power.
I am a retired EE. I really like the way you explained the reflection polarity at the end of the line. I never thought about it that way. Smith charts don't give you an intuitive understanding of what is going on. Thanks!
Thanks - I really appreciate that! I was so lucky to have a number of great EM Profs when I was a student and thats where I got that intuitive approach from.
I actually do not understand that explanation. Is not that the free wire acts like a crude capacitor and the shorted end makes a loop for an inductance? Inductance makes a counter voltage. Capacitor just charges up and send back the charge when voltage is zero.
Excellent video! Been in the field for 40+ years and that is the BEST explanation I've seen for a neophyte.
Thank you so much! I was really trying to do it without the math to make it understandable to almost anyone.
I was a metrologist in the Air Force for 6 years. I always wondered why the cables were 50ohm or 75ohm. It’s been about 13 years since and I finally understand. Thank you.
Your welcome! Glad I solved that mystery!
@@ElectromagneticVideos you're*
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@@NoNameAtAll2 Do you have some videos?
@@NoNameAtAll2 god not the grammer not zee. Math and physics is his thing. We are not here for the spelling content!
@@rwood1995 I can’t imagine how you native english speakers can mix up your and you’re. I know it’s pronounced the same, but with a bit of logic you can differentiate them.
I must say, after the (incredibly excellent) explanation of the reflections when open and shorted, and after the WONDERFUL direct practical demonstration of "Look, if we put a resistor at the other end that 'matches the rated impedance', the reflection disappears..."
When you then brought back the longer pulse and explained "The first bit is before the reflection, so we first only see the characteristic impedance as if the line were infinitely long"... I thought to myself, "This video is genius!"
Really love the super practical aspect of it! I've learned about the theory of transmission lines before, but it's amazing to see it so concretely demonstrated with real, simple, physical electric equipment like this.
Great work! Thank you for your clear explanations! ❤
I so appreciate your comment - thanks! I'm quite thrilled at how well this approach to explaining it has worked for so many viewers. I did a very similar experiment in a lab as a student and its one of the very few that I was so taken with I have always remebered. It was great fun to do it again after so many years.
More things like this to come - mirrors, reflections from transparent objects like glass, speed of light ... . All with as simple equipment as possible.
Yeah, I loved that bit.
That is brilliant: I now understand what I was doing for all those years cabling up the college where I worked with thinnet (50 ohm) coax cables! Thank you!
Your welcome! For what its worth I did my share of thinnet way back as well. Seemed so fast back then when files were small :)
Still new to ham radio and working with 50 Ohm coax; this really helps me understand what's going on with the signal. Thank you so much! 😎👍
What cool hobby - something I always think I should get into when I have time. I have done long range digital HF communications at work in the past - quite amazing the distance that can be achieved even just a few hundred watts if the conditions are right.
Just so that anyone reading this understands, while my example was 75 Ohms, what I did also applies to 50 Ohm cable or any other characteristic impedance cable.
At some point I will cover SWR and quarter wave transformers to match different impedance's which is also pretty neat and related to this. Look it up if you haven't already! Thanks for the comment!
I have never understood reflections either in college or technician explaining to me and now I know what it is because your class were clear, objective and easy to me understand. CONGRATULATIONS, Thank you.
Thanks so much! I'm glad you found my way of explain easy to understand - certainly what I try and do!
"That's one of the greatest videos I've ever seen. Anyone who wants to deep dive into High-Speed PCB design should watch this video and the one about measuring light speed. I truly admire the video creator's efforts to expand our knowledge."
Wow - thank you so much! High-Speed PCB design - I should actually dig out one particular circuit board from years ago where I designed a couple of long impedance controlled transmission line traces for 100Mbps twisted pair Ethernet which is a great example of transmission lines on circuit boards.
I've used 75 Ohm BNC cables for networking and then digital audio cumulatively for 40 years and never got an explanation of it. Thanks for the clear and well thought out video! I do feel its better to say that 'cable capacitence rounds off the square wave signal' rather than saying it 'messes things up'. Square wave is every harmonic of a sine wave so it makes sense that the tiny capacitence will filter the upper harmonics away. Great video though - thanks! :D
Well I'm glad you liked it and found it informative.
In terms of loosing the higher frequency harmonics, for a well matched/terminated system, the cause is the changing characteristics of the components of the cable. That includes the insulation plastic between the center and outer conductor becoming more RF absorbing at higher frequencies, , and the increasing effects of skin depth making the conductors more and more restive as the frequencies go up. That being said, many cables today commonly used in our or wireless devices work remarkably well in the GHz region although the spec sheets generally show the loss/distance increasing with frequency.
Finally, now i understand reflection in even other types of cable like RS485 networks. But also the whole 75 Ohm cable spec was unclear before. Thanks, this was a clear and complete explanation! Edit: Just fixed the Afri-English 😆
Thanks! I'm so pleased the video helped provide an explanation of reelections and characteristic impedance.
I’ve been selling Video cables for 15 years , Finally Some one explain it in such a clear and easy to understand way , Thank you sir
You welcome! So glad I was able to help! This was actually inspired by a lab I did in a course I took as a student in university. I always thought looking at reflections like this really helps understand what is going with matching and reflections.
Absolutely awesome demonstration on the importance of impedance matching! Not what I was expecting when I clicked on this video but I'm glad I did. Thank you and subbed :)
Thank you so much! Welcome abord to EM Videos!
I'm getting into amateur radio, and setting up my antenna. This is how I got into impedance, and impedance matching, which was a mysterious topic to me. Your excellent video made this so much clearer. Thanks for sharing it.
So glad it helped understand impedance matching. Since you are getting in to amateur radio, consider getting a NanoVNA. For about $100 this amazing devices does the job of what used to be a multi-thousand $ piece of test equipment. It great for measuring impedance of things like antennas, determining SWR at various frequencies etc. It really brings all that stuff to life!
For some reasons this bit of TDR part of my electronic course at uni always stuck with me. Good to "reflect back" the explanation of this again.
And also I better put on the the to do list, get upto the loft and replace the two bits of cables tied together 😅 and put a splitter in. 👍 Thanks for a informative video.
Glad you found it useful! What I remember about the university lab we almost always did the lab before the theory which was too bad in terms of understanding it.
You know, if your setup works, I wouldn't go to the effort to change the connection to a splitter... "dont fix it if it aint broke" :)
I know this is an obscure topic, but great job explaining it in simple terms without fancy equipment!!!!!
Thanks! Keeping things simple was hat I was trying to do!
@@ElectromagneticVideos I think your 2ch HP scope is quite fancy 😊
@@sophya5796 Actually it is even for a 25 year old scope! I specifically bought it (used) a few years ago troubleshoot some glitches on an i2c bus - it did a wonderful job on that and many other things.
Something like a 100MHz analong scope would be fine for this demo. Or slower with a longer cable.
Such a good demonstration of transmission line theory and conservation of energy. They should use this example in EE undergrad courses!
Thanks! Actually the video was inspired by a lab a million years ago whne I was an undergrad. I always thought that lab was an eye opener!
@@ElectromagneticVideos come to think of it, I just took a class where we did a very similar demo. Since the class was online though, the voltage waves were simulated in Matlab’s Simulink. If you have any more demos from a million years ago, I’d love to see them!
@@kurtttttttt More million year old demos to come :) It get having to use simulations for online courses, but I always feel you get a better sense of things with real experiemnts. And they are more fun!
Thank you. Similarly to other ppl - I wished this was part of the explanation and demonstration I was given in my classes.
You know, I'm really amazed some variant of this wasnt done in demos or labs for the appropriate classes You may have read my other replies - this video was inspired by a lab I did as a stundent.
Thank you for this detailed video. It's always useful to know why you should use splitters and what they actually do.
Your welcome! Yes! Its a lot harder to justify the nuisance of a splitter without the "why".
This is the best explanation of characteristic impedance I've ever heard. Well done!
Thanks! Apreciate that!!!
This is an awesome demonstration of transmission lines. I'm going to make a lab for my students based on your methods. Such a powerful use of YouTube!
That made my day! This was a reduced version of a lab I did as an undergrad. If you want to make it more challenging, non-resistive loads (capacitor or inductor) can be used at the end of the lab. I think we even had to identify the contents of some mystery loads.
@@ElectromagneticVideos I stress the concept of Max Power Transfer beginning on the first day of the course. These are technology students, so not sure how they would do with the reactive dummy loads, but will consider it. I would also look to have them identify the impedance of other cables (twin lead -300ohm, and come 50ohm coax) to validate the method. This brings home that the concept of line impedance is really and not something made up.
I also like the time-domain measurements that brings home the concept of matching and reflection as it relates to open and shorted lines.
Last thing, I'm curious if given enough of cable run, could the students also identify the velocity factor for a specific cable?
Awesome! I could never understand fully the signal and reflection and why use termination resistor of a certain ohm value till now. Great work on the explanation!
Thanks so much! Glad my explanation was helpful!
Very neat, now I understand the importance of terminating open ends of unused outputs with 75 Ohm terminal resistors!
Glad you liked it. A warning thought - different cables have different impedance. 50 Ohms is also very common, so you have to make sure you are using the right value terminator.
This video has answered several questions I've had as a HAM radio enthusiast.
Thank you for the wonderful explanation.
Thanks! I'm so glad it was useful and answered a few things.
I use an old Anritsu Sitemaster in TDR mode at work to find breaks, although it usually isn't required. The break is usually in the vacinity of a completely oblivious plumber or domestic electrician with a hammer in his hands.
"The break is usually in the vacinity of a completely oblivious plumber or domestic electrician" oh that's so funny! And I don't doubt it for a moment.
Plumbers make sure that electricians have jobs.
@@jannejohansson3383 :)
Got to love those. We had similar problems with crushed flexible waveguide sections on B-52s from crew chiefs (usually) standing or climbing on them. All of them were marked in day-glo yellow paint "NO STEP". You would think that would be easy to read.
Superb practical demonstration of the importance of impedance matching, and the impact of not performing impedance matching correctly. Needed another project to work with, and I've got an idea for making an impedance measuring tool now. Thank you.
Thank you so much! If you want an incredibly cheap but capable tool for that sort of thing, look at the Nano VNA if you havnet already seen it - does smith charts, measures RF impedance etc, all at a cost of about $100 or so. There are numerous variants of it available - I got one that was in $150 range - larger screen and up to 3GHz operation - best RF toy ever!
That’s a very clear explanation, great work ! ❤
Thank you so much! I really appreciate it!
Great explanation, well organized presentation! Somehow you targeted my level of comprehension to explain something that I've been struggling with for a long time. Thanks!
Well thank you Mike! I'm glad that somehow the way I presented it worked for you. The video was based on a lab experiment I did as student and I have never forgotten it. Hopefully soon something similar with light ...
I taught myself electronics pre internet. The hours that I spent fretting and poring over equations and diagrams trying to understand this topic. I could have made so much more progress if born 30 years later. Let alone the price of equipment.
p.s. channel w2aew has some videos that will complement this explanation very nicely and he covers many other topics too.
You know I often wonder what things would have been like for me if all the internet resources were available. On the one hand the weath of resources we have today making understanding stuff so much easier. But then the constant distraction students have with messaging etc.
And yes - the price of equipment and parts was such a killer. I used to go out on large item garbage day and pick up things like old TVs and radios and strip them of parts like resistors and capacitors. Restors now cost 100 for $1 or better!.
Just searched w2aew and found it and subscribed. Thanks for pointing it out to me!
Yeah but if you were born 30 years later you wouldn't know what good music sounds like. Or good movies.
@@johnclawed Ha! Yes!!!
I worked installing transmission lines in school's while in NC. I knew how to balance the loads per classroom, but never understood why... Until now! Thanks! 👍
I so glad I solved the mystery! The setup is taken from a lab from a 3rd year EM course I took. We had a great prof and he created that lab to help make the subject understandable. So really he deserves much of the credit (and he has seen this video!).
Awesome video!! I enjoyed every second of it! Explained well with also a real demonstration!! 👏👏👏
Thanks you so much!
I came to watch that video telling myself "I know the answer but I want to see how it's explained.", well it turned out I also learned a lot! Very well explained, thank you! It reminded me the video from Veritasium about transmission lines, I also recommend!
Thank you! I'm glad you found my explaining worked - I always find that with each of us havinig a slightly different prespective on things, sometimes one or another explanation works best. The veritassium poynting vector one? Its another neat look at similar stuff - a bit missleading but that may just be my perspective. I am actually thinking of recreating it with an experiment. Course being a small channel I cant afford 300,000 km of transmission line so it will be more like 300 feet :)
Great job explaining this all! It makes it easy to understand how detectors for broken or shorted cable actually work and how network wire analyzer measures the length of the wire.
Thank you so much - really appreciate your comment. Have you ever used it for non-RF cable? I have wondered how well it works with cable not designed or installed with constant impedance in mind.
@@ElectromagneticVideos I haven't but if I remember correctly, ethernet cabling has effective 100 ohms impedance and that's the most commonly used cable for such tests.
I would expect that you can measure any cable type as long as you can tune the signal generator: you only need steps in the signal. The speed of light is always close enough to 1.0c that you get distances roughly correct without knowing the true impedance.
And since the sign of the voltage is enough to separate broken wire from shorted wire, that's good enough, too, without knowing the true impedance.
@@MikkoRantalainen Your right about the Ethernet cable! I should really experiment with some 14/2 house wire - I wonder what its impedance is (should be easy to measure) and how constant it is when it is near other cables since it has no shield or twist.
@@ElectromagneticVideos house wire is intended for very low frequencies (50-60 hz), so no thought is given to impedance.
More interesting would be twisted pair cables used for lighting systems (DMX).
@@TomCee53 Just looked up DMX. Looks like its similar or even repurposed RS-485 so maybe 120 Ohm cable?
Wow. This has been bothering me for YEARS. Only, not intensely enough to wade into div/grad/curl type of explanations (that explain nothing really)
So this very real, practical explanation is a dream come true (I pinched myself - ouch - to verify)
THANKS
Subscribed immediately
"not intensely enough to wade into div/grad/curl" :) Actually the neat thing is you can do the transmission line math with plain old differental equations becuase its really a one dimensional object from the wave standpoint.
Butther eis nothing like seeing a real experiment to understand as you said. Glad you liked it and welcome to my videos!
Absolutely fascinating, packed with so many facts that I did not know.
So glad you found it so interesting! It is really stuff!
I've never had the chance to learn some of these fundamentals despite making tons of professional cable/dealing with signals in this world. Thanks for making a video about it
Your welcome! Glad you found the fundamentals behind things like impedance and termination interesting!
A passive splitter (built with a resistor network) does not increase the output signal amplitude, as was stated in the video. In fact, it results in lower amplitude compared to just connecting the three cables together:
When connecting the three cables without a proper splitter, 1/3 of the signal is going to each of the two outputs, and 1/3 is reflected back.
With the splitter, only 1/4 of the signal is sent to each output, and half the signal is absorbed by the resistors in the splitter.
Even though the splitter reduces the amplitude of the signal, it still increases the signal quality (and in case of TV leads therefore to better reception) by eliminating the reflections.
Very nicely stated. I will add you can get non-resistive splitters generally with limited bandwidth that do a very good job of both impedance matching and transferring close to have the signal power to each of the two outputs.
Even though I already had a bit of an understanding of what was going on, it's always good to get a new explanation. You never know when something new might click. And, indeed, the need for a splitter was never something I'd actually ever attributed to impedence matching.
"it's always good to get a new explanation. You never know when something new might click." How true!!!!! I know for me often one explanation really give me a sense of how something works, and another (equally good one) may not click with me.
@@ElectromagneticVideos❤
My learning about these things started, and nearly ended with the math. Luckily there was the lab exercise that returned the issue to a practical level! After that, the understanding has been just like you explain it. But the real "fun" starts when you add some stubs at different distances from one or the other end of the cable. In effect, you can start changing or matching different cable impedances. Just a hint, the antenna impedance is not necessarily the same 75 ohms that your cable is. By the way, in addition to the end termination, it is possible to deduce the cable impedance from the attenuation of the supplied signal by the series resistor. You obviously had 75 ohm source, as the 75 ohm cable received nicely half of the signal generator output pulse amplitude. Good job in any case!
Thanks! Your points are bang on! I purposely did not get into all the nuances of antenna impedance and mismatches to try and keep the video accessible to everyone with the math kept at a minimum. You mentioned stubs - I do intended to do a video on quarter wave transformers in the context of anti-reflection filters on glass sometime, so that sort of thing is not going to be forgotten in terms of future videos.
this adds confusion for me, so the cable resistance as shown in the video is not 75 ohms, and he is simply matching the signal generator impedance? If this is the case (as I understand it), then why are we calling the cable 75 ohm? Is this so we hook this particular cable to a signal generator with 75 ohms of output resistance, in series with the 75-ohm termination resistor? Thanks!
@@nv2134 Sorry that the video further confused things. Let me try and briefly explain:
If you had an infinity long cable and measure the resistance at the end you would get 75 Ohms. Thats the definition of characteristic impedance. That resistance is the cable appearing to absorb power as it carries the energy away along the cable at almost the speed of light.
With a shorter line (in my case 100 feet) the power going down the line hits the open end and gets reflected back. When it gets back to the source end (the meter) it raises the voltage to what we would have seen with an open circuit. In other words it takes a few hundred nanoseconds for before the refection arrives and we find out that the end is open (or closed or anything else). In that brief time before the refection gets back, we can measure the impedance of the line and it is 75 Ohms.
Since an infinite line looks like a 75 Ohm resistor in every way, and being infinite never sends a refection back, we can also say a 75 Ohm resistor looks like an infinite line. So of we put a 75 Ohm at the end of shorter line (ie 100 feet) the line thinks its connected to a further length of line that is infinity long and so no reflection comes back.
Hope that helps!
Wow, that is the finest explanation of TDR on youtube. Thank you.
What a wonderful complement! Thank you so much!
@@ElectromagneticVideos :)
Excellent video. Helps in understanding the importance of the terminating resistors in all the communications in the industry I have worked with for almost 30 years now. From Allen Bradley DH, Remote I/O or even if it was ControlNet using Coax all those reflections from an open line could cause a lot of comm issues for sure.
Thanks! Yes - so important but also something that is really not understood by most except people in communications or ham radio types. Since you have been doing this as long as I have, do you remember when when Ethernet was really Ethernet and used 50 Ohm coax in both thin and thick formats? And 10mbps was so incredibly fast who could imagine needing anything more!
@@ElectromagneticVideos
I really don't know much about the earlier format. I just did not understand much about it then. You are leap years ahead of me in the science of it and the signals. I absolutely thought 10mbps was more than we would ever need since even hard drives back in the 90's were only 250MB and back then I was using RS232 protocol for most systems at 9600 to 57.6Kbps Max.🤯
@@ThriftyToolShed I'll have to try and get some old Ethernet cards and do a video showing the signals. The idea was (in the most primitive form) all the computers were attached to one long coax (the Ether) and to send data they simply wrote data chunks (packets) on the cable. If two computers wrote at the same time it was detected and re-transmitted at a random time later. Cheap, simple, and effective as long as the network wasn't close operating at full load. And originally came from U of Hawaii where they used satellite link operating in a similar way to link the Islands.
I sure remember pushing RS232 it to the higher speeds. And hard drives - paid $1000 for a 20M one for an early PC - yikes!
@@ElectromagneticVideos That would be a great video!
@@ThriftyToolShed I think so - thanks for making me think of it! I'll have to look for some old network cards and motherboards to do something like that. I just did a quick scan - not much of that stuff available anymore.
I remember when I was in high school I was always told fibre optic cables were better than coaxial because “light is faster”, which I now know is not the true story, and it’s actually because of these TL characteristics that limit the bandwidth of a cable. Your video is so easy to understand, I wish I was shown this when I was younger
Glad you found it easy! Actually fibre is more similar to coax than may be obvious. The speed of light in glass is ususally 2/3 that of light in free space, so similar to the coax and for the same reasons! And, optical fibers do have bandwidth limitations too, but at the much higher frequencies of light, bandwidth is so huge its mush less restricting. And - relfections at the ends and matching impedance to prevent loss of optical power is all the same stuff just done with glass or other transparent materials. I have a few videos planned with all of that stuff - stay tuned!
Great demonstration! Very informative. I have always known, since the days of MFM encoding for computer hard disks, SCSI bus interfaces, coax network cabling, PLC buses, and PROFI instrument bus, that either end of the transmission line needed resistors installed or activated or reflections would result and therefore system function would be unstable and unpredictable; but I never understood the mechanism of the reflection, nor have I ever seen the time delay introduced by the resistor or extended cable length. I just assumed that the added length diminished signal strength accordingly, it’s impact on timing never occurred to me. This fact would/could affect analog systems, but would seem to affect digital transmission even more so.
Thank you so much! MFM and SCSI - I remember those so well, and squeezing 50% space out of a disk with RLL controllers. It actually does have a significant impact on analog and there will be a future video on that topic. In the analog world, consider reflections at different frequencies (=wavelengths) going back and forth on the transmission line if not perfectly terminated. At certain wavelength the reflections may produce constructive interference = greater signal amplitude, at others, destructive and lower amplitudes. So get filtering effects increasing and decreasing the power at certain frequencies. Often a nuisance, but sometimes used to advantage!
@Tiberius Tchaikovsky My first computer featured a 20 Megabyte 1/2 height, which I upgraded to a 30 MB full-height drive. I loved that drive, but like the Seagate, it failed prematurely, in my opinion. Thanks for responding, and congratulations on recalling the model number. 🙂
@Tiberius Tchaikovsky I have several examples of the form factor you reference, with magnificently powerful magnets. I think you can see where I’m heading. Too expensive in that larger format (and then they went to 2.5”, and now just memory chips). I’m like you, holding a full-height, 5.25” disk drive, you felt that you had something substantial. But then I worked with techs who remember the original ‘Winchester’ drives big as washing machines. People have turned their disks into coffee tables, so there is that; progress and all. Again, thanks for the response; enjoy the conversation.
Sir, thank you! A great contribution to the world!!! It really is - I'm not joking. How many people have wondered about that and have found no decent explanation ... now we can get an excellent explanation. Once again: thanks!
I am so honored to have received so many comments like yours. Its funny - I was rushed doing the video and its certainly not the quality I wanted it to be and never would have guessed it would get the attention that it did. It is based on a 2nd or 3rd year lab experiment I did as a student decades ago. My challenge in this video was to try and show how transmission lines works without the math - I'm thrilled that I seem to have succeeded! Thanks so much for the kind comment!
Would have been interesting to put a resistance box on the end of the t-line to watch the effect of a range of values around 75 ohms on the scope.
I was so close to doing that with a potentiometer. In the end I decided the video was just getting too long. I will do a future one about Ethernet twisted pair so it may be a neat thing to do in that video.
Fanbloomintastic, this was something that had foxed me for decades, excellently explained and demonstrated many thanks!
Thanks you so much! Glad to be able to shed some light on it for you!
Great explanation. In earlier days I used this for fault localization in telephone cables between our office buildings
Thanks! Cool that you actually used it!
You have a very interesting chanel, and you explained this better than any of my proffesors once did.
Thanks! I was fortunate to have great profs in the subject who explained it to me!
Kind Sir, I am a retired Electronic Technician and Instructor; with RCA for 33 yrs; and then 12 yrs doing it by "subbing" all around America. So I say this to you kind Sir: You are truly a "Cut-Above"; and you just solved a long waning question; "WHY do we have to put a 75 Ohm resistor at the end of the cable?".
And you showed me why easily! Wow! Thank you kind Sir. Because you are an incredibly, awesome instructor. I shant ever forget this! May Jesus continue to bless you and yours always.
What a wonderful complement! Thanks you so much - I really really appreciate your kind words. I have to say that I owe this intuitive understanding to a wonderful EM Prof I had in university who made EM fields and waves one of my favorite subjects. Have a wonderful day!
Finally I understand why I had to use a terminator when connecting computers in a coax lan. Thanks!
Your welcome! Yes - thin and thick Ethernet and other obsolete lans from the old days!
@crackedmagnet - 2023-05-23
Thanks for this. I've never really understood, why termination resisters were required. You've explained in a way that is both objective and easy to process.
@ElectromagneticVideos - 2023-05-23
Thanks!