That is so cool! I've never seen RF power measured at different positions along a transmission line like that. I guess it's no surprise that it works, but it's always nice to see theory shown with an oscilloscope probe and a steady hand.
***** If only there was a means by which you could collaborate and share data in real time over long distances. I long for the day when that becomes reality.
Thanks Allen. I remember when we did this experiments when I was an undergrad during one of my courses in the lab, I had a "WOW!" moment which changed my perspective forever. :)
***** Can you make a video explaining type 1 through 3 op amp compensation networks used for switch mode power supplies? Also, what does the "s" mean in (s+1) term in control theory for compensators and what does the H(s) function stand for?
EETechs H(s) is the transfer function of the loop expressed using LaPlace transforms. This would be a more complicated topic and would likely have to encompass a few videos. I'll put it on the list though.
This video is the absolute gold standard for demonstrating standing waves. A wiki page is one thing, but seeing the RF power minima and maxima being measured on an actual transmission line is quite another. Fantastic work W2AEW! 73 de W6PGS.
Your organized thought, visual aids and practical application is spot-on to me. So many You Tubers want to race through a subject without coming up for air. Also, you don't seem to have the tendency of diving down rabbit holes to the point where I feel I've been through a spin cycle! I rarely have to pause your videos either. I revisited this vid because memory is always the first thing to go!
That was a terrific visualisation of an effect I understand, but had never witnessed so clearly. I've watched many of your fabulous videos, Alan, but this one has somehow eluded me. Glad to see it today. Thank you.
Fantastic presentation. Back in the day, we saw similar demos using "Letcher" wires. Nothing can be more informative than actually seeing the reality of the electric waves in their natural habitat, so to speak. Well done. In all of your videos, you have proven yourself to be a great teacher.
Excellent practical demonstration of SW. I used to have students experience wave additions in the floor with 'super' slinkies. Similar wave behavior in different wave mediums makes understanding wave behavior so interesting.
I'm starting to think that utube is much better form of education than the universities. They never explained it this good at my U. In my case it was like here take this bunch of formulas, plug in numbers, calculate, congrats here's your bachelor's.
Unfortunately, I wasn't a very good student (and that's my fault), but I do think Alan's videos are much more educational than any lectures I received in my EE baccalaureate degree program; my lab classes didn't even use real oscilloscope probes (and, I had no knowledge what additional influences a home-made probe would have introduced , when high frequencies were used in a circuit- besides, we didn't have the equipment to generate anything over 200 megahertz, anyway, if my memory serves me well). We just used coaxial cables, stripped to the conductor on one end, with a BNC connector on the other end. It's embarrassing for me to admit, how much basic electronic science I've learned from Alan's videos - that I should have already know from my formal education as a EE student. I sure hope my university does a better job today educating future engineers - but, I doubt they do, since I can't imagine the lab classes having expensive modern electronic test equipment in them (too expensive). It blows my mind to think of the money wasted in education, when they could just buy some decent test equipment, and pay the correct people to demonstrate how to properly use the test gear - and what the equipment is measuring, and why; And I do believe in the idea of public education, and consider myself to be very much a liberal. However, just today I was reading, in our local paper, about our local convention center receiving an education award for educating children middle school and high school children- not sure of the exact grades; this convention center puts on these very silly plays, which are supposed to be comical (I guess), for which they bus in thousands of kids to watch, during the school day, and call this science education. I started watching one of the plays concerning science, and it was a joke (truly a joke), IMHO.
Amazing demo on Standing waves. Actuall measurement of the standing wave pattern on the live transmission line is absolutely unique. Much appreciated W2AEW !
Wow, I've never been able to see the effects of impedance matching as directly as you measured it here. It's very cool that you were able to measure the nodes and antinodes. Time to break out some PCBs and give it a try!
Ohhh man! That was awesome to see. I am literally learning about standing waves in my electromagnetics class right now. My brain started hurting so I stopped to watch some youtube. I just happen to type in standing waves and got this video. So nice to see what they are talking about in my EM book. All the equations and derivations just make you blind. This really helps me understand what is going on.
To be able to "see" practically what the text books teach, is just amazing. It really helps to take the information on board. Thanks. You have so much content, I'm working my way through. Watch one, think and absorb, watch another... I could be half educated by the end of this! LOL
Brilliant!! Never really understood this. Now I do !! You definitely have a gift for breaking apart the complex and showing us what is really going on.
Nice demo. I have a large slotted line and an HP 415E I use to demonstrate SWR. If the crowd is very large, I hook a small amp and speaker to the amp output of the 415E so the 1000Hz tone can be heard around the room. I usually get some comments about the archaic equipment. Then I like to point out that this is the type of network analyzer that was used to design the equipment that brought back TV signals from the moon walks.
I've had great trouble in the Electromagnetics course because while using Smith charts, we never went over what the transmission and reflection coefficients, nor the VSWR actually represented visually. Thanks so much for clearing this up.
I don't know how you do it. I could read for a week on all this voodoo magic, and you Sir, sum it all up in 10 minutes. Your channel is one of the best I have have subbed.
Amazing video as always, I was having a discussion with a friend of mine as to why the normalized impedance would repeat itself on a transmission line every half wavelength. This proves it. Also that is true for any termination except matched termination aka 50 ohms in most cases. Sits really well with the smith chart also. Alan you are true genius.
Thanks for letting me see this phenomena. Although yuur previous video spoke of it, it was tough to believe it. This video gives the edge. Thank you so much.
Well I feel like an infant listening to their first words! OMG! Some cool stuff here! I understood about 50% of it, but I took what I could from your words of wisdom! Thanks!
Thanks so much again Alan. I wish I had your vids and ideas for test fixtures when I was teaching Engg. Techs. Standing waves are one of the hardest ideas to grasp so I sent this link to my basic ham class. And so happy that the classic Similarities of Wave Behavior is on TH-cam too. Cheers
Hello Alan. Thank you for your time spent making these videos. In your summary at 10:15, if we can relate that a mis-termination of a transmission line as being an antenna that does not have a reactive, capacitance property of 50 Ohm at the frequency of operation. As well as not having our transmission line from the antenna to our SWR meter at 1/2 wave increments to the Velocity Factor of the coax, would only mean that we would not be able to truly see what are VSWR actually is. For example, if our main run is a random length of coax and we tune an antenna to the coax with the swr meter in our shack, although the swr meter is showing a reasonable swr it doesn't mean our antenna is a 50 ohm termination. Effectively not radiating power from the antenna but the coax as well diminishing the Effective Radiated Power. On the other hand, if we were to have our main run from the antenna to our SWR meter in increments of a 1/2 wave length at the frequency of operation to the Velocity Factor of the coax. Only then would we be able to tune our antenna to the lowest VSWR possible (or as close to 50 ohms as possible). Maximizing our Effective Radiated Power. Make sense?
First point of clarification - there is a mis-termination if the antenna does not have a *RESISTIVE* impedance that is equal to the transmission line impedance (not a reactive or capacitive property). If the antenna presented a 50 ohm capacitive reactance, it would have a very high SWR. The "magic" of a half-wavelength transmission line is that the impedance looking into the line will be equal to the impedance looking into the load (antenna in this case). In other words, the load impedance is replicated when the line is a multiple of half wavelengths long. A longer or shorter transmission line will result in a change in the impedance looking into the line, but does NOT change the SWR on the line. This is a very important statement to grasp - changing the line length does NOT change the SWR, it just changes the complex impedance seen looking into the line. This is illustrated pretty well in one of my videos on the smith chart (th-cam.com/video/ImNRca5ecF0/w-d-xo.html). Of course, this is assuming negligible loss in the transmission line. So, having the transmission line be not equal to a multiple of half-wavelengths long *only* means that the input impedance does not match the antenna impedance - but the SWR is still unchanged! The SWR is a function of the antenna impedance and the transmission line's characteristic impedance, *NOT* the line length. Therefore, if you make adjustments to an antenna to minimize SWR, the result will work fine regardless of line length (as long as the transmission line is not electrically part of the antenna). Bottom line, it is NOT necessary to have the transmission line be a multiple of 1/2 wavelength in order to get maximum radiated power (maximum power transfer to the antenna).
@@w2aew Clarification noted! If I am reading this correctly (Cuz I aint lernt to good) The "magic" of a 1/2 wave length coax is, that it is no longer seen by the transmitter. Poof, It disappears (not taking into account losses in the coax) Ta-da! Magic! To my understanding, the transmitter now sees the antenna only, and if the antenna dos not have a 50 ohm impedance a Standing Wave will be educed on the line and seen by the swr meter. You have made it clear in other videos that a 1/4 wave transmission line looks like a short.(it is also mentioned in the MFJ analyzers manual) Which means the impedance value of our transmission line does change BASED ON length of line with respects to frequency applied. so using a random length of coax while making a 1/4 wave ground plane antenna affects the tuning of the antenna, while trying to make the antenna terminate our line at 50 ohms. Making the coax electrically part of our antenna. (an experiment that everyone can do) If I build a 70 cm 1/4 wave ground plane antenna with a random length of coax, which works perfectly fine with minimal SWR at my house and I give it to my friend for Christmas (cuz i'm such a nice guy) and he uses his own coax, which is a different length and Velocity Factor (then what I made and tuned the antenna with) his SWRs will be totally different than mine. Now, the stipulation to this (in my opinion) is if I give him the antenna WITH the coax (in which i'm really not that nice of a guy) that i made and tuned the antenna with originally and he needs to extend the length of the coax to get to his swr meter. He would need to make a coax jumper in increments of half wave lengths. That would mean that the coax I give him IS Electrically part of the antenna (which is now a 50 OHMs to the end of the coax not to the connection at the antenna) and the extension he needs to make will not be seen by the swr meter because it is in 1/2 wave multiples. Poof, Ta-Da, Magic !! If I would have originally used a length of coax that was multiples of 1/2 wave lengths to the frequency of operation and velocity factor of the coax, The home brew 1/4 wave ground plane antenna, would have an impedance of 50 ohms at the connector.
(End experiment) To to sum it up (in my opinion), 1/2 wave increments of coax is the only way to do things, when setting up a station. Unless an antenna tuner is used. Even then, the work done in the transformation of impedance by a tuner would be less if 1/2 wave increments were used to and from the tuner (the transmitter will not see the coax only the tuner and the tuner would not see the coax, just the impedance mis-match of/at the antenna) That could only be proven with a VNA in which I don't own. Alan, I would like to make something clear. I am not trying to negate, manipulate, argue, dispute anything you are saying or showing us in your video. I really do appreciate the work you have done and your time spent making your videos. The one thing that you have done is sparked further curiosity and ambition on working towards discovery and understanding. I thank you for that.
@@sevenoseven8494 I'm afraid you misunderstood me! The *only* thing that the 1/2 wavelength increment gives you is a replication of the antenna impedance at the coax input. There is NO REASON to restrict yourself to this!! You can use any length you need. If the antenna is matched to the coax, all is well with any length coax. If the antenna is not matched to the coax, the input impedance to the coax won't be 50 ohms, but can still be matched at the transmitter end with a tuner, regardless of coax length. The SWR on the line in this case will be the same - regardless of the coax length. Restricting to 1/2 wavelength increments does not make the tuner's job any easier. The complex impedance looking into the coax will vary with the coax length, but the SWR is the same.
@@w2aew I'm sure I do understand you, It seems we are standing a quarter wave apart on a smith chart looking towards the center, both trying to achieve a 50 Ohm Impedance. 1) you state, The only thing that the 1/2 wavelength increment gives you is a replication of the antenna impedance at the coax input. my response, that's my point! I want to make sure that my antenna has an impedance of 50 Ohms not my coax and antenna! 2) you state, There is NO REASON to restrict yourself to this!! My response, It's not a restriction, having the main run to the antenna in 1/2 wavelength increments, would not be adding the complex impedance of the random length coax to the miss-matched complex impedance of the antenna, further complicating the total complex impedance of the antenna and coax. Why make things more complex then what they should be. the antenna impedance would be the cause of the miss-match on the system creating the SWRs. 3) you state, If the antenna is matched to the coax, all is well with any length coax. My response, If that's the case, I can use RG6 75 Ohm coax trough out my shack. Adjust the angle and length of the ground plane of my antenna to compensate for the impedance and affect the angle of radiation of the antenna. Rendering the antenna useless. Achieving a flat SWR. 4) you state, If the antenna is not matched to the coax, the input impedance to the coax won't be 50 ohms, but can still be matched at the transmitter end with a tuner, regardless of coax length. My response, That's exactly what the purpose of doing things this way. Now I know that the antenna needs adjusting. That's it! Using a half-wave length of coax ensures that the transmitter/SWR meter see the antenna and not the coax and the antenna matches to the transmitter. 5) you state, The SWR on the line in this case will be the same - regardless of the coax length. Restricting to 1/2 wavelength increments does not make the tuner's job any easier. The complex impedance looking into the coax will vary with the coax length, but the SWR is the same. My response, I quote "The only thing that the 1/2 wavelength increment gives you is a replication of the antenna impedance at the coax input." any SWRs will be caused by the impedance miss-match of the antenna. It will show a similar complex impedance miss-match value (of the antenna) on a MFJ analyzer, as if the analyzer was plugged into the bottom of the antenna with no coax or separated by one 1/2 wavelength of coax or separated by multiples of a half-wave length of coax. Intern, showing me how close the antenna is to a 50 ohm impedance. Because the coax becomes less of a factor, aside for the losses of the coax. in which we have no control of.
My point is that you’re not adding any complexity when using non 1/2 wavelength increments. When using 50 ohm coax, minimum SWR occurs when the antenna is adjusted to 50 ohms, regardless of coax length. You don’t have to use 1/2 wavelength increments to know when you’ve adjusted the antenna to 50 ohms.
Excellent demonstration. This would have been good at my tech-college. It took me ages to understand this straight off a blackboard, in a class of semi-interested students.
In the '70s I saw a professor run a light bulb between two conductors energised with a WWII 10 cm radar unit and watched the bulb get brighter and dimmer as he traversed the length. Now I've seen the effect twice!! Cool!!
I wish we had videos like these when they taught waves & antennas in college. All the test equipment were costly, we did not have time, and the teacher wasn't really interested because we could not visualize it! but it looks so cool :)
Very innovative way of showing this Alan. This is where you shine. You had once, awhile back, linked me to an older film produced by bell labs, I think. That film did a great job at flipping my light switch on this subject. You did an excellent job here as well however, your vid was not available then. The only down side of your videos is, I sometimes begin to think I know what I'm doing with RF.....that tends to be dangerous....LOL!
***** Yes, that's the one. Both of you show the exact same concept but in different ways and I agree, both are brilliant, indeed! I think persons tend to see electrical waves as a mysterious electrical process that clouds their judgement. The thing that really put my feet on the ground is seeing, in both the AT&T film and your probing the open line with the detector, that it is a physical process taking place inside the conductor. I think this distinction is what was needed to grasp the subject, at least for me it was. Again, great job as always.
One more great video, Alan. I did not read all comments, but perhaps the answer to your "sliding waveforms" question is: use a trigger source that is independent of any of the two channels shown (ext trigger, for example) and is only slightly different in frequency when compared to the other two waveforms. Also, the frequencies between the two channels are also slightly different but not perceptible. I think this would be enough to create the "travelling wave" effect in both directions, wouldn't it? Well, perhaps there is a simpler approach to it.
Rafael Souza Yep - brownie points to you too. I made one slightly higher than the trigger frequency, and one slightly lower (by the same amount). Thus, when you summed them up, the result was stationary with respect to the trigger source.
At 9:27... So, it seems that where one places the SWR meter along the transmission line affects the reading. How could I find the optimal point and what about added jumper length? Now my head hurts.
Awesome videos. Like others comments TH-cam has become a much greater source of practical learning than university ever was. In this video or one linked in the description you briefly mentioned the RF “jumping” off the transmission cable/antenna. I thought you mentioned that the RF voltage off the antenna was much greater than the cable voltage and a source of the reflected wave. Is there more to it than that? If so a video would be much appreciated.
Hi, This was my favorite video on standing waves that I've been able to find. Quick question, though: At 4:10, where you had the fwd and ref waves at the same amplitudes, what would the resulting standing wave's ratio, therefore be? Specifically, 100% of the voltage is being reflected back, right? I'm just a little confused about infinity, unity, 1:1, 2:1 and other ratios. I would like to see what these waves all look like to connect them to these terms. In fact, I need to see it, because I'm not understanding how 1:1--if this means there is no reflection, and impedances are matched--how on earth can you even call it a standing wave ratio if there are no reflections to create a standing wave? Am I gonzo here or does my question make sense to anyone out there?
Think of it this way... The reflected wave adds to the forward wave and results in causing the RF envelope to have different amplitudes depending on where you are on the line (see the red waveform at 5:43). The VSWR is the ratio of the peak of this envelope (Vpeak) to the trough (Vtrough) of the envelope, such as Vpeak/Vtrough. As the reflected waveform gets worse, the trough gets closer to zero, and the ratio grows. When the trough approaches zero, which would be a reflection from an OPEN or SHORT at the end of the line, the SWR approaches infinity (due to the divide by zero). When there is no reflected waveform, then the RF envelope is constant (Vpeak=Vtrough), so the SWR is 1:1
@@w2aew Actually, that makes perfect sense. I figured it out by working a few equations last night, then saw this this morning. I saw that the 0 was infinity, and it made sense. By using p=1+(coefficient of maxes) / 1-(coefficient of maxes) I could visualize the sizes of the waves and the resulting sums a lot easier. I worked out different ratios. I feel like I've nailed this down pretty good, now, because everything that came previous makes perfect sense to me now. For example, a lamp getting brighter and dimmer along a line. Makes perfect sense to me now! This was awesome. Thanks for the video, it unlocked all of this for me.
No I think it's just the matching of the termination that determines SWR. You can use a slotted line to measure SWR, you do that by sampling the standing wave at different points.
That bouncing/jerking motion of the sum of the two unequal amplitude waves (the partial standing wave) is very important to the proper operation of devices (like an EMdrive and Mach Effect thrusters) it appears, which I think are achieving very weak gravitational induction.
I have so many questions I could ask. Are you able to help visualize how RF travels down a transmission line in the sense of TV/radio towers? I can't wrap my head around how it would look like on a hollow conductor
I don't understand why the 50 ohm termination matters (8:30 mark). It seems to be its not completing the circuit, so its just 50 ohm on top of an open circuit (so negligibly higher, very high impedance). What am I missing? Its only a single trace leading out to the terminator right?
The piece you're missing is the fact that the transmission line *is made up of two conductors*, the trace and the ground plane beneath it. The 50 ohm termination connects the "trace" to the ground plane at the end of the line - so it certainly has a dramatic and expected effect on the reflections and standing waves on the line.
Thanks for the fast response. Really loving the videos. So just to be sure I understand the coax signal line (inner conductor) connects to the outer sheathing via the terminator?
Hi, first of all fantastic explanation!. I would like, if possible, I apreciate if you can clarify the following: theoretically there are not electromagnetic field outside the coax cble shield... then could not be possible to sample signal from it. but it is used! why the coax shield is grounded only in one end and could be ossible use only the central conductor removing the shield??? tks a lot Dario
Hi Alan! I am a big fan of yours and your videos have helped me alot for understanding RF MW. Thanks alot of the great stuff. I tried your probe it works great but I have a few questions. 1. When visualizing the standing waves on the transmission line did you use a modulated signal or just an RF tone @ 2.4 GHz ? 2. This probe worked well for your previous video for detecting the envelope. But I wanted to observe the standing waves on the transmission line. Could you please explain your setup for the second part?
I used a single, unmodulated RF tone. For the second part, I simply have an exposed 50 ohm transmission line that is unterminated. Because it is unterminated, the RF signal will be reflected, resulting in a standing wave on the line.
So basically, you can double the receiving Power of a receiver, by making the receiving line open-ended, and then adding a tap just 1/4 wavelength away (where there is a maximum)? Or is it something I missed?
I love this video!!! Great job. Do you have any resources on how to help me calculate the correct feeder length on a simple 1/4 wavelength dipole antenna? I make dual band antenna tuned to 155Mhz and then a resonant UHF harmonic of around 430Mhz appears. But my antenna are 1.3m long. I'd like them to be short (these are body worn flexible coax antenna by the way) Thanks in advance
That is so cool! I've never seen RF power measured at different positions along a transmission line like that. I guess it's no surprise that it works, but it's always nice to see theory shown with an oscilloscope probe and a steady hand.
Brilliant demo and explanation. Thanks for sharing!
Applied Science Ben and Alan, you guys should meet up and do a video.
Frédéric Dutrey I would *LOVE* that - too bad we're at opposite sides of the country!
***** If only there was a means by which you could collaborate and share data in real time over long distances. I long for the day when that becomes reality.
Yeah, maybe when someone figures out this inter web thingy ;-)
Outstanding demonstration of standing waves!
Thanks Allen. I remember when we did this experiments when I was an undergrad during one of my courses in the lab, I had a "WOW!" moment which changed my perspective forever. :)
The Signal Path Blog Yes, there's nothing like "seeing" things like this - really makes it "click!" Thanks for the comment!
***** Can you make a video explaining type 1 through 3 op amp compensation networks used for switch mode power supplies? Also, what does the "s" mean in (s+1) term in control theory for compensators and what does the H(s) function stand for?
EETechs H(s) is the transfer function of the loop expressed using LaPlace transforms. This would be a more complicated topic and would likely have to encompass a few videos. I'll put it on the list though.
Greetings from India, Alan you are my greatest RF guru
This video is the absolute gold standard for demonstrating standing waves. A wiki page is one thing, but seeing the RF power minima and maxima being measured on an actual transmission line is quite another. Fantastic work W2AEW! 73 de W6PGS.
one more animated visualization from me (made in HFSS) for coax line
ypylypenko.livejournal.com/42947.html
You are very good at explaining somewhat hard to understand "things" in a way that makes them understandable. Thank you.
Michael Lloyd I agree. I've never saw anyone else do it as good as he does.
Your organized thought, visual aids and practical application is spot-on to me. So many You Tubers want to race through a subject without coming up for air. Also, you don't seem to have the tendency of diving down rabbit holes to the point where I feel I've been through a spin cycle! I rarely have to pause your videos either. I revisited this vid because memory is always the first thing to go!
That was a terrific visualisation of an effect I understand, but had never witnessed so clearly. I've watched many of your fabulous videos, Alan, but this one has somehow eluded me. Glad to see it today.
Thank you.
This was fascinating. As a newish ham I never really got SWR until watching this. Thanks!!
Best demonstration of standing waves so far, and a useful function of persistence on a digital scope. Thank you.
Fantastic presentation. Back in the day, we saw similar demos using "Letcher" wires. Nothing can be more informative than actually seeing the reality of the electric waves in their natural habitat, so to speak. Well done. In all of your videos, you have proven yourself to be a great teacher.
Excellent practical demonstration of SW. I used to have students experience wave additions in the floor with 'super' slinkies. Similar wave behavior in different wave mediums makes understanding wave behavior so interesting.
I'm starting to think that utube is much better form of education than the universities. They never explained it this good at my U. In my case it was like here take this bunch of formulas, plug in numbers, calculate, congrats here's your bachelor's.
You just have to find youtube instructors that explain things in a way that works for YOU! I'm glad that my videos do that for you.
Unfortunately, I wasn't a very good student (and that's my fault), but I do think Alan's videos are much more educational than any lectures I received in my EE baccalaureate degree program; my lab classes didn't even use real oscilloscope probes (and, I had no knowledge what additional influences a home-made probe would have introduced , when high frequencies were used in a circuit- besides, we didn't have the equipment to generate anything over 200 megahertz, anyway, if my memory serves me well). We just used coaxial cables, stripped to the conductor on one end, with a BNC connector on the other end. It's embarrassing for me to admit, how much basic electronic science I've learned from Alan's videos - that I should have already know from my formal education as a EE student. I sure hope my university does a better job today educating future engineers - but, I doubt they do, since I can't imagine the lab classes having expensive modern electronic test equipment in them (too expensive). It blows my mind to think of the money wasted in education, when they could just buy some decent test equipment, and pay the correct people to demonstrate how to properly use the test gear - and what the equipment is measuring, and why; And I do believe in the idea of public education, and consider myself to be very much a liberal. However, just today I was reading, in our local paper, about our local convention center receiving an education award for educating children middle school and high school children- not sure of the exact grades; this convention center puts on these very silly plays, which are supposed to be comical (I guess), for which they bus in thousands of kids to watch, during the school day, and call this science education. I started watching one of the plays concerning science, and it was a joke (truly a joke), IMHO.
@@WECB640 I think Walter did a great job with the pendulum explanation.
Wow, I never thought that I'll ever witness RF Standing wave so practically.
You are a teacher that can do magic. Thank you for the effort you put in these teachings. Have you considered a teaching career?
Maybe I'll teach after I retire...
I dont think any body can explain this any better and any clearer than you have Alain. You've nailed it.
Great video! SWR explained without getting drowned in formulas and theory etc. Loved every moment of it. Thanks!
So great to see someone who knows the material explaining it well!
Great video! Really helped me in understanding transmission lines for a project I'm completing for my PhD. Much appreciated!
Amazing demo on Standing waves. Actuall measurement of the standing wave pattern on the live transmission line is absolutely unique. Much appreciated W2AEW !
Wow, I've never been able to see the effects of impedance matching as directly as you measured it here. It's very cool that you were able to measure the nodes and antinodes. Time to break out some PCBs and give it a try!
Ohhh man! That was awesome to see. I am literally learning about standing waves in my electromagnetics class right now. My brain started hurting so I stopped to watch some youtube. I just happen to type in standing waves and got this video. So nice to see what they are talking about in my EM book. All the equations and derivations just make you blind. This really helps me understand what is going on.
To be able to "see" practically what the text books teach, is just amazing. It really helps to take the information on board. Thanks. You have so much content, I'm working my way through. Watch one, think and absorb, watch another... I could be half educated by the end of this! LOL
Genius again.
The best video i saw about stand waves.
Clear, direct and simple.
Sorry about my rusty english.
Huge 59 S9+40
73 DE pu2srz
This an excellent demonstration of VSWR. Thank you!
Brilliant!! Never really understood this. Now I do !! You definitely have a gift for breaking apart the complex and showing us what is really going on.
You make it so much easier to understand transmission line theory. It's pretty hard to visualise it just by reading about it.
In 10 minutes you've completely demystified standing waves for me. Thank you!
That is the coolest thing i have ever seen. Illustrating standing waves with this trick was just amazing. Great thinking! Bravo!
Nice demo. I have a large slotted line and an HP 415E I use to demonstrate SWR. If the crowd is very large, I hook a small amp and speaker to the amp output of the 415E so the 1000Hz tone can be heard around the room. I usually get some comments about the archaic equipment. Then I like to point out that this is the type of network analyzer that was used to design the equipment that brought back TV signals from the moon walks.
I've had great trouble in the Electromagnetics course because while using Smith charts, we never went over what the transmission and reflection coefficients, nor the VSWR actually represented visually. Thanks so much for clearing this up.
Thanks for sharing! I watched this video with my 9 year old and it got him excited to break out his littlebits.
Awesome visualization of standing waves as well as a description of constructive/destructive wave interference.
Wow, your visualization methods help a lot to understand. Thank you!
Amazing to actually see it in action! TY
Thanks man. This does not only made me understand standing wave, but i now visualize the concept of transmission
Definitely booking marking this
I don't know how you do it. I could read for a week on all this voodoo magic, and you Sir, sum it all up in 10 minutes.
Your channel is one of the best I have have subbed.
Yes, yes exactly!
Thank you for taking the time to demonstrate standing waves. Great work!
Amazing video as always, I was having a discussion with a friend of mine as to why the normalized impedance would repeat itself on a transmission line every half wavelength. This proves it. Also that is true for any termination except matched termination aka 50 ohms in most cases. Sits really well with the smith chart also. Alan you are true genius.
Thanks for letting me see this phenomena. Although yuur previous video spoke of it, it was tough to believe it. This video gives the edge. Thank you so much.
Thumbs up! Great video to visualize standing waves on TML. The first time that I have seen such a kind of visualization.
This is such a great intuitive explanation of things. Thanks so much!
Well I feel like an infant listening to their first words! OMG! Some cool stuff here! I understood about 50% of it, but I took what I could from your words of wisdom! Thanks!
So awesome!! I'm studying about Transmission Lines right now and it's so cool to actually see all the theory presented in the books!
Thanks so much again Alan. I wish I had your vids and ideas for test fixtures when I was teaching Engg. Techs. Standing waves are one of the hardest ideas to grasp so I sent this link to my basic ham class. And so happy that the classic Similarities of Wave Behavior is on TH-cam too. Cheers
Hello Alan. Thank you for your time spent making these videos.
In your summary at 10:15, if we can relate that a mis-termination of a transmission line as being an antenna that does not have a reactive, capacitance property of 50 Ohm at the frequency of operation. As well as not having our transmission line from the antenna to our SWR meter at 1/2 wave increments to the Velocity Factor of the coax, would only mean that we would not be able to truly see what are VSWR actually is.
For example, if our main run is a random length of coax and we tune an antenna to the coax with the swr meter in our shack, although the swr meter is showing a reasonable swr it doesn't mean our antenna is a 50 ohm termination. Effectively not radiating power from the antenna but the coax as well diminishing the Effective Radiated Power.
On the other hand, if we were to have our main run from the antenna to our SWR meter in increments of a 1/2 wave length at the frequency of operation to the Velocity Factor of the coax. Only then would we be able to tune our antenna to the lowest VSWR possible (or as close to 50 ohms as possible). Maximizing our Effective Radiated Power.
Make sense?
First point of clarification - there is a mis-termination if the antenna does not have a *RESISTIVE* impedance that is equal to the transmission line impedance (not a reactive or capacitive property). If the antenna presented a 50 ohm capacitive reactance, it would have a very high SWR.
The "magic" of a half-wavelength transmission line is that the impedance looking into the line will be equal to the impedance looking into the load (antenna in this case). In other words, the load impedance is replicated when the line is a multiple of half wavelengths long. A longer or shorter transmission line will result in a change in the impedance looking into the line, but does NOT change the SWR on the line. This is a very important statement to grasp - changing the line length does NOT change the SWR, it just changes the complex impedance seen looking into the line. This is illustrated pretty well in one of my videos on the smith chart (th-cam.com/video/ImNRca5ecF0/w-d-xo.html). Of course, this is assuming negligible loss in the transmission line.
So, having the transmission line be not equal to a multiple of half-wavelengths long *only* means that the input impedance does not match the antenna impedance - but the SWR is still unchanged! The SWR is a function of the antenna impedance and the transmission line's characteristic impedance, *NOT* the line length. Therefore, if you make adjustments to an antenna to minimize SWR, the result will work fine regardless of line length (as long as the transmission line is not electrically part of the antenna).
Bottom line, it is NOT necessary to have the transmission line be a multiple of 1/2 wavelength in order to get maximum radiated power (maximum power transfer to the antenna).
@@w2aew
Clarification noted!
If I am reading this correctly (Cuz I aint lernt to good) The "magic" of a 1/2 wave length coax is, that it is no longer seen by the transmitter. Poof, It disappears (not taking into account losses in the coax) Ta-da! Magic!
To my understanding, the transmitter now sees the antenna only, and if the antenna dos not have a 50 ohm impedance a Standing Wave will be educed on the line and seen by the swr meter.
You have made it clear in other videos that a 1/4 wave transmission line looks like a short.(it is also mentioned in the MFJ analyzers manual) Which means the impedance value of our transmission line does change BASED ON length of line with respects to frequency applied. so using a random length of coax while making a 1/4 wave ground plane antenna affects the tuning of the antenna, while trying to make the antenna terminate our line at 50 ohms. Making the coax electrically part of our antenna.
(an experiment that everyone can do)
If I build a 70 cm 1/4 wave ground plane antenna with a random length of coax, which works perfectly fine with minimal SWR at my house and I give it to my friend for Christmas (cuz i'm such a nice guy) and he uses his own coax, which is a different length and Velocity Factor (then what I made and tuned the antenna with) his SWRs will be totally different than mine.
Now, the stipulation to this (in my opinion) is if I give him the antenna WITH the coax (in which i'm really not that nice of a guy) that i made and tuned the antenna with originally and he needs to extend the length of the coax to get to his swr meter. He would need to make a coax jumper in increments of half wave lengths. That would mean that the coax I give him IS Electrically part of the antenna (which is now a 50 OHMs to the end of the coax not to the connection at the antenna) and the extension he needs to make will not be seen by the swr meter because it is in 1/2 wave multiples. Poof, Ta-Da, Magic !!
If I would have originally used a length of coax that was multiples of 1/2 wave lengths to the frequency of operation and velocity factor of the coax, The home brew 1/4 wave ground plane antenna, would have an impedance of 50 ohms at the connector.
(End experiment)
To to sum it up (in my opinion), 1/2 wave increments of coax is the only way to do things, when setting up a station. Unless an antenna tuner is used. Even then, the work done in the transformation of impedance by a tuner would be less if 1/2 wave increments were used to and from the tuner (the transmitter will not see the coax only the tuner and the tuner would not see the coax, just the impedance mis-match of/at the antenna) That could only be proven with a VNA in which I don't own.
Alan, I would like to make something clear. I am not trying to negate, manipulate, argue, dispute anything you are saying or showing us in your video.
I really do appreciate the work you have done and your time spent making your videos. The one thing that you have done is sparked further curiosity and ambition on working towards discovery and understanding.
I thank you for that.
@@sevenoseven8494 I'm afraid you misunderstood me! The *only* thing that the 1/2 wavelength increment gives you is a replication of the antenna impedance at the coax input. There is NO REASON to restrict yourself to this!! You can use any length you need. If the antenna is matched to the coax, all is well with any length coax. If the antenna is not matched to the coax, the input impedance to the coax won't be 50 ohms, but can still be matched at the transmitter end with a tuner, regardless of coax length. The SWR on the line in this case will be the same - regardless of the coax length. Restricting to 1/2 wavelength increments does not make the tuner's job any easier. The complex impedance looking into the coax will vary with the coax length, but the SWR is the same.
@@w2aew
I'm sure I do understand you, It seems we are standing a quarter wave apart on a smith chart looking towards the center, both trying to achieve a 50 Ohm Impedance.
1) you state,
The only thing that the 1/2 wavelength increment gives you is a replication of the antenna impedance at the coax input.
my response,
that's my point! I want to make sure that my antenna has an impedance of 50 Ohms not my coax and antenna!
2) you state,
There is NO REASON to restrict yourself to this!!
My response,
It's not a restriction, having the main run to the antenna in 1/2 wavelength increments, would not be adding the complex impedance of the random length coax to the miss-matched complex impedance of the antenna, further complicating the total complex impedance of the antenna and coax. Why make things more complex then what they should be. the antenna impedance would be the cause of the miss-match on the system creating the SWRs.
3) you state,
If the antenna is matched to the coax, all is well with any length coax.
My response,
If that's the case, I can use RG6 75 Ohm coax trough out my shack. Adjust the angle and length of the ground plane of my antenna to compensate for the impedance and affect the angle of radiation of the antenna. Rendering the antenna useless. Achieving a flat SWR.
4) you state,
If the antenna is not matched to the coax, the input impedance to the coax won't be 50 ohms, but can still be matched at the transmitter end with a tuner, regardless of coax length.
My response,
That's exactly what the purpose of doing things this way. Now I know that the antenna needs adjusting. That's it! Using a half-wave length of coax ensures that the transmitter/SWR meter see the antenna and not the coax and the antenna matches to the transmitter.
5) you state,
The SWR on the line in this case will be the same - regardless of the coax length. Restricting to 1/2 wavelength increments does not make the tuner's job any easier. The complex impedance looking into the coax will vary with the coax length, but the SWR is the same.
My response,
I quote "The only thing that the 1/2 wavelength increment gives you is a replication of the antenna impedance at the coax input." any SWRs will be caused by the impedance miss-match of the antenna. It will show a similar complex impedance miss-match value (of the antenna) on a MFJ analyzer, as if the analyzer was plugged into the bottom of the antenna with no coax or separated by one 1/2 wavelength of coax or separated by multiples of a half-wave length of coax. Intern, showing me how close the antenna is to a 50 ohm impedance. Because the coax becomes less of a factor, aside for the losses of the coax. in which we have no control of.
My point is that you’re not adding any complexity when using non 1/2 wavelength increments. When using 50 ohm coax, minimum SWR occurs when the antenna is adjusted to 50 ohms, regardless of coax length. You don’t have to use 1/2 wavelength increments to know when you’ve adjusted the antenna to 50 ohms.
Simply the best, this should be the supplementary video material for any transmission / Power related EE course
As usual you make a complex topic so much easier to visualize in real life! 73 - Dino KL0S
Excellent demonstration. This would have been good at my tech-college. It took me ages to understand this straight off a blackboard, in a class of semi-interested students.
Best RF teacher on the planet!
Thanks Alan, every time I watch one of your vids I know more about RF theory, I really like your practical illustrations of this sort of thing.
Thank you Gerry!
Just saw saw this excellent video. I hope you are compensated in some way for all of your vauable videos! I have learned so much from you.
I like this practical literal demonstration of SWR stuff.
In the '70s I saw a professor run a light bulb between two conductors energised with a WWII 10 cm radar unit and watched the bulb get brighter and dimmer as he traversed the length. Now I've seen the effect twice!!
Cool!!
Fantastic video. so much knowledge compressed in only 10 mins.
I can't stop watching that video over and over again. you are the master of desk experiment.
Beautifully and perfectly illustrated.
Best explanation and practical demonstration ever! 4 thumbs up.....
Another awesome demonstration, Alan. - Thank you! - Jim
I've been following whenever; your way and material of explaining things is it. Thank you Sir. ae.
Perfect demo, 5 stars (cannot be explained better).
Awesome video. The visualization really helped me understand SWR
Fantastic video. Please continue making these informative videos on RF!
Nice Video. I clicked like and shared with some coworkers who are learning about this for FM Broadcast (Low power).
really great way to explain SWR.
Great video! It's so nice to see things come together in reality after studying this stuff on paper.
Incredible! I've been struggling to understand SWR and this helped me a lot!
That is an excellent video on standing waves.
I wish we had videos like these when they taught waves & antennas in college. All the test equipment were costly, we did not have time, and the teacher wasn't really interested because we could not visualize it! but it looks so cool :)
This is really awesome. Really appreciate the demo.
Brilliant demo!
Very innovative way of showing this Alan. This is where you shine. You had once, awhile back, linked me to an older film produced by bell labs, I think. That film did a great job at flipping my light switch on this subject. You did an excellent job here as well however, your vid was not available then. The only down side of your videos is, I sometimes begin to think I know what I'm doing with RF.....that tends to be dangerous....LOL!
bain5872 Yes, this is the video you were speaking about - totally brilliant!
th-cam.com/video/DovunOxlY1k/w-d-xo.html
*****
Yes, that's the one. Both of you show the exact same concept but in different ways and I agree, both are brilliant, indeed! I think persons tend to see electrical waves as a mysterious electrical process that clouds their judgement. The thing that really put my feet on the ground is seeing, in both the AT&T film and your probing the open line with the detector, that it is a physical process taking place inside the conductor. I think this distinction is what was needed to grasp the subject, at least for me it was. Again, great job as always.
amazing, really helped me understand standing waves
Great job on bringing my Emag class to life. Thank you.
Well done indeed- reminds me of the old HP slotted lines we used at Solitron Microwave. 73
One more great video, Alan.
I did not read all comments, but perhaps the answer to your "sliding waveforms" question is: use a trigger source that is independent of any of the two channels shown (ext trigger, for example) and is only slightly different in frequency when compared to the other two waveforms. Also, the frequencies between the two channels are also slightly different but not perceptible. I think this would be enough to create the "travelling wave" effect in both directions, wouldn't it?
Well, perhaps there is a simpler approach to it.
Rafael Souza Yep - brownie points to you too. I made one slightly higher than the trigger frequency, and one slightly lower (by the same amount). Thus, when you summed them up, the result was stationary with respect to the trigger source.
Very nice and clear to see how it behaves
I like your video, thank you so much for this video😊
Really good demonstration!
Your videos are always great. I'll be waiting for the next one !
Brilliant video! Did you create the sliding waveforms by triggering on a sine wave of a slightly different frequency to the forward and reflected?
Precisely.
Clever way to show VSWR, many thanks, from M3KQW. 73s
At 9:27... So, it seems that where one places the SWR meter along the transmission line affects the reading. How could I find the optimal point and what about added jumper length? Now my head hurts.
thank you so much for these videos
you make understanding electricity sooo much easier..
you're doing amazing work
Awesome videos. Like others comments TH-cam has become a much greater source of practical learning than university ever was. In this video or one linked in the description you briefly mentioned the RF “jumping” off the transmission cable/antenna. I thought you mentioned that the RF voltage off the antenna was much greater than the cable voltage and a source of the reflected wave. Is there more to it than that? If so a video would be much appreciated.
A very clever demonstration
It was a shame I discovered this video today. I would have save a lot of questions.
It´s great.
Thanks
Hi,
This was my favorite video on standing waves that I've been able to find. Quick question, though: At 4:10, where you had the fwd and ref waves at the same amplitudes, what would the resulting standing wave's ratio, therefore be? Specifically, 100% of the voltage is being reflected back, right? I'm just a little confused about infinity, unity, 1:1, 2:1 and other ratios. I would like to see what these waves all look like to connect them to these terms. In fact, I need to see it, because I'm not understanding how 1:1--if this means there is no reflection, and impedances are matched--how on earth can you even call it a standing wave ratio if there are no reflections to create a standing wave? Am I gonzo here or does my question make sense to anyone out there?
Think of it this way... The reflected wave adds to the forward wave and results in causing the RF envelope to have different amplitudes depending on where you are on the line (see the red waveform at 5:43). The VSWR is the ratio of the peak of this envelope (Vpeak) to the trough (Vtrough) of the envelope, such as Vpeak/Vtrough. As the reflected waveform gets worse, the trough gets closer to zero, and the ratio grows. When the trough approaches zero, which would be a reflection from an OPEN or SHORT at the end of the line, the SWR approaches infinity (due to the divide by zero). When there is no reflected waveform, then the RF envelope is constant (Vpeak=Vtrough), so the SWR is 1:1
@@w2aew Actually, that makes perfect sense. I figured it out by working a few equations last night, then saw this this morning. I saw that the 0 was infinity, and it made sense. By using p=1+(coefficient of maxes) / 1-(coefficient of maxes) I could visualize the sizes of the waves and the resulting sums a lot easier. I worked out different ratios. I feel like I've nailed this down pretty good, now, because everything that came previous makes perfect sense to me now. For example, a lamp getting brighter and dimmer along a line. Makes perfect sense to me now! This was awesome. Thanks for the video, it unlocked all of this for me.
At 4:41...Does that mean coax length is critical for both reducing SWR and where you insert the meter to read SWR?
lol, i asked the same question. not as straight forward as you did, but the same question none the less.
No I think it's just the matching of the termination that determines SWR. You can use a slotted line to measure SWR, you do that by sampling the standing wave at different points.
Oh nvm, he pretty much makes a slotted line with the strip line later in the video. good demo.
That bouncing/jerking motion of the sum of the two unequal amplitude waves (the partial standing wave) is very important to the proper operation of devices (like an EMdrive and Mach Effect thrusters) it appears, which I think are achieving very weak gravitational induction.
I have so many questions I could ask. Are you able to help visualize how RF travels down a transmission line in the sense of TV/radio towers? I can't wrap my head around how it would look like on a hollow conductor
I don't understand why the 50 ohm termination matters (8:30 mark). It seems to be its not completing the circuit, so its just 50 ohm on top of an open circuit (so negligibly higher, very high impedance). What am I missing? Its only a single trace leading out to the terminator right?
The piece you're missing is the fact that the transmission line *is made up of two conductors*, the trace and the ground plane beneath it. The 50 ohm termination connects the "trace" to the ground plane at the end of the line - so it certainly has a dramatic and expected effect on the reflections and standing waves on the line.
Thanks for the fast response. Really loving the videos. So just to be sure I understand the coax signal line (inner conductor) connects to the outer sheathing via the terminator?
Yes, the 50 ohm resistor in the terminator connects the center conductor to the outer conductor.
Another fantastic and informative video. Keep them coming. Karl
RF black magic, utterly fascinating.
Hi, first of all fantastic explanation!. I would like, if possible, I apreciate if you can clarify the following: theoretically there are not electromagnetic field outside the coax cble shield... then could not be possible to sample signal from it. but it is used! why the coax shield is grounded only in one end and could be ossible use only the central conductor removing the shield??? tks a lot Dario
Hi Alan! I am a big fan of yours and your videos have helped me alot for understanding RF MW. Thanks alot of the great stuff.
I tried your probe it works great but I have a few questions.
1. When visualizing the standing waves on the transmission line did you use a modulated signal or just an RF tone @ 2.4 GHz ?
2. This probe worked well for your previous video for detecting the envelope. But I wanted to observe the standing waves on the transmission line. Could you please explain your setup for the second part?
I used a single, unmodulated RF tone. For the second part, I simply have an exposed 50 ohm transmission line that is unterminated. Because it is unterminated, the RF signal will be reflected, resulting in a standing wave on the line.
So basically, you can double the receiving Power of a receiver, by making the receiving line open-ended, and then adding a tap just 1/4 wavelength away (where there is a maximum)? Or is it something I missed?
I love this video!!! Great job. Do you have any resources on how to help me calculate the correct feeder length on a simple 1/4 wavelength dipole antenna? I make dual band antenna tuned to 155Mhz and then a resonant UHF harmonic of around 430Mhz appears. But my antenna are 1.3m long. I'd like them to be short (these are body worn flexible coax antenna by the way) Thanks in advance
Magic explanation and demo!