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- Опубліковано 8 вер 2024
- This video shows some simple ways to measure the velocity factor (VF) and impedance of a known length of coax. (Please pardon the simple decimal point error in the metric conversion - see the annotations inline in the video - sorry about that!)
It uses a simple TDR technique using a cheap homemade fast-edge pulse generator, as detailed in this video:
• #88: Cheap and simple ...
which is based on the simple schmitt trigger oscillator shown in this video:
• #87: Schmitt Trigger O...
Here is a companion video that shows how to measure the length of a piece of coax, assuming that you know the VF:
• #37: Use a scope to me...
Your explanations are really good. Keep making such educational videos. Much appropriate for me
Wow, love this! Just a very simple application of physics to calculate something very useful!
great stuff! Discover the vitals of an unknown hunk of coax using a scope and a fast pulse gen
Nobody explains things quite like this man
VERY nice work and explanations. I wish we had your videos when we were doing similar stuff in our college labs. Just getting across the idea of how and where to terminate takes effort. Nice new scope - drool! I have a similar roll of flea market coax to test but may also use the MFJ-169. It might be cool to look at scope patterns at the same time, matched and not. Cheers.
Once again, very practical and informative. I've noticed that other have already pointed out that your cm is about ten times larger than everyone else's :). Great video.
Love your hands on videos. Clear and easy to follow.
Thank you for teaching us in such simple words. You have an real gift in teaching skills, Sir
That was bloody awesome thank you very much please don’t delete this video
Liked this, and liked the nice simple easy to understand way you explained as well.
Nice video! Using the same techniques here. I can only disagree with the last comment in the video. One of important parameters for HAMs using 100 MHz or higher is also the loss of the cable over different frequencies. 73! :)
Another good refresher video. Thanks Allen.
Thanks Alan.. Off now to build me that TDR and measure some of my own coax cables with it.
This is the Bugger Factor. Amazing video.
The cable seems to be longer than a meter to me. 88cm is way less than a meter (100cm). So I converted 29 feet to centimeters it comes out as 883.9 cm.
Yeah, sorry, I moved the decimal point too far...
@@w2aew I had just noticed that as well. Glad someone else addressed it. Thank you for these videos! They are exceptionally helpful.
thank you Alan, good video as always; is 50 ohms the impedance of the wire, or the other side of the cable, the pulse generator, and the potentiometer is matching it?
The pulse generator has a 50 ohm output impedance. The coax has a 50 ohm characteristic impedance (at RF), and the potentiometer has a 50 ohm resistance when it is adjusted to minimize the reflection. The point here is that the reflection goes away when the termination impedance (the potentiometer in this case) matches the characteristic impedance of the coax.
***** what if the characteristic impedance of the cable is not 50 ohms?
mdev1206 Then, the reflection will flatten out at a different potentiometer setting - matching the coax impedance. The generator impedance won't matter much, as long as it's low enough to drive the coax impedance.
Very interesting, great lesson! Thank You!
Genius, simple , sharp and perfect.
Tks for share.
73
PU2SRZ
SAO PAULO. BRAZIL
Second that!
How about measuring velocity factor of single wire line for your next lecture. Thanks to your nice videos on you tube!
Amazing!
Thanks for this video.
I haven't seen you make a video lesson about "directional cable". What Direction Cables are is one end the cables shield is connected to the circuits ground and the other end of the cable the shield is LIFTED not connected. This eliminates causing group loop issues. Not sure when would a circuit need to use directional cabling with only one shield is ground compared to non directional cabling with both shields grounded to the circuit?
Thank you sir...
I think you might have been an order of magnitude off with those centimeter measurements there...
Yeah - bonehead mistake! But, since I made it twice, the math worked out! I added some annotations to fix my screw-up.
*****
It happens! I enjoyed the video though as always!
Tom McLeod Rassum, tassum, curses and what was it? Oh yes, BUGGER! Our host might have been hanging around some Brits!
984francis Hah - I figured "bugger" was a lot "cleaner" and politically correct that what was going on in my head!
***** Truth to tell, "bugger" is actually very naughty being a noun for a person who indulges in a very specific version of a act beginning with F! Brits use (or used to) the b word quite a lot. Even my mother used to use it and probably still would if she were still around:) Whatever way Sir, I find your presentations extremely helpful.
You're my RF Herooooooo!!
Ummm... 348 inches = 8.8392 meters.. .that's ~ 884 cm... you have a error somewhere in you paper :D
Oh Bugger! You're right, duh! I added some annotations to "repair" it...
First thing I spotted too! Then my ironic side started to assemble a joke in my head about Americans and the metric system! Informative video, nonetheless. 73!
Im curious how you would have gone about measuring the characteristic impedance of the coax cable by using the measured step voltages directly from the scope. I believe TDR capable sampling scopes use this method, though Im curious how the characteristic impedance values are measured or extrapolated.
You can calculate the impedance from the size of the step.
thank you, great video!
This is known as a TDR setup which stands for Time Domain Reflectometry.
Thank you!
Thanks for many videos that you put and share to us. They are very educative and enjoyable. May God bless you more. 73 DE YB1NWP
In practical terms is there a sensible minimum length of co-ax you can work with to get the VF? Great video Alan, thanks as always!
ChrisWilson5006 It depends on two things: how fast is the edge on the pulse generator, and what BW is the scope. If the edge on the pulse is too slow, then you won't be able to separate out the incident edge and the reflection. With this 2ns edge, it will start getting difficult to see a delay that is less than about 10ns round trip (with the ringing, etc.), which corresponds to 5-6" or so. And, the accuracy of the delay measurement suffers. Of course, the scope has to have sufficient BW to not filter and slow the edge.
***** That's great, I have about 5 foot sample of unknown Heliax type cable I'd like to measure. If it's suitable for my needs I have been offered a part drum of it, cheaply. Hopefully this will be possible then, thanks Alan.
Hi Alan, thanks for another well done lecture. Have you done a video on determining the quality of older coax that has been in service, especially, outside? If not can uou put that topic in your Que..
I have some hard-line I have had for 25 years. It looks great still but I suspect it has some loss even at HF by now.
73,
Glenn WA4AOS
Excellent method and well explained, except, of course, your centimeters should really be meters.
Sorry..not meters but decimeters
This cable looks longer than 88.4 cm. Are you sure it wasn't 8.84 m ?
It was 8.84m - I missed a decimal point when I put the video notes together. You can see at the beginning of the video that I noted the cable is 29 feet long.
Will cable impedance change with frequency? If so, to match the terminating resistance, it may make sense to build a small circuit with BNC male and female connectors and a variable resistor so you can more precisely terminate the cable at the measurement point (i.e. the oscilloscope) for the frequency under test. Alternatively (or in addition to), it may be possible to get rid of the ringing in the fast rise pulse generator with this device.
Transmission line characteristic impedance does not change with frequency.
Any way to do this on the cheap (i.e. without the scope)? It seems the voltage doubling of the reflection could be exploited as a start/stop or trigger. For example, have the signal start a small (pf) cap charging and then stop charging when the reflection doubles the voltage. Another might be to start with very short pulses and then increase the duration until voltage above the signal is detected?
It would be possible, but not particularly easy. You'd need some very fast switches or comparators. This method of charging a cap to measure a short time interval is a common method to measure short time duration events. Using short pulses is another approach, but generation of single-digit ns wide pulses isn't particularly easy either, and would still require some fast comparators, switches, etc. It's all possible and has been done.
***** Thank you for the response! As i get older, time and money seem to be trading places on the value graph...twiddling with $10 in parts for ten hours to get uncertain results vs $400 and ten minutes is almost enough alone...add in the fact that you still have a good scope after the fact, and the choice is pretty clear. :)
Well, you've gone andmade me dig up piles of old, dirty coax from the boxes in my basement. Totally worth it, though.
Why does coax cables have a Velocity Factor? what is causing the coax cable to have a velocity factor?
Could you please include a bom for your fast edge pulse generator?
This video has the details: ua-cam.com/video/NuXitMK3HSA/v-deo.html
Great way to explain both VF and Impedance... that was a really big help. Thanks... Do you have any tutorials on how to use your Oscilloscope and other benchtop equipment? I want to take some field cable for a project I'm working on and get these two pieces of data to use a TDR to estimate cable length accurately... Do you have an e-mail address I can contact you at?
Yes, I have DOZENS of videos on how to use benchtop equipment. Even a couple on TDR alone. The easiest way to find the videos you'd like are to use my video index file (www.qsl.net/w2aew/youtube/W2AEW_video_index.pdf). The videos are arranged numerically in the first half of the index, and arranged by topic in back half of the file. Since it is a PDF, you can search for key words of interest. Every video entry is a link to the youtube page for that video.
Hi Alan, the above link doesn't work anymore. Is there another link to access the index?
great video
trying to find impedence of those fixed Attenuators I have did you do any videos or tell me how Thank you
+DENNIS N Determining the impedance of a fixed coaxial attenuators is pretty easy. Resistive attenuators will have an input resistance equal to the designed impedance when terminated in that same impedance. Thus, a 50 ohm attenuator has a 50 ohm input impedance when the output is terminated in 50 ohms. Same story for 75 ohm attenuators. The vast majority would be 50 ohms, and some might be 75 ohms. The easiest way is to put a 50 termination (resistor) on one side of the attenuator, and measure the resistance at the other end to ground. If it is 50, then it is a 50 ohm attenuator. If not, then try using a 75 termination and measuring the input - it should measure 75 if it is a 75 ohm attenuator.
Thankyou
Hi
I have 1.5mm silicon insulated copper wire I want to use for a wire antenna. How can I find the VF of this wire?
insulated wire for an antenna will typically affect the length by only a few percent.
Great vid on the subject as usual. As I pondered this, at first, it seemed the coax impedance would change with increased frequency however, I suspicion that frequency would have a negligent effect up and until its time base approached the cables propagation delay. In this case being 85ns. At this point the delay would appear as increased impedance with respect to the load. Am I correct in making this assumption?
bain5872 The impedance can appear to increase or decrease, depending on the frequency and length of the line. For example, a quarter-wavelength long line that is shorted at the far end will appear as an open circuit. If that same quarter wavelength line is open at the far end, it will appear as a short at the input.
***** Thanks Alan. I remember your very interesting vid about using quarter wave length as a switch in a trans/receive mod. What I'm really trying to get at here is the cause and effect of the cables changing impedance. It appears to me as a function of the cables capacitance/inductance which change with length, for the cause. It seems intuitive that the reason for the cable looking like a open circuit is due the cables propagation delay being equal to wave length of that particular frequency as the effect. Is this correct ? Please forgive; I'm trying to obtain a very firm understanding of what is actually happening in the coax.
bain5872 The distributed capacitance and inductance along the line gives rise to the line impedance, which is a constant. The impedance looking *into* the line looks like the line impedance *only* when the the line is terminated in it's characteristic impedance. If it is not properly terminated, then the reflected signal from the termination will add to the forward wave, resulting in an apparent input impedance that is a function of frequency and line length (because these affect the phase and amplitude of the reflected wave). You may want to review this video:
ua-cam.com/video/g_jxh0Qe_FY/v-deo.html
and this one:
ua-cam.com/video/DovunOxlY1k/v-deo.html
***** Thanks for the links and explanations Alan, they were helpful. This confirms my theory. I think I was confusing the issue by not indicating that my mental experiment was being performed with an unterminated piece of coax hanging open at the end of the line. Once the line is terminated there is no reflection. Again, I appreciate your time and patience.
A short unterminated coax looks complex/capacitive until the line is a quarter wavelength long. Then it looks complex/inductive until it is a half wavelength long. Then it repeats, etc.
Hi my name is David Grey,
My question is what input impedence were you using on the scope was it the usual 10meg Ohm or was it terminated using the 50 ohm termination on the scope.
73's ZL1UTS
The scope was set to 1Mohm.
Rassum, tassum, curses and what was it? Oh yes, BUGGER! Our host might have been hanging around some Brits!
u have dam good vidio.... nicw knowledge fow new hobies... i think it will be grate to add last one video (ore first one =)) about proses in coax while signal gose thru .. what isus will be ih we will have bad termination in coax .ore wave line ...
P.S.sory for inglish ... its not my native lang.
I already have videos online on those topics:
Transmission Line Terminations for Digital and RF signals - Intro/Tutorial
Use a scope to measure the length and impedance of coax
Your scope has certainly a generator BF that can do square signals, duty cycles, ... How could you use it instead of that specially made circuit ?
Yes, I could. My specially made circuit has faster rising/falling edges than the generator built into the scope which gives better distance resolution. Plus, since most scopes do not have a built in signal generator, I wanted to show how a simple external circuit could be used.
Many thanks for your answer. I'm trying to get an amateur license (in France, sorry for my poor English), and learning the basis of electronics, so it's easyer for me to use what I have, as I'm unable to design such an external circuit. I like your channel, thanks to you again
I wonder if a 555 timer could be used as the pulse generator...
Unfortunately no. The output edge speed (rise time) likely won’t be fast enough. And the output likely won’t be able to drive a 50 ohm transmission line.
How would you go about measuring the loss per 100 feet or similar?
Pick your test frequency. Measure power at generator, convert to dBm. Connect coax to generator. Measure power at dummy load connected at far end of coax. Convert to dBm. Subtract output power from input power, divide by actual length, and multiply by 100. This gives you loss per 100ft.
***** That sounds so obvious now, thanks for the help.
You are a little bit off with the cm so 20'. (Just) misplaced the dot. So don't work on construction sides. We are off 20'. However details matter. No offense.
intresting :))) I prepare my trashbin for a shit cables :))))
So did they name a school after you yet?
Ha! Not yet!