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 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*
:(
@@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.
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 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.
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.
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 :)
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!
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!
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'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.
That’s a very clear explanation, great work ! ❤
Thank you so much! I really appreciate it!
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!
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.
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.
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 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".
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" :)
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'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!
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!
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.
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!
This is the best explanation of characteristic impedance I've ever heard. Well done!
Thanks! Apreciate that!!!
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.
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 ...
"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.
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!
Absolutely fascinating, packed with so many facts that I did not know.
So glad you found it so interesting! It is really stuff!
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!
Awesome video!! I enjoyed every second of it! Explained well with also a real demonstration!! 👏👏👏
Thanks you so much!
Excellent demonstration. Perfectly clear. Thank you.
Thanks so much!
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.
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!
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❤
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
Glad you liked it. Certainly keeping reflections to a minimum will help with both receive and transmit powers!
One of the best arguments for fiber optic internet service I have seen.
I'm sure old cable wiring with unterminated splices drives cable company installers nuts when they try and get cable modems going!
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 :)
Thank you very much! You explained characteristic impedance very clearly and I learned so much from your video.
Your welcome! Glad you felt seeing the video was worthwhile!
Really neat to see an oscilloscope in use after seeing one at restore
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!
Good explanation, passed it through to some friends, who still thinks that cables from 50R suffice for 75R cables :)
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.
Excellent explanation. Your are a teacher par excellence, Sir!
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!
Amazing test setup.
Thanks! I hope by amazing you mean amazingly simple. Should be easily duplicated at home or in a classroom with relatively inexpensive equipment.
Very good and clear explanation to this topic. Help me a lot about what’s really going on.Thanks.
Thanks! Glad it helped!
Great quality of explanation. I am missing experiment with two 75 Ohm resistors on the ends of split cable
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.
Very nice video, studying for a transmission lines exam for next week!
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!!!!!
What a tremendous explanation! Thank you!
Thanks yo so much!
Great practical demonstration! Thank you!
Thank you!
Amazing explanation, I don't think I've had this explained to me before, thanks.
Thank you so much! Glad you liked it!
@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!