I used my Rig Expert AA-600 to check feed lines to find out its impedance. I did not use the Smith Chart but still found the correct impedance. The way you just showed is so much easier and spot on. No hunting needed. Now I use my NanoVNA H 4. Super simple. Thanks for posting such a great informative video. Barry, KU3X
Interestingly enough the new Zoom analyzers have a cable impedance measurement and it works quite well. You only need a short and open, no load. Spot on with what the NanoVNA measures. Like yours, my AA-600 lacks the new feature.
Some folks have a wonderful knack of conveying information. Alan is certainly an expert in the field of teaching difficult concepts. And to think he does this as a labor of love. Tektronix certainly has one heck of an employee working for them!!! PS: I would love to see SignalPath, eevblog, and Alan team up for a live stream fireside chat.
@@w2aew Hi thanks for the wonderful video. I have a problem, I am not getting the sweep like the one you showed. I have used the 50ohm termination that I used to calibrate with rg174 of 0.21m
I've just done it with a short test length of 75 Ω coax. It works perfectly. But now I need to measure the Z₀ of a short length of twin flat-line. If I hook it up to two connectors, will it also work? Update: I'm able to answer the question myself. Yes, it does work, and quite well, too. The only conditions for an accurate measurement, though, is to lay the short piece of twin flat-line out, away from metal objects, and also make sure it is not exposed to electric or magnetic fields. I measured the Z₀ of a 1.5 Metre long piece of common twin bell wire. BTW, it came out at 110 Ω.
😅Very well explained and to the point. Brevity is good coupled with the equations to work through ourselves. For those who are a bit lost redoing the exercise in person is a revelation.
Ha! You're a few weeks too late. I had to figure that out myself when I found a bunch of unknown coax. I actually used your poor mans TDR with the 9V battery method to get the length of the coax. And use the NanoVNA similarly to work it out. Great video!
Thank you for all your videos. Though I understand about half, that half gets larger every video, helping me understand previous videos and helping me be able to wrap my head around things I have read. Know your making a difference. I used to be a ham ke4wjz. Since being disabled about 20 years ago . Right before my divorce 15 years ago my ex either sold or destroyed my 2 meter radio. I have rediscovered electronics and enjoy learning about RF after my divorce. Friends and family give anything electronic headed for a landfill. If I can't fix it I scavenge most the discrete parts and IC's from. This hobbie has pulled me from a dark place and has put alittle purpose in my life. Thanks again for sharing your knowledge and experience!
I managed to do it, although the pattern displayed on my Smith chart was a very tight circle, so the resistance didn't change much away from 50 ohms as I scrolled through the resistance. Quite glad about this, since it was a 50 ohm cable! I'll try again with a piece of 75ohm cable. Thank you for that. One comment: in your videos, you very often "take" values to perform operations on them. It's unnecessary, and in British English, "take" can be interpreted as "subtract". This first time I heard it, I thought "take it from what?" Hope this is useful. 🙂 Thanks again.
Another excellent presentation, well done. I must admit I was thinking along the lines of the TDR and a long cable terminated with a 100 ohm potentiometer. Maybe that would be an interesting experiment? Great to see the NanoVNA being used for more than SWR plots, thank you for your great videos.
It can be done that way too. In fact, I did a video on that several years ago using a TDR technique with a scope and a potentiometer at the far end. ruclips.net/video/Il_eju4D_TM/видео.html
Thank you for explaining how to measure the impedance of a coax cable. I would like to measure a parallel lined ladder flat cable. I did the same thing using 1 m long 1.2mm diameter cupper wire (emamelled) and it seems to be 45-50 ohm. Is this correct? I would appreciate very much if you can provide the method how to measure the impedance of a paralell-lined-2-wire (a ladder line).
Alan, thanks. I'd hoped to set up something recurring, couldn't see a way to do that on paypal. Anyway as an ex navy electronic tech, wish that the internet and you were around when i was learnibg. Thanks so much.
You've got some useful info on that VNA. Thank you. Couple of questions: If I have an unknown antenna, will the NanoVNA tell me exactly what frequency the antenna is best resonant, or best optimized for? If I have a dipole, can I use the device to determine if the "ground" half is too long or short? If I have an antenna with loading coils, can this be used to determine the "electrical length"?
Alan, I have been watching your videos for some years now. I would like to contribute to your project, do you have a patreon account set up? Thanks Rick Mall
Hello Rick, I greatly appreciate that! I don't have a Patreon account, mainly because I feel that I don't publish content often enough to give member their value. Instead, I do have a "donate" button on the channel's main page that allows people to contribute if they choose to. It is; www.paypal.com/paypalme/w2aew
Two videos in two days. Good to see you making 2021 better than 2020. I wonder how you rate the NanoVNA against the Rig Expert or the MFJ ant. analysers? I am sure the Nano is more complicated to use and requires some learning but if you could only have one for antenna work which would you keep?
I think the RigExpert is more suited to antenna specific work, especially when working outdoors (rugged, great display). I found that the NanoVNA display was difficult to see in bright sunlight. For indoor work, the NanoVNA is great.
You read my mind... Yesterday I decided to replace the cut short coax from a Furuno NX-3H Navtex antenna. Furuno uses their own coax cables without any marking that are slightly larger than classic 50 ohms coax but smaller than 75 ohms. So I found your recent post and measured my piece of coax. Now you forgot to mention that if the NanoVNA indicates a perfect 50 ohms dot in the middle, then the cable is 50 ohms indeed; correct?
Correct. If your cable is 50 ohms, and you terminate it in 50 ohms, you'll just have a "dot" in the center. In fact, if you have 75 ohm coax, and terminate it with 75 ohms, you'll get a "dot" at the 75 ohm point in the chart.
Thanks for the OSL tip. Is the 50 ohm load a toggleable feature in NanoVNA? Do you think the margin of error is small enough to verify if cheap 75 ohm cable has impedance more like 77.5 ohm versus a good 74.5?
This is a great video and is a simpler technique than that used by other RUclips videos I've seen. Can the NanoVNA be used to find the value of an inductor without creating a resonant circuit (i.e. capacitor and the inductor in parallel as done in your video using a pulse generator and oscilloscope)? I have effectively used your pulse generator and resonant circuit system to calculate inductance, but was wondering if you could do the same by just using a NanoVna in series/parallel with the inductor under test, without making a resonant circuit. Thank you for all your videos. I have learned so much from your great videos over the years. Regards. -WI1B
You can, if you do it carefully. Make yourself a fixture that is basically a SMA connector with pin sockets on one end, to connect to your inductor - from center to gound, and connect to the S11 port. You don't want to use long clip leads as that will affect the results particularly for smaller value inductors. You can test at frequencies where the inductor is intended to be used, and use a marker on the Smith Chart to read off the inductance value. Be sure to calibrate properly.
@@w2aew Thank you so much Alan. Lead length may have gotten me when I tried. I appreciate your quick response. You should be writing books. You can always make a technical, difficult topic easy to understand. That's a rare gift. 73, Ken
@@w2aew thanks a lot. You do lots of great videos so that I forgot you made a video about what I asked for. Again thanks does not do you justice, really thanks a lot.
And what benefit will this give me? Why would i need the impedance reading? just interested, sorry for stupid question.. Ive subbed for years btw!. thanks for your efforts
You get best power transfer to a load like an antenna when it's input impedance matches the transmission line impedance. Impedance matching is important to prevent signal reflections.
Besides what Alan said, if you have a box full of coax from different sources and you want to sort through it to find how many 50 ohm pieces you have, this works like a charm.
Thank you for a very great video. If I want to use this method to measure the impedance of twisted pair cable (e.g. USB2 data lines), what is the length to use? The original cable length before twisting or the length of twisted pair?
So is it safe to assume that the same can be done with ladder line thinking would need a carbon film pot resistor for load. Thinking of home made ladder line.
Hi Alan, Could you do the similar things with some pcb impedance coupon? Is the frequency range of this device enough to observe impedance uniformity of the transmission line or fiber weave effect with TDR mode.
Is it the characteristic impedance of the coax that we are measuring? Also, is the impedance value strictly for 44MHz as seen on the VNA? What if I want to use the coax to pass 300MHz through it? (with some load on the other end)
Yes. The characteristic impedance of the coax is independent of RF frequency. So, 50 ohm coax at 44MHz is still 50 ohm coax at 300MHz. Now, if the coax is not terminated in it's characteristic impedance, then the impedance looking INTO the coax WILL vary with frequency (even though the coax is still 50 ohms).
Very cool. These little things have so many uses, it's amazing. My problem, I have a coax, buried under the sod. If I put a 50ohm load on one end, and connect the nano on the other, what should I see if the coax is good to use? Thank you.
Probably a better way to test it, is to start with leaving the far end open. When sweeping the frequency, the trace should hug the outside of the smith chart. This will indicate that there is low loss. Then put the 50 ohm termination on the far end and verify that the smith chart stays at the center.
Very nice! I have thought about getting a Nano VNA. How do you like using the Rocketbook notebook? I haven't purchased one yet, but it looks like a promising replacement for my old Neo Smartpen notebooks.
I like the Rocketbook. Seems to suit my purpose pretty well. It's nice that it uses readily available Pilot FriXion markers. Pretty easy to edit, etc. before scanning.
Thank you for your work!! That was an amazing and really informative video. In 1:13, you estimate the stop frequency. Where do the 75 and 246 constants come from?
It is computed from a simple free-space wavelength calculation. Speed of light is 300,000,000 meters/sec. So, you can calculate the free-space wavelength of an RF signal in meters as 300/Freq(in MHz). Then divide this by 4 to get the free-space quarter-wavelength. Thus the formula is 75/F(MHz). You can do a similar calculation for the value in feet, which gives you 246/F(MHz).
I used this technique to measure my various teflon coax. TC-18 got me 18.26 ohm, blue 25 ohm coax was 27.3 ohm and some unknown coax with only 0.15mm insulator between center conductor and shield was 15.2 ohm. But surprising outlier was TC-12 that measured at 16.8 ohm and supposed to be 10.7 ohm per datasheet. That may explain why my TLT transformer was not working properly.
Dear Sir, I have question, do you have any idea how to measure reflected power from microwave waveguide powered with magnetron 2.45ghz, few KW? Any special devices? Please advise 🙏 best regards
This is awesome! BTW, this works perfectly on my NanoVNA, but not on my Rigol RSA3015N. I've reached out to Rigol to determine what's going on. The Smith chart is incredibly noisy and unmeasurable. I can't explain it, but I'm hoping that Rigol can.
Thanks for this! I've been trying to measure resonance with different antenna materials using the loss/SWR on the nanoVNA with inconsistent results. It hadn't occurred to me to look for features on the smith chart.
Loss and SWR plots can't show you resonance because they don't include phase. Remember, resonance is when the reactive component is 0 (phase is zero). Resonance is NOT necessarily the frequency of minimum SWR.
Just for fun, I used this quarter wave method to look at a short piece of unknown cable I had. I expected 75 or 50, but kept getting a value around 95 ohms, and for a long time I scratched my head looking for a source of error. In desperation I finally Googled "95 ohm coax?", and there it was, probably RG62 - seems it's a rare "standard" value - and not one I'd ever encountered before! ( I've no idea where I got the cable, or its intended use!)
It is possible that it is actually 93 ohm. I found this info on wikipedia: RG-62 is a 93 Ω coaxial cable originally used in mainframe computer networks in the 1970s and early 1980s (it was the cable used to connect IBM 3270 terminals to IBM 3274/3174 terminal cluster controllers). Later, some manufacturers of LAN equipment, such as Datapoint for ARCNET, adopted RG-62 as their coaxial cable standard. The cable has the lowest capacitance per unit-length when compared to other coaxial cables of similar size.
@@w2aew Thanks Alan! 93 ohm and "has the lowest capacitance per unit-length" make sense: the dielectric looks like polyethylene - however the Vc seemed higher than 0.66, (and I forgot to record it !). The inner conductor/outer braid diameter ratio certainly looks smaller than in 75 ohm RG59. So now I have 1 metre of 93 ohm cable available to perform some interesting impedance transformations! As ever, the VNA is our friend! :)
This is definitely a tough topic. From this video, the measurement said the impedance of this cable is 75 ohm. If the same coaxial cable is double the length (2 meter), is the impedance going to be 150 ohm?
The transmission line impedance is not the same as the resistance of the wire. The line impedance arises from the distributed inductance and capacitance of the line, which determines the relationship between the voltage and current *as the signal wavefront is traveling* down the line. Therefore, the line impedance is a constant, regardless of length.
Right at the VNA, because the measurement plane is the *input* of the unknown cable. If you calibrate at the end of the unknown cable, then it's effects would be removed from the measurement, which is NOT what we want.
@@w2aew thanks a lot for that timely and helpful reply. Oh another question: is there an ideal length of cable to measure this? Because when I tried with a 4m piece I just got a tiny circle in the center of the Smith chart. I might have done something wrong though.
Sure, provided you can terminate it well, you can couple to it cleanly with the VNA, and your VNA can go high enough in frequency to be quarter-wavelength long.
How would one "couple to it cleanly with the vna"? I'm guessing that the coax between the vna and the transmission line affects the measurement just as much? What happens if the coax impedance doesn't match the impedence of the transmission line? Thanks for yet another great video!
@@albindennevi9829 Yes, the coax from the VNA to the PCB does make a difference. You would need to perform the calibration at the end of this coax, ideally at the interface to the PCB. This will "remove" the coax from the measurement.
@@w2aew Thanks for great video. So why do coupons from PCB manufacturers always come with close trace without the option to close it with 50Ohm resistor?
@@lancecruwys2177 Yes, if you knew the approximate frequency you could sweep closer to the crossing point. The sweep time in the NanoVNA won't change much, so the only benefit is getting finer granularity of frequency in the sweep.
When measuring the DC resistance/impedance from ground reference to the +Vcc positive voltage rail and measure the DC resistance, then measure from ground to -Vee or the negative voltage rail. Some circuit will measure from 10 ohms to 100 ohm and other circuits will have 100ohm or more for the positive rail and negative rail DC resistance/impedance. Is this the power supply impedance measurement? even those impedance is measured in AC but they often call this measurement the power supply impedance which is measured in DC resistance. Question#2 is if the positive rail measure 30ohms and the negative rail measures 20 ohms will this have a LOWER signal to noise ratio because current is easier to flow to ground compared to a circuit that has positive rail at 100 ohms and a negative rail at 100 ohms? The higher the DC resistance on the power rails means that the signal to noise ratio gets worse because the current can't SINK to ground which creates noise?
No, this is NOT the power supply impedance. A DC measurement of a power supply rail to ground is meaningless (unless it shows a dead short which would indicate a problem).
@@w2aew What is this power supply measurement called? because EE engineers do measure the DC resistance from ground to positive VCC rail and the DC resistance from ground to the negative VEE rail. It has to d with something about how the current is sinking to ground, the higher the resistance from ground to positive rail or ground to negative rail will take longer for the current to sink to ground? it might have something to do with leakage current measurement to measuring the leakage current.
Hello w2aew thanks for your very useful videos. I own Agilent sine wave generator with sine output up to 3GHz. How Can I convert the sine wave signal to 1 half cycle pulse which I Can use as TDR source
Generally not that simple - however, many professional signal generators have a Trigger output which is synchronized to the RF output - it would likely make a nice TDR source.
The free-space wavelength of an RF signal is: Length in meters = 300 / frequency_in_MHz. Thus, the 1/4 wavelength is simply 75/f(MHz). Since the propagation speed in coax is slower than free space (velocity factor
Very cool, can't wait to demo to myself and others. I suppose it would work for open line also. Maybe you would terminate it with 450 ohms (for window line) .
No, you'd still want to terminate it with 50. If you terminate 450 line with 450 ohms, you won't get a reflection. That's OK too, you'll just get a tiny spot or spiral right at 450 ohms on the Smith Chart.
@@w2aew ok, so if I understand correctly , the termination needs to be different than what you think the actual impedance of the line might be. So, since you had 75 ohm coax you were able to get a reflection with a 50 ohm load. So perhaps with a 50 ohm coax you would use a 75 ohm load, and the 450 anything but 450 ohm load ect. and in all instances you would have a healthy reflection. I found it very interesting that you could check twisted pair ect. I had always thought they were going for 600 ohms with that type of cable . Thanks for the video.
@@californiakayaker Actually, I used a 50 ohm termination because the NanoVNA is a 50 ohm instrument. If the unknown coax was 50 ohms, there’d be little/no reflection and the resulting trace would be just a small dot or spiral at he center and you’d know it’s 50 ohm coax. Twisted pair cable is typically 100 ohms.
Can you use an insulation tester Megger to test RF coax cables, audio cables, speaker cables, to find breaks in the conductor wire or its only made to test if only the insulation has breaks? because when using an insulation tester at 200v, 500v, 1000v you might measure .5 ohms which is considered a bad cable. The Guy that invented Coax cables made a differential math formula to be able to run the telephone signal in the center conductor using a differential signal so there will be no crosstalk. I'm not sure how you can send two different signals down one center conductor but that is why they used coax cables which he invented. The 50ohms was picked as a value because of power transfer. I'm not sure if they are talking about True Power or VAR Reactive power , or VA absolute power that coax was better at transferring power at 50 ohms.
No. The impedance of a transmission line is composed of distributed capacitance and inductance for AC signals and this impedance is not the real dc resistance between conductors.
@Jonathan Alvarado thanks for this question; I thought I knew were the 276 value came from, since I originally thought it was a conversion from meters to ft - yet, 246 ft would be the conversion from 75m, so it isn't a conversion, after all. Therefore, like you: were do the values (75) and (276) come from in the "stop" frequency calculations?
@@schwinn434 I think the 75 came from the calculation of a quarter of the wavelength, the wavelength is the speed of light divided by the frequency, 300,000,000m/s divided by the frequency, in MHZ is 300/frequency so the quarter wavelenght is (300/4)/frequency = 75/frequency. For miles i don't know, if I do the same calculation the result is about 246/frequency (in MHz)
Speed of light is 300 million meters per second, or 300e6 m/s. If we divide that by the frequency we get wavelength in meters. Thus, if we take 300/4, then divide by the frequency, we get the length of 1/4 wavelength. If we express the frequency in MHz instead of Hz, we can drop the 10^6 or e6 multiplier. Thus, 75/F(MHz) = free space 1/4 wavelength in meters, which can be rearranged to say 75/Length(m) = the frequency in MHz whose 1/4 wavelength is Len(m). We can do the same think with length given in feet. Speed of light is 984 feet/sec. So we divide 984 by 4 and get the 246 factor.
@@stefanolini279 thanks for the reply. I guess what the values of 75 and 246 represented should have been obvious. I really kinda-of stay lost when talking about impedance matching at radio frequencies -especially, when the concept of using inversion of impedance at 1/4 wave values are involved. :( Just when I think I kinda-of understand how the SmithChart can be used for impedance matching, I realize I'm totally lost once again.
Assuming you know the length of the line, the ratio of the free-space quarter-wavelength calculation (shown in the video) to the actual frequency at the quarter-wavelength point (from the measurement) will give you the velocity factor.
I just started working in a laundry mat. They have a bunch of debit card boxes going out on their washers and dryers. They are a small business so it would be hard for them to replace them due to cost. I don't have a set-up and I know you have the best. If you don't have the extra parts laying around, I am sure that they probably wouldn't mind if you had to cannibalize a box or two. I know the price to fix each box might vary, but let me know if you might be interested. If interested, I will check back here and let me know how to get my phone number and email address to you. I didn't see any contact information.
Of course - terminating a 50 ohm coax with 50 ohms will result in a single point (ideally) in the center of the chart. This will verify that the coax is 50 ohm.
I **did** learn something! Thanks!
You know a channel is legit when one of the most knowledgeable engineers on YT puts a comment on it.
I used my Rig Expert AA-600 to check feed lines to find out its impedance. I did not use the Smith Chart but still found the correct impedance. The way you just showed is so much easier and spot on. No hunting needed. Now I use my NanoVNA H 4. Super simple. Thanks for posting such a great informative video.
Barry, KU3X
Interestingly enough the new Zoom analyzers have a cable impedance measurement and it works quite well. You only need a short and open, no load. Spot on with what the NanoVNA measures. Like yours, my AA-600 lacks the new feature.
Thanks. Your clear, concise videos save both my time and sanity.
Some folks have a wonderful knack of conveying information. Alan is certainly an expert in the field of teaching difficult concepts. And to think he does this as a labor of love. Tektronix certainly has one heck of an employee working for them!!!
PS: I would love to see SignalPath, eevblog, and Alan team up for a live stream fireside chat.
SignalPath and I have talked about doing something together - we live very close to each other. Maybe we will once COVID restrictions are lifted.
@@w2aew Hi thanks for the wonderful video. I have a problem, I am not getting the sweep like the one you showed. I have used the 50ohm termination that I used to calibrate with rg174 of 0.21m
Alan, thank you for this followup video. It si mindblowingly (I hope I spelled that correctly) simple!
Hi Alan,
Didn't know it was that easy to determine the impedance using the NanoVNA. Stay safe. 73 WJ3U
Always learn something (usually many things actually) from your videos!
really educational and something even i (a non technical person) could undeestand or apply in some way.
.
Thank you so much, sir!
I've just done it with a short test length of 75 Ω coax. It works perfectly. But now I need to measure the Z₀ of a short length of twin flat-line. If I hook it up to two connectors, will it also work?
Update: I'm able to answer the question myself. Yes, it does work, and quite well, too. The only conditions for an accurate measurement, though, is to lay the short piece of twin flat-line out, away from metal objects, and also make sure it is not exposed to electric or magnetic fields. I measured the Z₀ of a 1.5 Metre long piece of common twin bell wire. BTW, it came out at 110 Ω.
Simply a great teacher !!! TNX & 73s.
Your videos are great. You are concise and knowledgeable. Thank you sir.
😅Very well explained and to the point. Brevity is good coupled with the equations to work through ourselves. For those who are a bit lost redoing the exercise in person is a revelation.
Great video & very useful for us hams!.
Wishing you a very happy new year for you, family & loved ones including health & happiness
Happy New Year, Alan! This should help people who don't have access to an O-scope, but have a VNA handy.
Great video Alan. Thank you for your work.
Ha! You're a few weeks too late. I had to figure that out myself when I found a bunch of unknown coax. I actually used your poor mans TDR with the 9V battery method to get the length of the coax. And use the NanoVNA similarly to work it out. Great video!
Thanks Alan, this video was most usefull to me, as many others you created. :)
I needed that. Thank you very much. Great video.
Great teaching techniques...👍👍honestly I have learnt a lot from your valuable explanations, 🙏🙏
another great video. could you show how we can use the nanovna to verify the frequency range of ferrites?
Thanks for these videos i learn so much
Allen is on fire this new year!!
Well, back to work (real job) this week, so video production will slow down again...
Thank you for all your videos. Though I understand about half, that half gets larger every video, helping me understand previous videos and helping me be able to wrap my head around things I have read. Know your making a difference. I used to be a ham ke4wjz. Since being disabled about 20 years ago . Right before my divorce 15 years ago my ex either sold or destroyed my 2 meter radio. I have rediscovered electronics and enjoy learning about RF after my divorce. Friends and family give anything electronic headed for a landfill. If I can't fix it I scavenge most the discrete parts and IC's from. This hobbie has pulled me from a dark place and has put alittle purpose in my life. Thanks again for sharing your knowledge and experience!
Alan, I wiuld love to see you do a video on the IGBT. There aren't any good videos online covering these....
Thanks for another great video !
I managed to do it, although the pattern displayed on my Smith chart was a very tight circle, so the resistance didn't change much away from 50 ohms as I scrolled through the resistance. Quite glad about this, since it was a 50 ohm cable! I'll try again with a piece of 75ohm cable. Thank you for that.
One comment: in your videos, you very often "take" values to perform operations on them. It's unnecessary, and in British English, "take" can be interpreted as "subtract". This first time I heard it, I thought "take it from what?" Hope this is useful. 🙂 Thanks again.
Sorry about the idiosyncrasies of the same language!
Fantastic video!
Thanks for the video, concise and usefull, as always.
Great vid, sir.
"I hope you learned something"... I learn something even when Alan says a random number.
Another excellent presentation, well done. I must admit I was thinking along the lines of the TDR and a long cable terminated with a 100 ohm potentiometer. Maybe that would be an interesting experiment? Great to see the NanoVNA being used for more than SWR plots, thank you for your great videos.
It can be done that way too. In fact, I did a video on that several years ago using a TDR technique with a scope and a potentiometer at the far end. ruclips.net/video/Il_eju4D_TM/видео.html
@@w2aew sorry, I must have missed that. Thoroughly enjoy your videos including revisiting the old ones.
@@w2aew thank you Alan, having watched that now I am amazed at how well it worked 73
Thank you for explaining how to measure the impedance of a coax cable. I would like to measure a parallel lined ladder flat cable. I did the same thing using 1 m long 1.2mm diameter cupper wire (emamelled) and it seems to be 45-50 ohm. Is this correct? I would appreciate very much if you can provide the method how to measure the impedance of a paralell-lined-2-wire (a ladder line).
Alan, thanks. I'd hoped to set up something recurring, couldn't see a way to do that on paypal. Anyway as an ex navy electronic tech, wish that the internet and you were around when i was learnibg. Thanks so much.
You've got some useful info on that VNA. Thank you.
Couple of questions:
If I have an unknown antenna, will the NanoVNA tell me exactly what frequency the antenna is best resonant, or best optimized for?
If I have a dipole, can I use the device to determine if the "ground" half is too long or short?
If I have an antenna with loading coils, can this be used to determine the "electrical length"?
Muchas gracias señor...! me sirve mucho su valioso conocimiento.
Very nice! And we can use the same method to measure your one meter coax's velocity factor as 45/75 = 0.6?
yes
Thank you Alan, for another great video! Very informative! 73 =)
I enjoy your explanations and diagrams TY 7 3
Alan, I have been watching your videos for some years now. I would like to contribute to your project, do you have a patreon account set up?
Thanks
Rick Mall
Hello Rick, I greatly appreciate that! I don't have a Patreon account, mainly because I feel that I don't publish content often enough to give member their value. Instead, I do have a "donate" button on the channel's main page that allows people to contribute if they choose to. It is; www.paypal.com/paypalme/w2aew
Two videos in two days. Good to see you making 2021 better than 2020. I wonder how you rate the NanoVNA against the Rig Expert or the MFJ ant. analysers? I am sure the Nano is more complicated to use and requires some learning but if you could only have one for antenna work which would you keep?
I think the RigExpert is more suited to antenna specific work, especially when working outdoors (rugged, great display). I found that the NanoVNA display was difficult to see in bright sunlight. For indoor work, the NanoVNA is great.
You read my mind... Yesterday I decided to replace the cut short coax from a Furuno NX-3H Navtex antenna. Furuno uses their own coax cables without any marking that are slightly larger than classic 50 ohms coax but smaller than 75 ohms. So I found your recent post and measured my piece of coax.
Now you forgot to mention that if the NanoVNA indicates a perfect 50 ohms dot in the middle, then the cable is 50 ohms indeed; correct?
Correct. If your cable is 50 ohms, and you terminate it in 50 ohms, you'll just have a "dot" in the center. In fact, if you have 75 ohm coax, and terminate it with 75 ohms, you'll get a "dot" at the 75 ohm point in the chart.
Thanks for the OSL tip. Is the 50 ohm load a toggleable feature in NanoVNA? Do you think the margin of error is small enough to verify if cheap 75 ohm cable has impedance more like 77.5 ohm versus a good 74.5?
The 50 ohm is not toggleable. There is likely enough resolution to see the few ohms difference.
So would the result have been the same if you used a 75ohm terminator on that " unknown" impedance length of coax ?
Cheers
This is a great video and is a simpler technique than that used by other RUclips videos I've seen. Can the NanoVNA be used to find the value of an inductor without creating a resonant circuit (i.e. capacitor and the inductor in parallel as done in your video using a pulse generator and oscilloscope)? I have effectively used your pulse generator and resonant circuit system to calculate inductance, but was wondering if you could do the same by just using a NanoVna in series/parallel with the inductor under test, without making a resonant circuit. Thank you for all your videos. I have learned so much from your great videos over the years. Regards. -WI1B
You can, if you do it carefully. Make yourself a fixture that is basically a SMA connector with pin sockets on one end, to connect to your inductor - from center to gound, and connect to the S11 port. You don't want to use long clip leads as that will affect the results particularly for smaller value inductors. You can test at frequencies where the inductor is intended to be used, and use a marker on the Smith Chart to read off the inductance value. Be sure to calibrate properly.
@@w2aew Thank you so much Alan. Lead length may have gotten me when I tried. I appreciate your quick response. You should be writing books. You can always make a technical, difficult topic easy to understand. That's a rare gift. 73, Ken
@@kenvez9309 I've heard that before, and have been asked a few times to write a book, I wish I had the time...
Every university should use this video for microwave courses...
This is wonderful as usual from you. But how can you measure it using Osilliscope and a function generator?
Here is one way: ruclips.net/video/Il_eju4D_TM/видео.html
@@w2aew thanks a lot. You do lots of great videos so that I forgot you made a video about what I asked for.
Again thanks does not do you justice, really thanks a lot.
And what benefit will this give me? Why would i need the impedance reading? just interested, sorry for stupid question..
Ive subbed for years btw!. thanks for your efforts
You get best power transfer to a load like an antenna when it's input impedance matches the transmission line impedance. Impedance matching is important to prevent signal reflections.
Besides what Alan said, if you have a box full of coax from different sources and you want to sort through it to find how many 50 ohm pieces you have, this works like a charm.
Thank you for a very great video.
If I want to use this method to measure the impedance of twisted pair cable (e.g. USB2 data lines), what is the length to use? The original cable length before twisting or the length of twisted pair?
So is it safe to assume that the same can be done with ladder line thinking would need a carbon film pot resistor for load. Thinking of home made ladder line.
very good artifice, very simple, thanks for the class, py1bys
Hi Alan,
Could you do the similar things with some pcb impedance coupon? Is the frequency range of this device enough to observe impedance uniformity of the transmission line or fiber weave effect with TDR mode.
The NanoVNA does not go high enough in frequency to see the short distance details needed to characterize the uniformity of a PCB test coupon.
Is it the characteristic impedance of the coax that we are measuring? Also, is the impedance value strictly for 44MHz as seen on the VNA? What if I want to use the coax to pass 300MHz through it? (with some load on the other end)
Yes. The characteristic impedance of the coax is independent of RF frequency. So, 50 ohm coax at 44MHz is still 50 ohm coax at 300MHz. Now, if the coax is not terminated in it's characteristic impedance, then the impedance looking INTO the coax WILL vary with frequency (even though the coax is still 50 ohms).
Would love to see the a comparison between Nano and Rhode Schwarz results to find accuracy (price of equipment) versus frequency
Very cool. These little things have so many uses, it's amazing. My problem, I have a coax, buried under the sod. If I put a 50ohm load on one end, and connect the nano on the other, what should I see if the coax is good to use? Thank you.
Probably a better way to test it, is to start with leaving the far end open. When sweeping the frequency, the trace should hug the outside of the smith chart. This will indicate that there is low loss. Then put the 50 ohm termination on the far end and verify that the smith chart stays at the center.
Very nice! I have thought about getting a Nano VNA.
How do you like using the Rocketbook notebook? I haven't purchased one yet, but it looks like a promising replacement for my old Neo Smartpen notebooks.
I like the Rocketbook. Seems to suit my purpose pretty well. It's nice that it uses readily available Pilot FriXion markers. Pretty easy to edit, etc. before scanning.
Thank you for your work!! That was an amazing and really informative video. In 1:13, you estimate the stop frequency. Where do the 75 and 246 constants come from?
It is computed from a simple free-space wavelength calculation. Speed of light is 300,000,000 meters/sec. So, you can calculate the free-space wavelength of an RF signal in meters as 300/Freq(in MHz). Then divide this by 4 to get the free-space quarter-wavelength. Thus the formula is 75/F(MHz). You can do a similar calculation for the value in feet, which gives you 246/F(MHz).
Lots of cheap coax adaptors out there. How can I measure to make sure the “adaptor” is 50ohm? Love your videos.
I used this technique to measure my various teflon coax. TC-18 got me 18.26 ohm, blue 25 ohm coax was 27.3 ohm and some unknown coax with only 0.15mm insulator between center conductor and shield was 15.2 ohm. But surprising outlier was TC-12 that measured at 16.8 ohm and supposed to be 10.7 ohm per datasheet. That may explain why my TLT transformer was not working properly.
Could you use this same method on twin-lead/ladder line? Would that require a balun or similar to go from the coax to balanced line?
Dear Sir,
I have question, do you have any idea how to measure reflected power from microwave waveguide powered with magnetron 2.45ghz, few KW? Any special devices? Please advise 🙏 best regards
I would imagine some sort of a directional coupler inserted in the waveguide.
@@w2aew thank you sir
Excellent!
Great show very informative thank 2e0ree
What about connectors? Do those matter? How would we measure those?
This is awesome! BTW, this works perfectly on my NanoVNA, but not on my Rigol RSA3015N. I've reached out to Rigol to determine what's going on. The Smith chart is incredibly noisy and unmeasurable. I can't explain it, but I'm hoping that Rigol can.
I wonder if the Rigol needs an external return loss bridge or directional coupler?
Thanks for this! I've been trying to measure resonance with different antenna materials using the loss/SWR on the nanoVNA with inconsistent results. It hadn't occurred to me to look for features on the smith chart.
Loss and SWR plots can't show you resonance because they don't include phase. Remember, resonance is when the reactive component is 0 (phase is zero). Resonance is NOT necessarily the frequency of minimum SWR.
Can we measure the impedance of NFC tag using nanoVNA?
Just for fun, I used this quarter wave method to look at a short piece of unknown cable I had. I expected 75 or 50, but kept getting a value around 95 ohms, and for a long time I scratched my head looking for a source of error. In desperation I finally Googled "95 ohm coax?", and there it was, probably RG62 - seems it's a rare "standard" value - and not one I'd ever encountered before! ( I've no idea where I got the cable, or its intended use!)
It is possible that it is actually 93 ohm. I found this info on wikipedia: RG-62 is a 93 Ω coaxial cable originally used in mainframe computer networks in the 1970s and early 1980s (it was the cable used to connect IBM 3270 terminals to IBM 3274/3174 terminal cluster controllers). Later, some manufacturers of LAN equipment, such as Datapoint for ARCNET, adopted RG-62 as their coaxial cable standard. The cable has the lowest capacitance per unit-length when compared to other coaxial cables of similar size.
@@w2aew Thanks Alan! 93 ohm and "has the lowest capacitance per unit-length" make sense: the dielectric looks like polyethylene - however the Vc seemed higher than 0.66, (and I forgot to record it !). The inner conductor/outer braid diameter ratio certainly looks smaller than in 75 ohm RG59.
So now I have 1 metre of 93 ohm cable available to perform some interesting impedance transformations! As ever, the VNA is our friend! :)
What fun! Thank you for this!
I am going to buy one of these NanoVNA, it should be very useful.
Does anyone know a good low price brand, up to 3 GHz?
Thanks if someone helps me
This is definitely a tough topic.
From this video, the measurement said the impedance of this cable is 75 ohm.
If the same coaxial cable is double the length (2 meter), is the impedance going to be 150 ohm?
The transmission line impedance is not the same as the resistance of the wire. The line impedance arises from the distributed inductance and capacitance of the line, which determines the relationship between the voltage and current *as the signal wavefront is traveling* down the line. Therefore, the line impedance is a constant, regardless of length.
How did you calibrate after you set the frequencies? Did you calibrate with the 50 Ohm load at the end of the mystery cable or right at the VNA?
Right at the VNA, because the measurement plane is the *input* of the unknown cable. If you calibrate at the end of the unknown cable, then it's effects would be removed from the measurement, which is NOT what we want.
@@w2aew thanks a lot for that timely and helpful reply. Oh another question: is there an ideal length of cable to measure this? Because when I tried with a 4m piece I just got a tiny circle in the center of the Smith chart. I might have done something wrong though.
@@matthiasmartin1975 If the unknown cable is 50 ohms, then the *expected* result will be a tiny circle in the center of the chart.
cool! Would this also work for measuring the transmission line impedance on a PCB?
Sure, provided you can terminate it well, you can couple to it cleanly with the VNA, and your VNA can go high enough in frequency to be quarter-wavelength long.
How would one "couple to it cleanly with the vna"? I'm guessing that the coax between the vna and the transmission line affects the measurement just as much? What happens if the coax impedance doesn't match the impedence of the transmission line?
Thanks for yet another great video!
@@albindennevi9829 Yes, the coax from the VNA to the PCB does make a difference. You would need to perform the calibration at the end of this coax, ideally at the interface to the PCB. This will "remove" the coax from the measurement.
Ahh! So obvious when you say it! I need to purchase a VNA and start learning these things. 😀 Thank you!
@@w2aew
Thanks for great video.
So why do coupons from PCB manufacturers always come with close trace without the option to close it with 50Ohm resistor?
Thanks vary much. Very handy skill to have... I would be lost without my NanoVNA 😅
Why do you have to sweep through to do the readings as opposed to just reading the frequency at the prime axis crossing?
You don't know what frequency will result in the crossing of the prime axis *until* you sweep it.
@@w2aew Could you not start the sweeping closer to the prime axis?
I'm new to this so I might be misunderstanding vital info.
@@lancecruwys2177 Yes, if you knew the approximate frequency you could sweep closer to the crossing point. The sweep time in the NanoVNA won't change much, so the only benefit is getting finer granularity of frequency in the sweep.
When measuring the DC resistance/impedance from ground reference to the +Vcc positive voltage rail and measure the DC resistance, then measure from ground to -Vee or the negative voltage rail. Some circuit will measure from 10 ohms to 100 ohm and other circuits will have 100ohm or more for the positive rail and negative rail DC resistance/impedance. Is this the power supply impedance measurement? even those impedance is measured in AC but they often call this measurement the power supply impedance which is measured in DC resistance. Question#2 is if the positive rail measure 30ohms and the negative rail measures 20 ohms will this have a LOWER signal to noise ratio because current is easier to flow to ground compared to a circuit that has positive rail at 100 ohms and a negative rail at 100 ohms? The higher the DC resistance on the power rails means that the signal to noise ratio gets worse because the current can't SINK to ground which creates noise?
No, this is NOT the power supply impedance. A DC measurement of a power supply rail to ground is meaningless (unless it shows a dead short which would indicate a problem).
@@w2aew What is this power supply measurement called? because EE engineers do measure the DC resistance from ground to positive VCC rail and the DC resistance from ground to the negative VEE rail. It has to d with something about how the current is sinking to ground, the higher the resistance from ground to positive rail or ground to negative rail will take longer for the current to sink to ground? it might have something to do with leakage current measurement to measuring the leakage current.
Does this method work if I have to measure the impedance of the antenna wire of an EFHW ??
You can simply use the NanoVNA to measure the complex impedance at the feedpoint by calibrating where the EFHW would connect.
Thanks.
Hello w2aew thanks for your very useful videos.
I own Agilent sine wave generator with sine output up to 3GHz.
How Can I convert the sine wave signal to 1 half cycle pulse which I Can use as TDR source
Generally not that simple - however, many professional signal generators have a Trigger output which is synchronized to the RF output - it would likely make a nice TDR source.
Hi Alan,
How did we use 75/length for fSTOP?
The free-space wavelength of an RF signal is:
Length in meters = 300 / frequency_in_MHz.
Thus, the 1/4 wavelength is simply 75/f(MHz).
Since the propagation speed in coax is slower than free space (velocity factor
@@w2aew Thanks Alan!
Awesome THANK YOU!
Thanks a lot...
Very cool, can't wait to demo to myself and others. I suppose it would work for open line also. Maybe you would terminate it with 450 ohms (for window line) .
No, you'd still want to terminate it with 50. If you terminate 450 line with 450 ohms, you won't get a reflection. That's OK too, you'll just get a tiny spot or spiral right at 450 ohms on the Smith Chart.
@@w2aew ok, so if I understand correctly , the termination needs to be different than what you think the actual impedance of the line might be. So, since you had 75 ohm coax you were able to get a reflection with a 50 ohm load. So perhaps with a 50 ohm coax you would use a 75 ohm load, and the 450 anything but 450 ohm load ect. and in all instances you would have a healthy reflection. I found it very interesting that you could check twisted pair ect. I had always thought they were going for 600 ohms with that type of cable . Thanks for the video.
@@californiakayaker Actually, I used a 50 ohm termination because the NanoVNA is a 50 ohm instrument. If the unknown coax was 50 ohms, there’d be little/no reflection and the resulting trace would be just a small dot or spiral at he center and you’d know it’s 50 ohm coax. Twisted pair cable is typically 100 ohms.
Great! Thx
Can you use an insulation tester Megger to test RF coax cables, audio cables, speaker cables, to find breaks in the conductor wire or its only made to test if only the insulation has breaks? because when using an insulation tester at 200v, 500v, 1000v you might measure .5 ohms which is considered a bad cable. The Guy that invented Coax cables made a differential math formula to be able to run the telephone signal in the center conductor using a differential signal so there will be no crosstalk. I'm not sure how you can send two different signals down one center conductor but that is why they used coax cables which he invented. The 50ohms was picked as a value because of power transfer. I'm not sure if they are talking about True Power or VAR Reactive power , or VA absolute power that coax was better at transferring power at 50 ohms.
No. The impedance of a transmission line is composed of distributed capacitance and inductance for AC signals and this impedance is not the real dc resistance between conductors.
Where did you get the 75 and 276 in the "stop" frequency calculation?
@Jonathan Alvarado thanks for this question; I thought I knew were the 276 value came from, since I originally thought it was a conversion from meters to ft - yet, 246 ft would be the conversion from 75m, so it isn't a conversion, after all. Therefore, like you: were do the values (75) and (276) come from in the "stop" frequency calculations?
@@schwinn434 I think the 75 came from the calculation of a quarter of the wavelength, the wavelength is the speed of light divided by the frequency, 300,000,000m/s divided by the frequency, in MHZ is 300/frequency so the quarter wavelenght is (300/4)/frequency = 75/frequency.
For miles i don't know, if I do the same calculation the result is about 246/frequency (in MHz)
Speed of light is 300 million meters per second, or 300e6 m/s. If we divide that by the frequency we get wavelength in meters. Thus, if we take 300/4, then divide by the frequency, we get the length of 1/4 wavelength. If we express the frequency in MHz instead of Hz, we can drop the 10^6 or e6 multiplier. Thus, 75/F(MHz) = free space 1/4 wavelength in meters, which can be rearranged to say 75/Length(m) = the frequency in MHz whose 1/4 wavelength is Len(m). We can do the same think with length given in feet. Speed of light is 984 feet/sec. So we divide 984 by 4 and get the 246 factor.
@@stefanolini279 thanks for the reply. I guess what the values of 75 and 246 represented should have been obvious. I really kinda-of stay lost when talking about impedance matching at radio frequencies -especially, when the concept of using inversion of impedance at 1/4 wave values are involved. :( Just when I think I kinda-of understand how the SmithChart can be used for impedance matching, I realize I'm totally lost once again.
@@w2aew thanks, once again, for the reply. I guess this should have been obvious, i.e, what the 75 and 246 values represented.
Zo=SQRT(Z(short circuit) x Z(open circuit)) is a better way to measure impedance of any transmission line with any length.
Here is another of your good instructional videos on this, using potentiameter :
ruclips.net/video/TpIIftvQPFM/видео.html
Since you know the frequency that it's a quarter wave, could this also be used to compute the velocity factor?
Yes it can - simply compute the ratio of the measure quarterwave frequency to the free-space q/w frequency.
Awesome
Is this possible with an antenna analyzer like AA600?
As long as you can determine the resistive portion of the impedance at the quarter-wave frequency.
Please, haw can I measure the velocity factor ?
Assuming you know the length of the line, the ratio of the free-space quarter-wavelength calculation (shown in the video) to the actual frequency at the quarter-wavelength point (from the measurement) will give you the velocity factor.
Why not use an open or a short to measure what frequency inverts the load? Use that same frequency to measure the input impedance for 50 Ohm...
sure, there's more than one way to skin the cat
@@w2aew Thanks Alan. I'm beginning to have some practical confidence in myself.
I just started working in a laundry mat. They have a bunch of debit card boxes going out on their washers and dryers. They are a small business so it would be hard for them to replace them due to cost. I don't have a set-up and I know you have the best. If you don't have the extra parts laying around, I am sure that they probably wouldn't mind if you had to cannibalize a box or two. I know the price to fix each box might vary, but let me know if you might be interested. If interested, I will check back here and let me know how to get my phone number and email address to you. I didn't see any contact information.
I’m sorry, but I would not be able to take on your repair job.
@@w2aew hey, thank you! I just knew you were good. I enjoyed watching your videos over the years.
🙏🙏🙏❤️🙏🙏🙏
Doing that all the time, when i need to measure line impedance.
Second !
This will not work on a 50 Ohm coax.
Of course - terminating a 50 ohm coax with 50 ohms will result in a single point (ideally) in the center of the chart. This will verify that the coax is 50 ohm.
goot
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