I am SO GLAD to see you back again ! When youtube announces me that you uploaded somwthing I am sure I will learn something new in a proper way. It's been a while ...
Who on Earth thumbs videos like this down? w2aew educational videos are so easy to understand thanks to the awesome and concise explanations of essentially complex physics phenomena that I'm sure he could explain quantum theory or thermal dynamics to a 5-year-old, and they would get it.
Merry Christmas - or seasons greatings. I don't know what's allowed anymore. You really make the top of my international youtube list, and now w2aew made a great entry. At any rate, greetings from OZ7AHR.
I wanted to suggest this to some people I was working with in the field at a radar site. They were a mixture of EEs and technicians, and had no idea how to determine the distance down the line, of a fault, without specialty test equipment. I have an ME, but have little EE coursework under my belt (but I have a lot of experience with oscilloscopes). I was not confident enough to suggest anything, and the people at site didn't want to hear anything from me anyway. Glad to see it might have worked.
No.. not really TDR is simple , it just SOUND complicated TIME DOMAIN REFLECTOMETRY sound scary SIMPLE TRANSLATION : A Little signal is sent all the way down a cable and when it gets to the end a reflection is caused , causing it to come all the way back to the beginning of the cable Now 1. We know how fast the signal travels and it travels at a constant pace 2. We can measure the time it takes to get there and back because time is a constant 3. so.... if we want to know distance, we just used DISTANCE = SPEED / TIME that's it Then because the signal went up and back, you divide the answer by 2 to get the length in 1 direction it's not really that hard, Just sounds fancy
I vouch from personal experience this technique to repurpose a 100m section of 19mm dia SHF triax feeder for a microwave link. The cable had been installed in a semi-permanent location but at the weather gland where the it entered the transmitter hut there had been some damage that had left the cable under unnecessary bending strain in direct sunlight during day eventually resulting in a dead short of the inner conductor to both outer screens. The run was replaced with new but shortened lengths from the 100meters can be be used on other jobs as cutdown spare lengths. Where to cut was the quandary and with no obvious recorded marking of the damage point nor any deformity i chose to use TDR. The result was that when i put the cable cutter through the calculated distance point it was right on the centre to outers short! That was a great feeling to see Physics & theory come together! There was a small section where the dielectric had softened probably in the heat of high summer over many cycles allowing the centre conductor to eventually move enough & finally touch the two concentric braided conductors.
Nice short and quick test method there Alan. If you have the scope available the rest of the parts are likely to also be there, and you just need the T piece and the wire ended resistor, the adaptor is not needed, as you can simply put the resistor end into the T socket, and touch the battery to both.
Wow.... all those years I scrambled to find a TDR at work.... and thought I could never measure length myself without a TDR! Thank you Alan... I can do it now right at my operating position! 73 KK0DJ
I just found the break in a guitar cable using this TDR technique :D The break was inside one of the jacks. The break was 2 mm before the very end of the center wire inside a coax cable, right next to the tab it was soldered to inside the connector. The delta x was 41 ns so the break should be a little more than 4 meters away, according to the formula. The cable was about 3,5 meters long, so it had to be at the very other end
Your channel has an excellent signal to noise ratio , which in the days of millienals consuming bandwidth on anti social media , with much noise and hardly any signal is a welcome reprieve .
You're very confident in your presentation, and thank you for taking the time to edit these videos. They each tell a GREAT story! I wish you had been my electronics instructor in high school, it would have been neato! :) good to see you back!
Wow. So simple. Can't believe I've not seen it done before. Much more accurate than cutting off an known length and measuring the capacitor ratio of both. Thanks for this tutorial.
That's about as simple as it can be! I was about to build the TDR based on the 74AC14 to test the integrity of some cables and it seems I no longer need it :-) Thanks for another very pleasant and informative video!
Hi Alan, Thank you so much for this series of videos! I’ve built your Schmidt trigger generator and it works like a charm! I used a 74AC14 smd chip and 1% resistors, and I put them in a shielded airplane junction box that I had lying about. I powered it up with 3V and I got a rise time of 4ns. Right now struggling with crappy cables. Thanks for the great videos! Cheers, Mark
Great demo! I love simple. Thanks! Years ago, I used a similar technique to resonate a coil and capacitor for 14.318MHz VCO in my very first video frame grabber. (The literature of the day was obsessed with 4fSC at that time) I wound a few turns of coil on a hunk of chopped off Bic pen, put 22pF cap (representing the varactor diode I planned to use) in parallel with it and AC coupled a pulse from a 555 timer running at a couple hundred Hz. I then looked at the ringing of the LC combo on the scope. The scope was triggered on ch2 directly from the 555 and ch1 showed the ringing of the LC. I tuned it 70nS by adding / subtracting turns and in less than 5 minutes, Bob was my uncle!
Fantastic video. I am a newbie electronic hobbyist. Good explaination on use of the oscilloscope. I'm going to make this my next project. Thanks for a very easy to understand tutorial.😁
In case folks are here for methods of estimating remaining cable on a spool, you can always weight it, subtract weight of an empty spool, and divide by weight of some known length of cable.
@@w2aew True. But you may well have an empty spool laying around to weigh. And if you don't, you might write on the spool you have a note reminding you to weigh it when it's empty :-).
@@Graham_Wideman If you have a buried cable with a break in it, could you not use this method to find the distance from the termination point that the break is at? It would make it easier for repair work.
Good video, subscribed. I had wondered about trying this on wires to my well pump to figure out how deep it was. Sadly the pump failed and I got to measure it the hard way.
Nice. You actually sending a step function signal along the line. No polarity change, only 0 to 9V! As it propagates and reflects - it looks like it only interferes constructively. Doesn't it? Very interesting.
@@eugeneeugene3313 This is the right answer. The coax is an open circuit, so once it is "charged" up to the full battery voltage, no more current flows - thus no voltage drop across the series resistor, and the voltage on the line equals the battery voltage.
A TDR application I like is on railroad crossings where it is used to calculate the speed of an approaching train so the gates can be down 30 seconds prior to passing the crossing, no matter the speed of the train. This is very handy for big mainlines with 65 mph freight trains and 100 mph passenger trains...
WoW, this is a great learningvid! Really clear, complete and helpfull! Or most great one this one is the supereasy super usefullness! Thanks for your knowledgesharing!
Nice, simple explanation and implementation. I had no idea that amateur radio would lead me to falling down the electronics rabbit hole. And its all your fault! M0YZT
I upgraded with a timer IC and a FET driver IC. Keep this device in my scope kit. I added a 500 ohm trim pot to find the nominal impedance of the cable. I found most cheap "DMX digital cables" are actually built with low impedance microphone cable. They measure 60 to 70 ohms instead of 110 to 120 ohms. This is important if you do stage lighting.
Depending on the scope, I’ve used the square wave probe calibration output on the scope itself as a signal source for tdr. Never thought of doing tdr for a roll of speaker wire or lamp cord. I now have an easy way of estimating how long my box of instrumentation cable is too, thanks!
Seeing you move around the cursors, i wonder why vintage analog scope manufacturers never made a mechanical variant of those. I got an old Tek 454a, once a really expensive beast. But while it has loads of features and bandwith that i won't use, i'm still stuck counting divisions by hand. It seems to me that it would have made sense for Tektronix, HP and the other big names of the past to deliver such high end things with slide rule like cursors, or caliper style with a nonius. Hell, perhaps even with a spinning dial like you see on some micrometer calipers. I would've expected it to be a major selling point. Hell, for the highest of high end stuff you could make some mechanical linkages between time/div switch and several scales to mechanically calculate the period time, perhaps even to indicate the frequency... Not as accurate as a real counter but i can imagine that it would definitely come in handy.
Alan, thanks again for another practical EE video. Never thought you could do a TDR measurement as simple as this. Hmmmm I wonder what happens when you try it on a 3 conductor wire like Romex...
Always love your perfect videos!!! Happy Holidays to you and your family! Merry Christmas and Happy New Year also. But I wish you mainly good health or at least better health & love!
I agree with all you showed. Have done that myself. But like in radars, you can do with some other way, often called Chirp. Too bad that goes to the opposite dirction from the basic idea of most simple instrument requirement. You would need some sort of sweep frequency source, although I speculate that I could do it by finding the different resonant frequencies using a grid dip meter. Or maybe that is too much of history by now. Maybe a FET dipper?
I was so disappointed to learn this can't be done with a single wire. I thought you found a magical mystical new way! I have spools of magnet wire I like to know the length of, without resorting to calculating it through resistance and cross section? that 9 feet something, was impressive resolution for such a simple procedure.
With enamel wire most of the mass is in the copper, I'd thing you should be able to get close enough just assuming the whole mess is 100% copper, weighing it, and calculating from there.
@@Broken_Yugo A quick look on Google and I find #24 wire weighs 0.124 pounds per 100 feet. The chart I found didn't say if that was bare wire or what sort of insulation. I'd assume bare, but probably looking at something other than the first hit Google brought up would give a result for enable wire or even DCC if that is what the OP has. The reel will weigh something, but for 1lb to 10lb spools this will probably be thin sheet metal or thin plastic, and add less than about 5% to the total weight. So a simple weight will probably be as or more accurate than either a resistance check (if you could get both ends of the wire on the spool) or some attempt at a TDR-like length determination.
I love the videos, but it kind of drives me up the wall to be reminded that I can't really use any of it with the equipment I have and the task I need it for. I like that you included the UTP notes though. Idk how much VF uncertainty is going to be a problem when dealing with waterlogged cables.
I would think that the VF would change on waterlogged cables since the dielectric constant between the conductors will change. While I show a high end scope here, the same can be done with a 50 or 100MHz digital scope (or an analog scope with a scope camera, or using a pulse generator instead of the battery and resistor).
@@w2aew Yeah, I have an old (10? 20MHz?) dumpsterized single-trace CRO. Trying to capture single-shot events is kind of a maddening gamble, and usually results in enough image distortion (from a digital camera) that time measurements are unreliable. I can use a pulse train, but I can't really zoom in on the leading edge of a single pulse, because the trigger delay will always cut it off at short timebases. I would need to make a synchronized trigger source to lead it ... but that would require a scope with a working external trigger input. There really isn't much to do about it. Frustration is a natural consequence when the people with the right tools for the job already gave up and walked away. I still really appreciate the videos though.
Allan great video. Using the same ideia you can put the open end in another channel it is possible to measure the velocity factor from the delay that it causes. I know that the propose of the vídeo are to measure maybe an cable that are installed and you dont know the length ... but as all yours videos are so didactic I thought that it will be an addendum to the subject. (chanels 1 and 3 are best when there are 4 channels because many scopes uses interleave in adjacent channels (1 and 2 for example)). thx for all the knowledge shared.
The real juvel here is that you can measure the real VF of a cable at hand - and I guess that you can find the caracteristic impedance of the cable just by changing the resistor and find the cleanest waveform. You can always use a tape measure to get the length, but finding the real VF (for baluns, filters etc.) is really the key, right? Knowing the lenght of the coax by measurement, we can easily calculate VF - and by changing the resistor you can find the true caracteristic impedance.
I was waiting for you to short the far end of the coax and for the reflection to invert...but the moment never came. IMO no TDR demo is complete without showing this inversion.
Love it! Glad I stumbled onto your channel. Here’s a thought. So you have two spoils of single conductor wire. One of known length and one unknown. If they are separated a good ways (super high L & C) I wonder if you could get a decent guesstimation length of the unknown spool? Thx. WD5JJT
Hi Alan This video like most of the others you have done on this subject relies upon look-up sources to obtain Velocity Factor figures. For transmission Line that is pretty easy to find. But believe it or not for more common types of conductor it is not so easy to find. For instance take common copper pipe. I can't for the life of me find it. Same thing can be said for 6061-T6 Aluminum. Is there something I'm overlooking ? Bob AA6XE (ex-K6TR)
Velocity factor doesn't depend on the conductor type, it depends on the dielectric constant of the insulator between the conductors. That's why you can't find it for copper pipe or aluminum. If the insulator is air, then the VF will be close to 1.
Excellent video, Alan! This video also explains why you can have a high SWR on your antennas, due to the reflected energy. I don’t know if you know much about physics, but now that I’ve seen your video, you have me thinking. Why does the energy reflect back into the wire, and not simply dissipate from the end? I’m guess that it has something to do with the wire being a better conductor than air. Any thoughts?
I am SO GLAD to see you back again ! When youtube announces me that you uploaded somwthing I am sure I will learn something new in a proper way. It's been a while ...
Who on Earth thumbs videos like this down?
w2aew educational videos are so easy to understand thanks to the awesome and concise explanations of essentially complex physics phenomena that I'm sure he could explain quantum theory or thermal dynamics to a 5-year-old, and they would get it.
I guess you can't please everyone. Thanks for the support!
It is so very enjoyable to listen to someone with such a thorough knowledge of electronics. Thanks.
W2aew is a master to me and i love to watch his tutorials
And the liking of experimentation.
Great presentation Alan. Merry Christmas to You and Yours!
Thank you Paul! It's an honor to know that you like my videos! Merry Christmas to you, and best wishes (and hopes) for a better 2021!
@@w2aew Two really smart guys admiring each other's work, love it!
@@patrickbouldinkm5l143 Nerd Smarts overload! ☺️
2 of my favorite geniuses
Merry Christmas - or seasons greatings. I don't know what's allowed anymore. You really make the top of my international youtube list, and now
w2aew made a great entry. At any rate, greetings from OZ7AHR.
I wanted to suggest this to some people I was working with in the field at a radar site. They were a mixture of EEs and technicians, and had no idea how to determine the distance down the line, of a fault, without specialty test equipment. I have an ME, but have little EE coursework under my belt (but I have a lot of experience with oscilloscopes). I was not confident enough to suggest anything, and the people at site didn't want to hear anything from me anyway.
Glad to see it might have worked.
Well, maybe you could just send them a link to this video before you head out to the field again ;-)
@@w2aew lol
Excellent video, TDR always seemed very mysterious and demanding technically and it’s encouraging to see it done so simply
No.. not really
TDR is simple , it just SOUND complicated
TIME DOMAIN REFLECTOMETRY sound scary
SIMPLE TRANSLATION : A Little signal is sent all the way down a cable and when it gets to the end a reflection is caused , causing it to come all the way back to the beginning of the cable
Now
1. We know how fast the signal travels and it travels at a constant pace
2. We can measure the time it takes to get there and back because time is a constant
3. so.... if we want to know distance, we just used DISTANCE = SPEED / TIME
that's it
Then because the signal went up and back, you divide the answer by 2 to get the length in 1 direction
it's not really that hard, Just sounds fancy
I vouch from personal experience this technique to repurpose a 100m section of 19mm dia SHF triax feeder for a microwave link. The cable had been installed in a semi-permanent location but at the weather gland where the it entered the transmitter hut there had been some damage that had left the cable under unnecessary bending strain in direct sunlight during day eventually resulting in a dead short of the inner conductor to both outer screens. The run was replaced with new but shortened lengths from the 100meters can be be used on other jobs as cutdown spare lengths. Where to cut was the quandary and with no obvious recorded marking of the damage point nor any deformity i chose to use TDR. The result was that when i put the cable cutter through the calculated distance point it was right on the centre to outers short! That was a great feeling to see Physics & theory come together! There was a small section where the dielectric had softened probably in the heat of high summer over many cycles allowing the centre conductor to eventually move enough & finally touch the two concentric braided conductors.
Nice short and quick test method there Alan. If you have the scope available the rest of the parts are likely to also be there, and you just need the T piece and the wire ended resistor, the adaptor is not needed, as you can simply put the resistor end into the T socket, and touch the battery to both.
Wow.... all those years I scrambled to find a TDR at work.... and thought I could never measure length myself without a TDR! Thank you Alan... I can do it now right at my operating position! 73 KK0DJ
I just found the break in a guitar cable using this TDR technique :D The break was inside one of the jacks. The break was 2 mm before the very end of the center wire inside a coax cable, right next to the tab it was soldered to inside the connector. The delta x was 41 ns so the break should be a little more than 4 meters away, according to the formula. The cable was about 3,5 meters long, so it had to be at the very other end
Not sure why this was recommended. Turned out to be much more interesting than I thought it might be. Will certainly look at more of your videos.
This method is much easier than unwinding, measuring, and rewinding a reel to get length. Very much appreciated, N1QDT
Your channel has an excellent signal to noise ratio , which in the days of millienals consuming bandwidth on anti social media , with much noise and hardly any signal is a welcome reprieve .
You're very confident in your presentation, and thank you for taking the time to edit these videos. They each tell a GREAT story! I wish you had been my electronics instructor in high school, it would have been neato! :) good to see you back!
Wow. So simple. Can't believe I've not seen it done before. Much more accurate than cutting off an known length and measuring the capacitor ratio of both. Thanks for this tutorial.
People will be referencing this very useful video for years.
It was simple and very educational.
School tells the theory. It sure is nice to see it demonstrated! And fun too. Your videos are awesome learning tools!
That's about as simple as it can be! I was about to build the TDR based on the 74AC14 to test the integrity of some cables and it seems I no longer need it :-)
Thanks for another very pleasant and informative video!
Hi Alan,
Thank you so much for this series of videos! I’ve built your Schmidt trigger generator and it works like a charm!
I used a 74AC14 smd chip and 1% resistors, and I put them in a shielded airplane junction box that I had lying about.
I powered it up with 3V and I got a rise time of 4ns. Right now struggling with crappy cables.
Thanks for the great videos!
Cheers,
Mark
Great demo! I love simple. Thanks!
Years ago, I used a similar technique to resonate a coil and capacitor for 14.318MHz VCO in my very first video frame grabber. (The literature of the day was obsessed with 4fSC at that time) I wound a few turns of coil on a hunk of chopped off Bic pen, put 22pF cap (representing the varactor diode I planned to use) in parallel with it and AC coupled a pulse from a 555 timer running at a couple hundred Hz. I then looked at the ringing of the LC combo on the scope. The scope was triggered on ch2 directly from the 555 and ch1 showed the ringing of the LC. I tuned it 70nS by adding / subtracting turns and in less than 5 minutes, Bob was my uncle!
Fantastic video. I am a newbie electronic hobbyist. Good explaination on use of the oscilloscope. I'm going to make this my next project. Thanks for a very easy to understand tutorial.😁
In case folks are here for methods of estimating remaining cable on a spool, you can always weight it, subtract weight of an empty spool, and divide by weight of some known length of cable.
Another great method - assuming you know the weight of the spool and everything can fit on a scale!
@@w2aew True. But you may well have an empty spool laying around to weigh. And if you don't, you might write on the spool you have a note reminding you to weigh it when it's empty :-).
True, but this method should be able to tell you how far away you have a break in a buried cable
@@patpalermo7629 I'm not sure your point. If you're looking for remaining cable on a spool, presumably it's not buried or broken.
@@Graham_Wideman If you have a buried cable with a break in it, could you not use this method to find the distance from the termination point that the break is at? It would make it easier for repair work.
Time was, scopes were unknowable to many - a scary icon of the arcane. Then there was the Alan Channel. Thumbs up.
Good video, subscribed. I had wondered about trying this on wires to my well pump to figure out how deep it was. Sadly the pump failed and I got to measure it the hard way.
Every time I look at your channel I find a gem. Thank you!
Nice. You actually sending a step function signal along the line. No polarity change, only 0 to 9V! As it propagates and reflects - it looks like it only interferes constructively. Doesn't it? Very interesting.
Yes, only constructively.
On the othe hand, the total voltage of the second step is ~9V=voltage of the battery, which also makes sense...
@@eugeneeugene3313 This is the right answer. The coax is an open circuit, so once it is "charged" up to the full battery voltage, no more current flows - thus no voltage drop across the series resistor, and the voltage on the line equals the battery voltage.
So pleased to see your return to UA-cam, Al. I learn each time.
Fun to learn and do so with minimal expense while learning a useful trick!
A TDR application I like is on railroad crossings where it is used to calculate the speed of an approaching train so the gates can be down 30 seconds prior to passing the crossing, no matter the speed of the train. This is very handy for big mainlines with 65 mph freight trains and 100 mph passenger trains...
WoW, this is a great learningvid! Really clear, complete and helpfull! Or most great one this one is the supereasy super usefullness! Thanks for your knowledgesharing!
Nice, simple explanation and implementation. I had no idea that amateur radio would lead me to falling down the electronics rabbit hole. And its all your fault! M0YZT
I upgraded with a timer IC and a FET driver IC. Keep this device in my scope kit. I added a 500 ohm trim pot to find the nominal impedance of the cable. I found most cheap "DMX digital cables" are actually built with low impedance microphone cable. They measure 60 to 70 ohms instead of 110 to 120 ohms. This is important if you do stage lighting.
Revisited #37 😝 from eight years ago, time flies when you have measured fun!
Oh boy - that's just such a useful and dependable technique. Thanks for all your video's Alan, you are a genius as always! 73 de GI8WFA.
Missed your videos, haven't done catching up for a while. Excellent as always!!!
Alan, you NEVER cease to amaze me. Thanks for the great presentation! 🤠
Depending on the scope, I’ve used the square wave probe calibration output on the scope itself as a signal source for tdr. Never thought of doing tdr for a roll of speaker wire or lamp cord. I now have an easy way of estimating how long my box of instrumentation cable is too, thanks!
It does depend on the scope - many scope probe compensation signals don't have a fast enough rise/fall time.
Surprised how well this works! Thank you :)
So... could you try this on last years Christmas lights to find where the bad bulb is? Brilliant information, thanks for sharing!
Thank you, once again, for a very helpful presentation. This will certainly help with approximating the coax lengths around my place.
This is fantastic. I love simple solutions using gear I already have. Cheers.
Seeing you move around the cursors, i wonder why vintage analog scope manufacturers never made a mechanical variant of those. I got an old Tek 454a, once a really expensive beast. But while it has loads of features and bandwith that i won't use, i'm still stuck counting divisions by hand.
It seems to me that it would have made sense for Tektronix, HP and the other big names of the past to deliver such high end things with slide rule like cursors, or caliper style with a nonius. Hell, perhaps even with a spinning dial like you see on some micrometer calipers. I would've expected it to be a major selling point.
Hell, for the highest of high end stuff you could make some mechanical linkages between time/div switch and several scales to mechanically calculate the period time, perhaps even to indicate the frequency... Not as accurate as a real counter but i can imagine that it would definitely come in handy.
Merry Christmas and thank you for your excellent content over the years.
Stay safe.
Learned something new today - Thank you for demonstrating / posting!
Love this video, Thanks Merry Christmas to you and your nice family from Seyed in Dubai.
The smart solution is the simplest! Thanks for sharing this! 🙂 👍
I have an old Tektronix with leaking capacitors (still alive to some extent) with which I would like to give it a try in measuring some cables.
Cool idea. I will try this. Thanks
you are a true inspiration to me when it comes to indepth electronics
Brilliantly simple. Great video. God bless you
Alan, thanks again for another practical EE video. Never thought you could do a TDR measurement as simple as this. Hmmmm I wonder what happens when you try it on a 3 conductor wire like Romex...
Awesome practical applications explained clearly, thanks for all the tips.
From Argentina I wish a happy Christmas to you and your family.
How to measure a transmission line, the McGuyver Method.
:)
Great stuff as always!
Cheers,
Super simple, super useful, just the right length! Thanks.--73, K2UA
Very nice presentation, very informative. I learned something new today.
Always love your perfect videos!!! Happy Holidays to you and your family! Merry Christmas and Happy New Year also. But I wish you mainly good health or at least better health & love!
So simple, but so clever. Big thumb up !
As usual, nice and informative video. BRAVO!
Thank you for the informative post. Please keep `em coming!
This is another great video i enjoyed watching Alan, absolutely genius how this works - thanks for sharing - enjoy your holidays and stay healthy
Thank you so much for all your videos, you do amazing job teaching us
I agree with all you showed. Have done that myself. But like in radars, you can do with some other way, often called Chirp. Too bad that goes to the opposite dirction from the basic idea of most simple instrument requirement. You would need some sort of sweep frequency source, although I speculate that I could do it by finding the different resonant frequencies using a grid dip meter. Or maybe that is too much of history by now. Maybe a FET dipper?
Thanks for sharing. Nice and useful technique.
Fantastic method; this also explains why some LED lamps never turn off using the same principle!
Very informative and simple.
Fascinating tip. Thank you! 👍👏🏻
Really so easy to use. Thank you.
Many thanks for this valuable info!
Thank you very much for this very clear explanation.
What would be different if you didn't use a resistor?
Would it not work?
A good follow up is on short circuit v open circuit or damaged cables
broken cable is pretty much the same. the tdr method tells you the distance to the break. thats all there is to know.
Thanks for sharing! This is really cool.
That's a great piece of information Alan,
ATB Adam UK
I was so disappointed to learn this can't be done with a single wire. I thought you found a magical mystical new way!
I have spools of magnet wire I like to know the length of, without resorting to calculating it through resistance and cross section?
that 9 feet something, was impressive resolution for such a simple procedure.
How about weight? Should get you within 10% or so.
With enamel wire most of the mass is in the copper, I'd thing you should be able to get close enough just assuming the whole mess is 100% copper, weighing it, and calculating from there.
@@Broken_Yugo A quick look on Google and I find #24 wire weighs 0.124 pounds per 100 feet. The chart I found didn't say if that was bare wire or what sort of insulation. I'd assume bare, but probably looking at something other than the first hit Google brought up would give a result for enable wire or even DCC if that is what the OP has. The reel will weigh something, but for 1lb to 10lb spools this will probably be thin sheet metal or thin plastic, and add less than about 5% to the total weight.
So a simple weight will probably be as or more accurate than either a resistance check (if you could get both ends of the wire on the spool) or some attempt at a TDR-like length determination.
I liked it. Thanks
This is so cool! Thank you for sharing!
Merry Christmas
Holy cow, this definitely deserves a like, share, and sub!
I love the videos, but it kind of drives me up the wall to be reminded that I can't really use any of it with the equipment I have and the task I need it for. I like that you included the UTP notes though. Idk how much VF uncertainty is going to be a problem when dealing with waterlogged cables.
I would think that the VF would change on waterlogged cables since the dielectric constant between the conductors will change. While I show a high end scope here, the same can be done with a 50 or 100MHz digital scope (or an analog scope with a scope camera, or using a pulse generator instead of the battery and resistor).
@@w2aew Yeah, I have an old (10? 20MHz?) dumpsterized single-trace CRO. Trying to capture single-shot events is kind of a maddening gamble, and usually results in enough image distortion (from a digital camera) that time measurements are unreliable.
I can use a pulse train, but I can't really zoom in on the leading edge of a single pulse, because the trigger delay will always cut it off at short timebases. I would need to make a synchronized trigger source to lead it ... but that would require a scope with a working external trigger input.
There really isn't much to do about it. Frustration is a natural consequence when the people with the right tools for the job already gave up and walked away. I still really appreciate the videos though.
Allan great video.
Using the same ideia you can put the open end in another channel it is possible to measure the velocity factor from the delay that it causes. I know that the propose of the vídeo are to measure maybe an cable that are installed and you dont know the length ... but as all yours videos are so didactic I thought that it will be an addendum to the subject.
(chanels 1 and 3 are best when there are 4 channels because many scopes uses interleave in adjacent channels (1 and 2 for example)).
thx for all the knowledge shared.
Yes, the assumption is that you might use this method when you *don't* have access to the other end (buried in a spool or at the top of a tower, etc.)
Pretty cool experiment could use this at my school.
Very nice tip!
Great video as always
Great to learn something new. Bravo sir, Bravo. Thumbs Up!
TNX 4 yet another great video !
73 N8AUM
simply mind blowned!
That's great video! And great explanation! Thx
Nice. For single conductor, would access to both ends plus a 2 ch scope be measurable. I will have to experiment next time i am in the workshop.
Great video
It kept telling me my 50ft cable was only 40 ft long?? Couldn't figure out why, then I measured it...Oops! Great video.
Could be due to a time warp ;)
They are here
I’m liking this!
Thanks Alan!
73’ de OZ2BO
This is SOOO useful !
Super 👍
The real juvel here is that you can measure the real VF of a cable at hand - and I guess that you can find the caracteristic impedance of the cable just by changing the resistor and find the cleanest waveform. You can always use a tape measure to get the length, but finding the real VF (for baluns, filters etc.) is really the key, right? Knowing the lenght of the coax by measurement, we can easily calculate VF - and by changing the resistor you can find the true caracteristic impedance.
Real good stuff!
Great video sir.
Excellent !! Hugs from Brazil ....
I was waiting for you to short the far end of the coax and for the reflection to invert...but the moment never came. IMO no TDR demo is complete without showing this inversion.
Amcia Apple, links in the description.
Well, sometimes you don't have access to the far end...
Love it! Glad I stumbled onto your channel. Here’s a thought. So you have two spoils of single conductor wire. One of known length and one unknown. If they are separated a good ways (super high L & C) I wonder if you could get a decent guesstimation length of the unknown spool?
Thx. WD5JJT
Hi Alan This video like most of the others you have done on this subject relies upon look-up sources to obtain Velocity Factor figures. For transmission Line that is pretty easy to find. But believe it or not for more common types of conductor it is not so easy to find. For instance take common copper pipe. I can't for the life of me find it. Same thing can be said for 6061-T6 Aluminum. Is there something I'm overlooking ?
Bob AA6XE (ex-K6TR)
Velocity factor doesn't depend on the conductor type, it depends on the dielectric constant of the insulator between the conductors. That's why you can't find it for copper pipe or aluminum. If the insulator is air, then the VF will be close to 1.
@@w2aew Thanks Alan
Excellent video, Alan! This video also explains why you can have a high SWR on your antennas, due to the reflected energy. I don’t know if you know much about physics, but now that I’ve seen your video, you have me thinking. Why does the energy reflect back into the wire, and not simply dissipate from the end? I’m guess that it has something to do with the wire being a better conductor than air. Any thoughts?
The same reason that the 120/240VAC mains power doesn't "dissipate" at an unused outlet/receptacle - there is no load.