Why doesn't a 75 Ohm cable measure 75 Ohms?

[@crackedmagnet]

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]

Thanks!

[@Jtstien]

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.

[@ElectromagneticVideos]

Your welcome! Glad I solved that mystery!

[@NoNameAtAll2]

@@ElectromagneticVideos you're* :(

[@qzorn4440]

@@NoNameAtAll2 Do you have some videos?

[@rwood1995]

@@NoNameAtAll2 god not the grammer not zee. Math and physics is his thing. We are not here for the spelling content!

[@TheDunai]

@@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.

[@joesilverbliss1721]

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!

[@ElectromagneticVideos]

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.

[@publikjuli]

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.

[@earthoid]

I'm also a retired EE and I never fully understood this until now. My experience at my university was that there were so many new technical concepts being thrown at me that there just wasn't time for my non-genius brain to grok them all, so some were just accepted as true and then I moved on to the next new thing. Thanks for the clear explanation.

[@TohidGadiri]

"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."

[@ElectromagneticVideos]

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.

[@charlesaeneas]

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!

[@ElectromagneticVideos]

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 :)

[@ronmcc100]

Excellent video! Been in the field for 40+ years and that is the BEST explanation I've seen for a neophyte.

[@ElectromagneticVideos]

Thank you so much! I was really trying to do it without the math to make it understandable to almost anyone.

[@aieousavren]

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! ❤

[@ElectromagneticVideos]

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.

[@tricky778]

Yeah, I loved that bit.

[@PrinceWesterburg]

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

[@ElectromagneticVideos]

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.

[@durvius2657]

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 :)

[@ElectromagneticVideos]

Thank you so much! Welcome abord to EM Videos!

[@ozanozdemir5515]

Thank you.. you solved my 30 years old problem. What a genius idea of explaining reflection.. you old wolf..🙏

[@ElectromagneticVideos]

Wonderful and thank you! I have been called a lot of things but never that before :) I hope it is a compliment :)

[@ozanozdemir5515]

@@ElectromagneticVideosDon’t worry.. :) In Turkiye, we call knowledgeable, talented people the old wolf. like the expression "old hand" in English.

[@kurtttttttt]

Such a good demonstration of transmission line theory and conservation of energy. They should use this example in EE undergrad courses!

[@ElectromagneticVideos]

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!

[@kurtttttttt]

@@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!

[@ElectromagneticVideos]

@@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!

[@tcpbox]

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.

[@ElectromagneticVideos]

Thanks so much! I'm glad you found my way of explain easy to understand - certainly what I try and do!

[@brianchandler3346]

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! 😎👍

[@ElectromagneticVideos]

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!

[@ahmadalkhatib8677]

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

[@ElectromagneticVideos]

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.

[@bernarddt]

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 😆

[@ElectromagneticVideos]

Thanks! I'm so pleased the video helped provide an explanation of reelections and characteristic impedance.

[@wkjagt]

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.

[@ElectromagneticVideos]

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!

[@timypp2894]

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.

[@ElectromagneticVideos]

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" :)

[@alexandershopov1669]

Thank you. Similarly to other ppl - I wished this was part of the explanation and demonstration I was given in my classes.

[@ElectromagneticVideos]

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.

[@jdaley197931]

Thank you for this detailed video. It's always useful to know why you should use splitters and what they actually do.

[@ElectromagneticVideos]

Your welcome! Yes! Its a lot harder to justify the nuisance of a splitter without the "why".

[@toastymotors]

This is the best explanation of characteristic impedance I've ever heard. Well done!

[@ElectromagneticVideos]

Thanks! Apreciate that!!!

[@t0nito]

Very neat, now I understand the importance of terminating open ends of unused outputs with 75 Ohm terminal resistors!

[@ElectromagneticVideos]

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.

[@kuensenloh8305]

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!

[@ElectromagneticVideos]

Thanks so much! Glad my explanation was helpful!

[@gbriceno]

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 KZbin!

[@ElectromagneticVideos]

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.

[@gbriceno]

@@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?

[@mikeclark1647]

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!

[@ElectromagneticVideos]

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 ...

[@digitalradiohacker]

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.

[@ElectromagneticVideos]

"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.

[@jannejohansson3383]

Plumbers make sure that electricians have jobs.

[@ElectromagneticVideos]

@@jannejohansson3383 :)

[@bobmorgan1575]

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.

[@jsanpe35]

Thanks for a clear explanation and a very well made video on this topic! Seeing this on the scope really helps understanding what's going on.

[@ElectromagneticVideos]

I'm so glad you liked it and the explanation. Its actually one of my favourite experiments - I first came across it as a student in an electromagnetics class one was one of the few experiments I never forgot.

[@anonymous.youtuber]

That’s a very clear explanation, great work ! ❤

[@ElectromagneticVideos]

Thank you so much! I really appreciate it!

[@captiveimage]

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.

[@ElectromagneticVideos]

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!

[@MikkoRantalainen]

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.

[@ElectromagneticVideos]

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.

[@MikkoRantalainen]

@@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.

[@ElectromagneticVideos]

@@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.

[@TomCee53]

@@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).

[@ElectromagneticVideos]

@@TomCee53 Just looked up DMX. Looks like its similar or even repurposed RS-485 so maybe 120 Ohm cable?

[@MrPatdeeee]

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.

[@ElectromagneticVideos]

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!

[@JanHartmann]

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.

[@ElectromagneticVideos]

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.

[@111000100101001]

Very helpful in understanding why a given cable is designated an Ohm value.

[@ElectromagneticVideos]

Glad you found it helpful. A lot of misconceptions about that!

[@ThriftyToolShed]

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.

[@ElectromagneticVideos]

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!

[@ThriftyToolShed]

@@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.🤯

[@ElectromagneticVideos]

@@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!

[@ThriftyToolShed]

@@ElectromagneticVideos That would be a great video!

[@ElectromagneticVideos]

@@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.

[@chopper5371]

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! 👍

[@ElectromagneticVideos]

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!).

[@grahamwhite75a]

Absolutely fascinating, packed with so many facts that I did not know.

[@ElectromagneticVideos]

So glad you found it so interesting! It is really stuff!

[@alphamike_1612]

This video has answered several questions I've had as a HAM radio enthusiast. Thank you for the wonderful explanation.

[@ElectromagneticVideos]

Thanks! I'm so glad it was useful and answered a few things.

[@virgilioconti2561]

Awesome video!! I enjoyed every second of it! Explained well with also a real demonstration!! 👏👏👏

[@ElectromagneticVideos]

Thanks you so much!

[@zaydabbas1609]

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

[@ElectromagneticVideos]

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!

[@InssiAjaton]

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!

[@ElectromagneticVideos]

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.

[@nv2134]

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!

[@ElectromagneticVideos]

@@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!

[@DanielLopez-up6os]

You have a very interesting chanel, and you explained this better than any of my proffesors once did.

[@ElectromagneticVideos]

Thanks! I was fortunate to have great profs in the subject who explained it to me!

[@jeremiahchamberlin4499]

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.

[@ElectromagneticVideos]

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!

[@jeremiahchamberlin4499]

@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. 🙂

[@jeremiahchamberlin4499]

@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.

[@StarkRG]

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.

[@ElectromagneticVideos]

"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.

[@ndr8469]

​@@ElectromagneticVideos❤

[@Hereford1642]

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.

[@ElectromagneticVideos]

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!

[@johnclawed]

Yeah but if you were born 30 years later you wouldn't know what good music sounds like. Or good movies.

[@ElectromagneticVideos]

@@johnclawed Ha! Yes!!!

[@DouglasFish]

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

[@ElectromagneticVideos]

Your welcome! Glad you found the fundamentals behind things like impedance and termination interesting!

[@James_Bowie]

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.

[@ElectromagneticVideos]

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.

[@jlinkels]

Great explanation. In earlier days I used this for fault localization in telephone cables between our office buildings

[@ElectromagneticVideos]

Thanks! Cool that you actually used it!

[@FPE93]

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!

[@ElectromagneticVideos]

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 :)

[@roliveira2225]

Excellent! I only feel lack of the part of the full pulse with a matched load.

[@ElectromagneticVideos]

Thanks!!!!!!!!!

[@garganega]

One of the best arguments for fiber optic internet service I have seen.

[@ElectromagneticVideos]

I'm sure old cable wiring with unterminated splices drives cable company installers nuts when they try and get cable modems going!

[@nudebaboon4874]

Fanbloomintastic, this was something that had foxed me for decades, excellently explained and demonstrated many thanks!

[@ElectromagneticVideos]

Thanks you so much! Glad to be able to shed some light on it for you!

[@tonypitsacota2513]

Wow, that is the finest explanation of TDR on youtube. Thank you.

[@ElectromagneticVideos]

What a wonderful complement! Thank you so much!

[@tonypitsacota2513]

@@ElectromagneticVideos :)

[@serialskeptic]

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

[@ElectromagneticVideos]

"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!

[@Bluelagoonstudios]

Good explanation, passed it through to some friends, who still thinks that cables from 50R suffice for 75R cables :)

[@ElectromagneticVideos]

Thanks! Just threw 50 Ohms and 75 Ohms into the reflection equation and the mismatch results in a 20% voltage reflection. Not a huge amount, but enough to loose signal power and smear things if a mismatch on each end of the cable.

[@LorencoLovepump]

I have been to a 2 day course about TDR, and this video taught me more

[@ElectromagneticVideos]

Thank you so much! Glad you found it useful. I`m complelty fine with any instructors playing the video to a class or distributing the link so if you would like to suggest it to the instructor, or suggest an in class demo is done based on the video, please feel free to do so. Any number of free online KZbin video downloaders could be used to ge the video file so a class does not get subjected to the ads that are normally inserted.

[@maxheadrom3088]

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!

[@ElectromagneticVideos]

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!

[@seb_gibbs]

thank you, after decades of messing around with electronics, I finally get it.

[@ElectromagneticVideos]

Sorry to hear it took that long :) Glad to hear that seeing this video helped!

[@ScottsSynthStuff]

I had that exact DMM for decades, it was my favorite...until one day a couple of years ago, it just outright died, would no longer turn on anymore. RIP favorite DMM.

[@ElectromagneticVideos]

Yes - its old and and wonderfully simple and mechanically rugged hardware store multi-meter. Sadly many of the more recent ones are so filled with functions that its a nuisance. I have one recent one that has a wonderful of array of functions but DC voltage is not on the dial switch. Voltage defaults to AC and you have to press another button. Almost like designed by someone who has never used one!

[@FilippSudanov]

Great quality of explanation. I am missing experiment with two 75 Ohm resistors on the ends of split cable

[@ElectromagneticVideos]

Thanks! That part was to show that if you just splice cables together, it looks like two resistors in parallel = half the impedance and you get a reflection. So you need an impedance matching device like a splitter sending the signal to two TVs for example.

[@MVVblog]

This is a great explanation! Fantastic video!!!

[@ElectromagneticVideos]

Thanks you so much! I just subscribed to your channel - looks wonderful - too bad I don't speak Italian but hopefully the KZbin captions translation function will work! Greetings from Canada!

[@youtubeaccount931]

This was good. I'll have to watch a few times to absorb it but it's something i've never really dug into but wanted to know as a radio enthusiast

[@ElectromagneticVideos]

Glad you liked it. Certainly keeping reflections to a minimum will help with both receive and transmit powers!

[@leonarperro]

Finally I understand why I had to use a terminator when connecting computers in a coax lan. Thanks!

[@ElectromagneticVideos]

Your welcome! Yes - thin and thick Ethernet and other obsolete lans from the old days!

[@RobertLBarnard]

I used to use a time domain reflectometer (TDR) on network cables back in the day. Length, kinks, shorts, and/or other issues showed up as well as the location. Nice tool.

[@ElectromagneticVideos]

So how obvious were things like kinks? Did the cable really have to be bent to show up?

[@bobmorgan1575]

@@ElectromagneticVideos Depends on the sensitivity of the instrument. We used a USM-430 TDR in the military that when properly calibrated could be used to located impedance changes as low as 10% of the nominal value. Larger discontinuities generated larger return pulses on the CRT display. It was also capable of measuring phase shift within .1 degree if memory serves, and we used that function to check the phase matched RF cables between the receivers and processor on the F-4G.

[@RobertLBarnard]

@@ElectromagneticVideos Yes, bends would show up as changes in impedance. It was pretty usable, it had a scope/screen, but also an option to printout a ribbon of thermal paper with length graduations along it. Every 2 1/2 meters, if a vampire tap/connector was present, there'd be a little blip. A sloppily performed tap may short, effectively ending the cable as for as the TDR would sense (as I recall, any signal traveling beyond the short or it's echo would be eliminated by the short). Distance to a node does have limited value. A story I can share is of a Sun OS "pizza box" (pre Slowaris) was recycled as a licence manager for a large distributed CAD site. IT lost track of were the little box was and when it was time to rebuild the Kernel, they tried to find it to no avail. Fearing bringing the LMS down and not being able to bring I back up the machine ran for well over 400 days before a custodian found "an old computer" hidden above a closet on a drop ceiling panel. Of course, a TDR doesn't tell you the MAC or IP of anything on a cable, just reflected signals/reactance.

[@ElectromagneticVideos]

@@RobertLBarnard So print a measing tape - that something I have never heard of. Interesting about taps going bad. I didnt come across that with thin ethernet but sometimes a cable connector would come off - either from the cable or the BNC not being attached properly. I have a bunch of BNC Ts from the old networking days - will have to see what type of reflections they produce whne attached to an old network card. A Sun hidden in a drop ceiling - thats a new one! I remeber those pizza boxes well - whne I was a grad student Sun was the standard workstation in our lab. How great they seemed back then and years ahead of PCs in terms of performance and a window system - wasnt the window sustem SunView. And and pre solaris their unix was SunOS.

[@jianbob]

Very good and clear explanation to this topic. Help me a lot about what’s really going on.Thanks.

[@ElectromagneticVideos]

Thanks! Glad it helped!

[@hanfo420]

14:23 Mathematically correct, but I would like to add that the positive pulse is now traveling on the negative line. our scope assumes that the negative line is always 0 and so it draws the pulse as a negative, since our positive line seems to dip when the negative line goes up. That also explains how the resistor works! It splits the pulse up to exactly reflect the pulse on both lines equally so that our scope can’t see the reflection. But in reality the reflection is still there, just on both lines.

[@ElectromagneticVideos]

Not quite sure that I agree with you interpretation - but we all have our own way of understanding things, so if it works for you (and others), great! Its always useful; to hear other perspectives - thanks for commenting!

[@clivegillham7910]

Excellent! Thanks for spending the time putting this together

[@ElectromagneticVideos]

Thanks! Really appreciate your comment!

[@fbn_vnslw]

I feel like you opened my eyes. It's kinda amazing how electricity works.

[@ElectromagneticVideos]

Your comment made my day! Thank you! Yes it is amazing - the thing that I found most amazing when I was studying it as a student was how you can measure static non-moving electric and magentic field characteristics and from that get the speed of light. And how light is just and electric and magnetic field - wounderfully amazing. And I will do a videe on that sometime!

[@margaqrt]

Good demonstration and explanation. I do believe the most common velocity factor for consumer grade TV coax is .66

[@ElectromagneticVideos]

Thanks! You could be right. I looked up the cable I used and it was 0.8 . There were others listed at 0.66 and some as high as 0.92 (that may have have been 50 Ohm).

[@kiweekeith]

What a Brilliantly explained vidclip, Thanks a Stack from ChCh, NZ

[@ElectromagneticVideos]

Your welcome from the other side of the planet - Canada!

[@kiweekeith]

@@ElectromagneticVideos Thanks ..... Appreciate your reply .... I am into CB / Ham type Radios and love building Antennas and Your Expanation just 'Hit the Spot' .... Best to You and Yours from ChCh, NZ

[@ElectromagneticVideos]

@@kiweekeith Thanks again - glad the explanation 'Hit the Spot' . Its really neat stuff - I'm an EE and this sort of stuff is covered in the EM fields and waves courses in university which I always liked. One of these days I'm going to get a Ham license - what a neat hobby!

[@LOLWUT281]

I should have subscribed when I saw your video the other day. Great job once again.

[@ElectromagneticVideos]

Thank you so much! Just so you know, I try to do a video a week but like right now, work or other things occasonally gets in the way. I do have one more video almost edited - with luck I'll get it up before I get back home. Thanks for subscribing!

[@THRASHMETALFUNRIFFS]

Very cool deep dive I never seen in action before! It all makes sense. This is why is may be useful to cap the unused fittings in a house if there's a weak antennae or cable signal to prevent "signal loss", I guess that'd be this noisy reflection you demonstrated that could get quite noisy with multiple potential outlets in a home or business connected to a splitter that may or may not be in use.. This is how the cable guy quickly turns on ur service by connecting a specific resistor cap to the incoming wire or how thieves invented black boxes to unlock certain blocked pay channels. I was waiting for you to talk about the benefit of using it to find breaks in a wire but it also has real world usage in everything from an alarm system reading the state of wires with an end of line resistor so the panel can know if you have a short, open or closed condition (did the wire break or cut, someone twist the ends together to bypass an alarm or is the window open, etc) Oh, this is how ur car knows if the taillight bulb is burned out and many other conditions of sensors, too!

[@ElectromagneticVideos]

Yes! I didnt get into this but the trick is to use connector caps that have 75 Ohm loads built into them prevent reflections. The reflections are actually worse than this video shows when there are multiple improperly matched connections or unterminated ends - when the cable distances between those ends is multiples of the wavelength of a particular frequency. the signal at that frequency can be completely wiped out due to destructive interference or greatly enhanced due to constructive interference. This results in all sorts of weird effects. This is actually used to make some RF filters. The real cost saving use is of course to find the location of faults in things like buried cables. And of course, if you take away the transmission line and replace it with an antenna, its RADAR.

[@Luzt.]

Excellent explanation. Your are a teacher par excellence, Sir!

[@ElectromagneticVideos]

Thank you so much! Its always a challenge to explain a mathematically intensive topic in an intuitive way with minimal math - I'm thrilled to apparently have succeeded!

[@sunshaking]

Thank you very much! You explained characteristic impedance very clearly and I learned so much from your video.

[@ElectromagneticVideos]

Your welcome! Glad you felt seeing the video was worthwhile!

[@davidrichter9164]

I love these kinds of videos, so many know it all speech makers.

[@John-uo4xl]

Great demonstration and explanation of impedance matching. Looking forward to your video on ethernet twisted pairs.

[@ElectromagneticVideos]

Thanks! The twisted pairs video is delayed - had to go on a suddden business trip. Back in 10 days so should be able to get back to videos!

[@rdhunkins]

My EE instruction in electromagnetic was abysmal. Thanks for the clear explanation.

[@ElectromagneticVideos]

Glad you found it useful! Its funny/sad how different instructors can taint ones understanding and enjoyment of a subject. I had the best profs for EM and ended up loving it. I had the worst prof for control theory - to this day I dont much care for the topic, but at least I have finally gotten a decent understanding of it over the years.

[@GoldSrc_]

Amazing explanation, I don't think I've had this explained to me before, thanks.

[@ElectromagneticVideos]

Thank you so much! Glad you liked it!

[@dachintya]

Wow, thank you very much sir. This was a mystery to me. I was looking for the why's of the signal reflection and characteristics impedance. As this concept forms basis of signal cabling. Mind-blowing explanation. Thanks a lot for making it easier to understand than what it really is!!

[@ElectromagneticVideos]

Wel thank you so much! "Easier to understand than what it really is" well maybe easier to understand that with the full blown math? Even with the math I find a good experiment like this really helps me undertand what going on.

[@FrankBitterlich]

Many thanks... this was a very insightful explanation. I always had a general understanding about the concept of impedance, but now I'm understanding it much better.

[@ElectromagneticVideos]

Well thank you! So glad I help you understand it a bit better!

[@FrankBitterlich]

@@ElectromagneticVideos What is the resistance of the resistor (-network) you used on the input side? Is that 75Ω?

[@ElectromagneticVideos]

@@FrankBitterlich So most signal generators are 50 Ohm outputs. The TV coax I used was 75 Ohms. To make them both see the right load, I used a 10 Ohm resistor to ground. The Signal generator was connected to that resistor though a 39 Ohm resistor so it would see about 49 Ohms (close enough). The coax was connected to the 10 Ohm resistor with a 68 Ohm resistor so it would see 78 Ohms, close enough to the 75 Ohm Characteristic impedance. Note that I am assuming the 10 Ohm resistor is so low compared to the others that any effects of the resistors on the other legs can be ignored. If you use some 50 Ohm cable - almost any cable other than TV - and a 50 Ohm signal generator - you don't need the matching network.

[@jtsiomb]

Excellent demonstration. Perfectly clear. Thank you.

[@ElectromagneticVideos]

Thanks so much!

[@Oreonee]

Ham radio operator here, unmatched impedance in antenna systems is what makes power outputs in radio transmitters fry. Reflected signal goes back and heats up amplifier transistor. There is a device measuring reflections in feed lines called SWR Meter (Standing Wave Ratio Meter) to tell us how properly matched is our transceiver - antenna system.

[@ElectromagneticVideos]

Yes! I didnt go there because I wanted a visual way to show people reflections rather that having to explain standing waves. I will probbaly do something with standing waves or interference patters soon.

[@superwaffle01]

Really neat to see an oscilloscope in use after seeing one at restore

[@ElectromagneticVideos]

I remember the one you probably thinking of, If must have sold and hopefully found a good home. This is a much more modern one - only 25 years old!

[@brandonkirisaki9708]

just did this with a homemade avalanche pulse generator. beautiful video. beautiful explanation.

[@ElectromagneticVideos]

That is so cool that yuu did it to. Its such a neat experiment! So glad you liked the video and explanation!

[@brandonkirisaki9708]

@@ElectromagneticVideos admittedly, I wanted to test a "security wire" with an unknown impedance. It ended up somewhere around 42 ohms before I gave up soldering so many SMD resistors onto the end. Something tells me that this wire isn't exactly designed to transmit high-speed signals. My original intention was to use it as RS-485, but I think I should get some dedicated wire for that instead.

[@ElectromagneticVideos]

@@brandonkirisaki9708 I see! Yeah not all cables are designed for higher frequencies. Great example is the old phone cables which were never meant for much more than voice and with incredible equaliuzation and error correction, we can force much more though them. But only for distances. You also cant depend on a cable impedance above its design frequency - you would have to measure its impedance at various frequencies above rated frequencies and to avoid reflections, design a termination with impedance that changes to match the line. I sure its has been done in some exotic application, but changing the cable would be easier. If you need a cheap high frequency cable, you cant beat Cat5 Etherent with an impedance of about 100 Ohms for each twisted pair.

[@brandonkirisaki9708]

@@ElectromagneticVideos actually, my next experiment is testing ethernet

[@FoulOwl2112]

I needed this. I reacently cut the cord. Ive been experimenting with different home brewed/hand built antennae designs. Which frankly, have exceded all my expectations and outperformed everone l know's expensive store bought antennae. BY FAR! I've finally settled on a design and moved on to the "permanent installation" phase. Then l started in on cabling....Uugghh somehow l just couldn't wrap my gead around this aspect of the job. My my numbers just weren't adding up. Readings wayyy off in respect to the numbers l expected to see. This helped to at least put my mind at ease so l get on with the job at hand. Thanks!

[@ElectromagneticVideos]

Glad this was useful! What a great project! If your experimenting with home made antennas and want to see in real time how bending/altering cutting parts of them alters the impedance and SWR, a $100 NanoVNA is an an amazing tool - incredible functionality for the fraction of the price of a name brand more accurate equivalent. I was just using one to tune a stub antenna I'm using as a signal sniffer. Also great for checking for impedance mismatches in the transmission line.

[@FoulOwl2112]

@@ElectromagneticVideos I appreciate that. I'll check that out. I've settled on a double loop design with a central feed. I forget the exact element length l ended up using. I split the difference between the frequencies of my two preferred channels. Then blammo! Was surprised to discover l was able to pull in channels that, according to EVERY ota prediction website, is wayyy outside my reception area. I thought to myself " Wow maybe l can make it even better with different cabling, IMTs, baluns etc". Whereupon the confusion and head scratching set in....

[@ElectromagneticVideos]

@@FoulOwl2112 A double lop antenna - how cool! I would be thrilled to see your design if you ever have time to make a video about it. I live near Ottawa and would be neat to be able to pull in some of the US TV stations from the Watertown/Syracuse area, all of which are predicted to be a bit beyond the reception range. One other thing you might want to consider - try a cheap LNA (many available with 20dB gain and a noise of about a dB up to a few Ghz for about $20) with the LNA located near your antenna. Might bring in a few more channels!

[@FoulOwl2112]

@@ElectromagneticVideos Ive been consitering doing a video on it. I gotta pull it down and put the new cable,, get it properly routed through the attic. Secure my mast to the home and get it grounded. My last little experiment might involve some form of deployable reflector. My favorite channel tower is 96.5 miles away. With a reflector l get it 10 outta 10 days. Without, l get it 8 outta 10 days. Invariably, when l really want to watch it, it ALWAYS seems to be in one of the TWO days l cant get it! Just my luck. And if l leave the reflector attached it interferes with many of my local stations as they are nearly 180° in the opposite direction. I might just look into that amplifier you mentioned. All this is sorta inconsequential.... Its just a personal hobby/ challenge Like always looking for that "Trophy" Bass or Musky....lol

[@FoulOwl2112]

@@ElectromagneticVideos BTW Just look up "Pennyloop UHF antenna" thats where my inspiration primarily came from. Ive tried numerous other designs. None come close to the overal quality and flexibility of that basic design.

[@oseyedian]

You are a passionate. Thank you very much for your explanation and actual testing. 👍

[@ElectromagneticVideos]

Thank you! I have always loved combining theory with actual real stuff - it really brings it to life!

[@5cyndi]

My parents had a VHF/UHF TV antenna installed at our house he built when I was a kid, yep no cable, and the installers gave him these neat little terminators that were so cool looking to my young self (and to my current self). My dad said they had a 75 ohm resistor at the end and they said to put the terminators on any unused outlet. They wired up 4 and I think we only used 2, and put terminators on the others.

[@ElectromagneticVideos]

So they knew what they were doing! I get the impression that many people beck them - even installers - really didn't appreciate the need for termination. On old analog TV the reflections can typically blur horizontal edges of things in the TV picture due to the image information arriving, and then arriving again a few nano-seconds later. Most old black and white TVs had reduced bandwidth and not very sharply focuses picture tubes, so it often wasnt noticeable. NTSC color was a different story - the delay would mess with the phase an and amplitude of the 3.58Mhz color subcarrier that was superimposed on the black-and-white image, and cause all sorts of interesting color issues.

[@santiagodeolaso4846]

Very nice video, studying for a transmission lines exam for next week!

[@ElectromagneticVideos]

Glad you enjoyed it! You must be taking a 2nd or 3rd year EM course - those were some of my favorite courses ever. Good luck on the exam!!!!!

[@tomdoyle813]

This is an excellent explanation, well done mate!

[@ElectromagneticVideos]

Well thank you! Nothing better than the explanation being called excellent!

[@markproulx1472]

What a tremendous explanation! Thank you!

[@ElectromagneticVideos]

Thanks yo so much!

[@aagrafio]

Bravo! The equations are OK, but in that video I learned much more than I learned in one semester about transmission lines. Math is a must in EE, but it will never replace the lab. Never. (And vice versa).

[@ElectromagneticVideos]

Thanks you! I am amazed how many other EEs never did this in a lab. The video was inspired by a lab I did as a second or 3rd year student. It sure made theory come to life.

[@ricban1950]

Great explanation. You mention reflections in glass.. I have always wondered how individual photons reflecting from different position of a flat surface are able to reflect in the same direction.

[@ElectromagneticVideos]

Thanks. So the glass reflection video will be from the classic EM standpoint: how the only solution to an EM wave at a surface dufferent impedance is some being reflected and some going though. Your photon question goes into quantum physis: Here is the very short explanation: Each photon has a wave function with highest value were it is most likly to be any point in time and space. The highest values are wher we expect it would go with classic physiscs. The wave funtion includes a complex terms (e to the iwy + theta) which carries the wave properties in terms of wavelength and phase. These probability wave funtions interact with the sheet of electrons in the metal mirror. Your could think of everry point on the mirror reradiating a probability wave with a phase releated to the phase of the impinging probability wave. And whne you sum it all, you end up with the highest probabilities along where we would expect the relfect ray/phototn to be. Put a gazillion photons together and all the tiny controbutions from each photon add to a very stable average to become the EM fields we are used to. So thats the verys quick handwaving answer that is intented to give a sense of whats happening as opposed to rigerous physics. Hpe that helps!

[@ricban1950]

@@ElectromagneticVideos Thank you very much. I appreciate the clarity of your explanation.

[@jazbinsek]

Great practical demonstration! Thank you!

[@ElectromagneticVideos]

Thank you!

[@tonysofla]

Impedance though is close to its cousin Resistance, it's at a certain higher Hertz, so it acts more like a resistive high band pass filter. Signals are often 75ohm in serais at source and 75ohm to ground at the appliance, so it's voltage divider that will stop signal bounce.

[@ElectromagneticVideos]

Great point about the voltage divider nature of 75 Ohm source - line - load. Impedance - right - and cal also act as low pass or other filter types depending on frequency and nature of the impedance caused by the device.

[@tonysofla]

@@ElectromagneticVideos yea actually meant low pass filter, noise higher than intended signal or DC path, you intentionally insert impendence, like a CHIP NOISE FILTER NFZ32BW, 43 milliOhm at dc, 6.8 Ohm a 1MHz, so it's the HZ that makes the impedance change.

[@joelsoncdma]

Oh my friend, i never see this explained...amazing; "look´like waltz of mathematics and physic"...thanks!!!

[@ElectromagneticVideos]

Thank you so much for that comment and what a wonderful way of describing it : "waltz of mathematics and physics". Thats such a great way of talking about electromagnetics - the intertwining of electric and magnetic fields to give us everything from light and radio waves to reflections in a transmission line. More of this type of stuff to come!

[@PixelSchnitzel]

This is an excellent explanation. For those who aren't fully satisfied by the demo and simple math of open/short reflections and want to understand what is physically happening, Walter Maxwell (W2DU) has an equally revealing explanation in section 3 of his book, "Reflections III". There, he explains what happens to the electromagnetic wave that results in what we're seeing here on the oscilloscope and the drawing.

[@ElectromagneticVideos]

Thank you so much! I just download the book you mentioned - wow - what a wonderfully practical book - I will definitely have to read it in detail sometime.

[@mgabrielle2343]

No one has been able to explain this phenomenon in such a clear detail, and most of my life I have never known what the hell or how the hell these antenna cables were referred to as 75 Ohm coaxial, or 50 ohm Coaxial, and thought it was to do with the end terminations like the BNC plugs and sockets, a bit late for this clarity, as my electronics carrier is now reached an end and I am retired OAP. I guess you would never not stop learning.

[@ShainAndrews]

It's in all the books. Explained roughly the same way.

[@ElectromagneticVideos]

So glad to solve a mystery! I was so lucky to have EM instuctors! I gompletely agree - never stop learning - the world is way too interesting! Hope your enjoying retirement!

[@dovhanimakhado2434]

All I can say is Wow! This gentleman deserves Bells! 🎉

[@ElectromagneticVideos]

Thanks you verry much!

[@sparkstarter]

Beautiful explanation of an often difficult to understand concept.

[@ElectromagneticVideos]

Thanks!

[@ZDM314]

This actually reminded me of working with SCSI in the 90s. You had to deal with terminating the SCSI bus with either active or passive resistors. Otherwise the reflection would cause signal/data loss on the bus and degrade the maximum transmission speed. Well put together video!

[@ElectromagneticVideos]

Thanks! Your not going to believe this, but I some SCSI cards, cables and terminators stored around somewhere. SCSI was so much better than IDE hard drive connections back then!

[@button-puncher]

Thank you! You are an excellent teacher. Things always make MUCH more sense with a demonstration. I always wondered if there was a cheap/quick/easy way to verify if the test cables that I got are really 50 ohm. I've got a Rigol and the same AWG. Time to do some testing! BTW, thanks for the tip about the AWG software. I'd never looked in to it.

[@ElectromagneticVideos]

It is neat experiment. If you want to try someting intersting, try a variabe resistor at the end of the line. As you turn it you can see the refelction go from positive to none to negative. One thing I didnt try was watching thereflections as really sharp bend is made or the line is crunched with pliers - might be interresting to look at.

[@button-puncher]

@@ElectromagneticVideos Yeah! I was thinking of trying a ganged 10-turn linear pot. So that you have two variable resistors that are pretty well matched on each end of the cable. Definitely, or what it'd look like if you have a 50-ohm coupler between two 75 ohm cables. Would something as tiny as a mismatched coupler show on a scope? Fun stuff!

[@ElectromagneticVideos]

@@button-puncher The other thing to try is capacitors inductors or some RLC combination at the end. That was part of the undergrad lab where I first came across the experiment as a student. We had mystery loads that we had figure out by looking at the reflections. Probably really tough for those who did the lab before the theory was covered in class (There were a bunch of different experiments. Each lab group did a differnt ine each week, so they werent always timed well with the lectures).

[@MickHealey]

Brilliantly explained, thank you.

[@ElectromagneticVideos]

Thanks you for that wonderful compliment!