nanoVNA - Coaxial Cable Measurement Methods (Characteristic Impedance and Cable Loss) - VE6WGM
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- Опубліковано 6 лип 2020
- 1. Calibrating the nanoVNA
2. The Smith Chart - Briefly (Begins at 8:24)
3. Measuring the characteristic impedance of coax cable. (Begins at 11:46)
4. Measuring coax cable loss. (Begins at 22:34)
*Correction 1: At 23:34 I stated that the nanoVNA has a directional coupler. I have since discovered that this is not accurate information. The nanoVNA actually uses a bridge to make it's measurements. My apologies. For more information on how this works, please see this excellent video here: • #502 NANOVNA Demystified
* Correction 2: I recently looked back at this video (June 2023) and realized that I was referring to the ports on the nanoVNA as “Port S11” and “Port S21”. I believe this is not the correct way to identify these ports. Port 1 is where one takes S11 measurements, and Port 2 is where one takes S21 measurements. S11 and S21 indicate the type of measurement being taken… Port 1 and Port 2 are where these measurements are taken. Listening to how I was referencing these ports in this video now sounds awkward to me given the progress and experience I’ve gained in the use of the nanoVNA since this video was created.
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Q - Can a piece of coaxial cable be resonant?
www.allaboutcircuits.com/text...
www.rfcafe.com/references/ele...
A - Yes. It can!
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References:
hamwaves.com/zc.measuring/en/...
rfcec.com/RFCEC/Section-3%20-%...
www.allaboutcircuits.com/text...
www.arrl.org/files/file/Techno...
• How to Measure Transmi... - Наука та технологія
Hi Gregg. Just a quick note of appreciation for your clear, well produced videos on the nanoVNA and antenna and components characterizations. I am recently retired and returning to Amateur Radio after a 25-year hiatus and I trying to brush up on my technical skills. Although I am a degreed Electrical Engineer with focus on Radio Communications, my 43-year career took me a different direction. My engineering knowledge is now several half lives decayed. Had my university instructors spent more time with students in labs doing the kinds of things you do in your videos, instead of trying to prove Maxwells equations, I might have stayed in an engineering role. Your videos have rekindled my early fascination with radio and antenna technology. Thanks for that. I have subscribed to your channel. 73. Roger K8YX
Thanks Roger. I have 15 more working years left until I get to play radio full time. Haha!
The videos are my attempt at sharing my own personal investigations and learning journey towards trying to answer the myriads of questions I have about the fundamentals of Amateur Radio. I’m glad that these videos are able to spark such positive responses from viewers! Perhaps you could join in on the fun and try to answer a few questions of your own, and share what you find with the rest of us!
In my case, the videos usually start out with me coming upon something that doesn’t quite make sense, or a simple question.. “why”. At that point I try to set up an experiment to see if I can figure it out, usually some research is involved, and further experimentation happens. When I reach a point where I feel I have a satisfactory answer in my mind, I then attempt to share what I’ve found.
At 16:25, the abrupt change in the phase of S11 does NOT indicate a resonance, it is simply the frequency at which the round trip delay of the cable exceeds 180 degrees. You should note that a +180 phase shift and a -180 phase shift give you the exact same result (inverting the waveform). Thus, the VNA "wraps" the plot at +/-180. A piece of coax does not have a resonance point. In this case, you went from 0 to 180 degrees in delay round trip, or 1/2 wavelength round trip, means that open-ended cable is 1/4 wavelength long. The cable will work perfectly well at that frequency, it is will simply invert the load impedance (open ended cable looks like a short, shorted cable looks like an open).
Thank you for the comments :-)
I understand that those points where the phase is 0 and 180 degrees are the points at which the inductive and capacitive reactance seen by the VNA is equal and opposite, which is a resonant state. Is this incorrect?
@@ve6wo Resonance is defined by having zero reactance. In this case, the load at the far end (either open or shorted) doesn't have a reactance component, so the impedance looking into the line at 1/4 wavelength (where the impedance in inverted) doesn't have a reactance component either. If you extended the line (or increased the frequency) to the point where the trace goes completely around the smith chart, the line would be 1/2 wavelength long and the impedance at the input of the line will match the impedance at the end of the line (minus cable loss of course).
@@w2aew It looks like VE6WGM was describing zero or 180*n deg phase, a reflection coefficient and resulting input impedance with no imaginary component as resonance, regardless of circumstance and then attributing that quality to the cable. The cable itself of course has no resonance, but the DUT composed of both the cable and the load (open or short) can, no? If I understand you correctly, you are asserting that without an actual frequency dependent load existing at the end of the cable that has some reactance to begin with, you can't call Zin having no imaginary component at the end of the cable a state of resonance. It's seeming pedantic to me, since I don't see why you can't just consider the whole network, cable included, to be the DUT. Then why should we not say it presents a resonant load that draws no imaginary power when this happens. I agree, it's wholly different from the idea of a radiating antenna structure or other complex load like a motor, but does electrical resonance not boil down to the load presented to a source and whether that presented load draws complex or purely real power, not the actual type of load itself?
So, in the context of measuring the cable’s capacitance and inductance, an open or short is placed on the end of the cable. Standing waves are created because the termination on the end of the cable does not match the Zo of the coax cable. In this condition, there will be resonance at frequencies determined by the physical length of the cable and the propagation velocity (see reference below). When the end of the coax is open, at 0 degrees phase the coax cable looks to the signal source to be 1/2 wavelength long and the coax behaves as a parallel resonant LC tank circuit. At 180 degrees phase the coax cable looks to be 1/4 wavelength long and behaves as a series resonant LC circuit. These properties can be tested and verified all day long, and in fact coax cables can be used as very hi q resonant devices in oscillator and filter circuits if one wishes.
Resonances in the coax cable need to be avoided when measuring the inductance and capacitance in order to calculate the Zo of the cable. Ideally the L and C readings should be performed at a frequency where the coax appears mostly capacitive when open and mostly inductive when shorted (90 degrees phase is perfect). This cannot be done if the chosen frequency happens to be one where the VNA sees an equally inductive and capacitive DUT.. these two equal and opposite vectors are the definition of resonance in an electrical circuit.
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Please refer to the following:
www.allaboutcircuits.com/textbook/alternating-current/chpt-14/standing-waves-and-resonance/
@@ve6wo Standing waves are created whenever the load doesn't match the line impedance. The extreme cases of an open and a short create a complete, non-reactive reflection. You'll note that if you take the capacitance and inductance measurements at a different frequency (except at the quarter-wavelength points), that the impedance calculation will still work (it doesn't have to be taken at the 12 and 6 o'clock positions). Under the open and short conditions, the line will appear resonant at all multiples of a quarter wavelength - this is due to the fact that the open and short loads are non-reactive. At even half-wavelength lengths, Zin=Zload. At odd-quarterwavelengths, the impedance ratio inverts around Z0 (the line impedance), such as Zload/Z0 = Z0/Zin. Thus, an open looks like a short, and a short looks like an open. This is because you rotate halfway around the smith chart with every quarterwavelength of line.
I have several videos on transmission lines, reflections and standing waves, and the smith chart:
visualizing standing waves: ua-cam.com/video/M1PgCOTDjvI/v-deo.html
transmission lines and terminations: ua-cam.com/video/g_jxh0Qe_FY/v-deo.html
basics of smith chart: ua-cam.com/video/TsXd6GktlYQ/v-deo.html
smith chart with transmission lines and swr: ua-cam.com/video/ImNRca5ecF0/v-deo.html
This is by far the best video on UA-cam if you want to learn how to use the nanovna to measure cable loss....thank you!
Thank you, I very much appreciate the kind words :-)
Making people smarter one video at a time. THANKS!
Amazing - very scientific and applying all the math returns excellent results - keep up the great work
Great explanation hope you will make more video
You have explained the basic concepts very well.
Thanks for this video Gregg, it's helped me a lot and I'll watch it over when I'm making some tests. It's a handy reference to save. I appreciate the time you have taken to make this.
Thank you.
Xtra nice video! ~ Very educational! ~ Thank You!
Thanks for your explanation about calibrating the Nanovna @ the antenna side of the coax as the reference point. This was exactly the information that I was seeking today. I was getting the strangest readings at the equipment side of the coax. I was trying to adjust my home-brew antenna frequency response curve and impedance by re-angling the ground plane elements. Your presentation of how to use the Nanovna to check coax for being in-spec was also super-helpful!
Thanks Jerry. I’m glad to hear my videos are helpful to people :-)
Thank you Gregg, I've recently bought the same NanoVNA as yours, so this is a valuable learning video.
Thanks for the feedback. I’m glad to be able to help.
Brilliant! I just picked up 200ft of used LMR 400 for $10. I now have a way to check loss and make sure it’s still good before using it in my projects. Thrill to find this vid!
Thank you. I’m glad you found the video helpful :-)
That was very good. You've inspired me to make some coax measurements with my NanoVNA.
Great video! Thanks for this tutorial. I plan to check my cables soon. VA3MFR. 😃
Ett fantastiskt this calculations you do😊😊😊😊😊😊😊😊😊😊😊
One comment that may or may not effect any of your conclusions as I don't know if the velocity factor is used for anything except finding problems at a specific distance from the source. My method is quite old fashioned and had about a 2% difference than the method you showed on the NANO VNA. Since I use four transmission lines, all LMR-400 and about 200 feet in length, I measured the length of the cable and then adjusted the Vf until that number was quite close. I also made two LMR-400 pigtails that connect to the VNA (mine is an AIM 4170D) and do all my calibrations with the pigtails so as not to damage the connectors on the VNA. This has enabled me to find faults within a couple of inches of the VNA TDR measurement. Great video, K7HIL
Thumbs up.. Great 48 minutes of knowledge..
Thank you.
thanks a lot to this video easy to understand calculation
Amen. Every time I think I'm starting to get this stuff I realize how little I understand.
Very interesting indeed😊
Great video and equal great comments. Thank you!
Thank you very much for this very informative video Gregg. Have a nice Easter weekend, PA0CMU
Thank you. I’m glad the video was helpful :-)
That big connector hanging on your nanovna is a good way to break it.
Yup.
Thank you for explanation. I just want to add than a point with +180 phase degree or -180 phase degree in not a resonance. It's just a point where phase goes more then 180 degree and nothing more. A device is made to show phase degrees whithin a range -179.99 to +179.99.
Thanks, well explained. I'll do that with my coax! from VE2DPS
I thought I knew how to calibrate a Nano VNA, but I wasn’t familiar with the “isolate” calibration. Thanks for this video. KC3JJH
To 43:00 - 0.77 dB loss is fine because each connector adds 0.1 dB roughly. So, you may have 0.2 dB loss in connectors and 0.57 dB loss in a cable. Which is very close to 0.6 dB/100 ft.
Awesome video. I am trying to figure out if there is any way to measure antenna gain with the nanoVNA to compare with what the antennas advertise. Any ideas? Thanks.
I am not aware of any way to do this with the nanoVNA. In my mind, a field strength meter would need to be used whereby readings of a transmitted signal are taken at various locations around the antenna and a radiation pattern diagram made from those readings.. something along those lines. I suspect some googling would provide a better answer for this question.
Thanks for this great job, for RL method, when you add both result and devide by 4. why you devide dB by 4, you would better add-6dB to the result? or conevert dB to linear and devide all by 4?
Hi Gregg-thanks for the video. I got halfway through before I realized I am missing the 3 adaptors shown at 24 seconds. They did not come with the Nano kit. I have the 4 small adaptors but not the 3 larger adaptors in your picture. Could you possibly let me know the part no or name of those 3? Thanks
You can take a shortcut here and use the Z11 values directly (down next to you S11). Take the geometric mean of both measured values (open and short end) and you will get your answer as well
Thank you Roel.
Great Video! This is exactly what I was looking for in terms of a long term plan to assess the status of 2 100' runs of LMR-400 going to my attic satellite antennas. I will definitely be testing out these methods myself with the NanoVNA V2 for this purpose.
I have a few questions for you:
(1) Why do you pick the most imaginary reflection coefficient points for the L/C calculation? Also, this can be done with any 2 points at the same frequency for reflection coefficient on the smith chart corresponding to a short and open termination, right?
(2) Is there a method that can be applied to measure characteristic R and G for the cable to get a complex characteristic impedance directly so as to know the cable loss in this figure given Zo = sqrt((R+jwL)/(G+jwC), or are we stuck with having to measure loss separately and then work backwards to calculate R and G?
(3) Are you familiar with the "transformation" functions available on the nanoVNA that allow operation similar to a TDR (Time Domain Reflectometer) to directly measure cable length? W2AEW used the "Low Pass Impulse" to do this in one of his recent videos (#316?), and the manual is silent on how this method works under the hood (see page 14: nanovna.rf.pl/Manuals/NanoVNA-User-Guide-English-reformat-Dec_9-19.pdf ). I was hoping for a good resource on understanding the function of that approach to a similar task to what you do here.
Thanks for the comments and questions.
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I will take a stab at the first question here.. why do I make the measurements where I do (at 90 degrees phase angle)?
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I wish to consistently use a frequency that is as far away from the points where the inductive and capacitive reactance vectors are equal and opposite (the point of resonance, 0 & 180 degrees phase angle). No meaningful measurements for capacitance and inductance can be performed at these frequencies using the nanoVNA in order to calculate Zo of the cable. At 90 degrees, the magnitude of the reactance is high in only one direction, thus enabling a good measurement. In addition, the VNA is most accurate when taking measurements at 0+j50 and 0-j50 ohms, which is at +90 or -90 degrees on the smith chart.
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If the method of measurement is standardized (ex: always taking your readings at 90 degrees phase angle), then you can be sure that any changes seen in the coax over time (if you have an annual coax health check schedule in your radio shack) will be due to actual changes in the coax and not because of differing methods of measurement. If you take a measurement at 90 degrees, calculate the Zo you will get a value. If you calculate from measurements taken at another phase angle (not at 0 or 180), you will likely find that the value you get is close to the first, but there is a slight difference due to various factors such as small errors in measurements due to resolution of the nanoVNA (which is why I like to grab the measurements at a point where the reactive vector magnitude is greatest and only in one direction). If you have a health check log and perform the measurements differently every time, can you be sure that you are seeing a change in the coax? Or is it because you have changed your measurement method?
I hope this answers your question :-)
Question 3 - I have not played with the TDR functions of the nanoVNA other than to follow the guidance in the video produced by W2AEW, which is excellent!
Question 2 - I am unable to answer off the top of my head. I do not recall using this formula when I was in college, I will have to go back in the text books and see if I can find it. My apologies.
If you play around with it and find a more accurate method that is relatively simple to measure Zo then please make a video and share with us. Please link to it here :-)
Regarding Question 2 - I did a bit of snooping regarding the formula you referred to and found this: www.translatorscafe.com/unit-converter/en-US/calculator/coaxial-cable/
The formula I am using assumes there is no dielectric or resistive losses in the cable. Using the formula you referred to is based on a more complete model of coax cable and would be more accurate. If your cable has developed an issue with the dielectric deteriorating or has some resistance change from a DC point of view then this calculation would show the change using the more complex formula.
DC resistance can be measured easily, but who has the ability to measure the dielectric constant?
Please correct me if I'm wrong, but if the dielectric constant changes would not also the measured capacitance? If this is true then one can deduce a change in dielectric properties over time based on the change in capacitance measurement. Therefore, the more complex formula would be unnecessary for the average operator who is monitoring their coax system over time for any kind of degradation, would it not?
@@ve6wo I really like this approach. If the coax is degraded as suggested by any reading, chuck it, because whatever the reason for degradation and whether indicated by change in dielectric constant (difficult to measure) or resultant change in capacitance (and therefore by extension characteristic impedance), it is likely no longer fit for purpose and there's no way it's gong to get fixed in a hurry.
Thanks for a very informative video. I recently bought a nanoVNA but it hasn't had much use yet apart from analysing my DIY antennae.
Keep up the good work. Julian Grammer. G1EKW.
Thanks for the excellent Video Gregg... Don, W5MML
Thank you, very useful. But think: the accuracy is not so high to say the cable impedance is 50,5687 Ohm. The accuracy not higher a 1% so the impedance is about 50,6 Ohm.
A fair observation. Thank you.
what power level is set at nano VNA during cable loss testing? can we really do loss measurement of a long length cable around 1000 meteres or so..using nano VNA? your help is appreciated. Thanks for informative video.
How did u get the numeric graph at the edge of your nano...my Nana vna _4 dosnt have that
I understand it depends on what NanoVna firmware is loaded.
Thanks for a very thorough presentation. You could help those whose hearing ain't so great any more (like me...) by using a headset/boom mike thereby holding your audio at a constant level as you turn your head. In this video, your audio is moving up and down as you turn your head away the desk or studio mike (I'm guessing here...) and that is hard for me to follow. Just saying.
Thanks for letting me know.
Do you need to enable a transform (low impulse pass) to take RL and DTF measurements?
At 48:21 is that impedance being displayed as -50.4 ohm on the nano S11 yellow? and what does the minus sign indicate? -50.4 [ohm sign]
That is the reactance, not impedance. A negative reactance tells you that the DUT is showing capacitive reactance at that frequency. If it were inductively reactive, the reading would be positive.
Hi, thanks for the video :-) I got a question : As the connector of NanoVNA is an SMA, its impedance is 50 ohm. So if we use a cable with not 50 ohm impedance, there will be mismatch between cable & connector. I would like to know if in this case the protocol you used is still valid for determining the caracteristic impedance of the new cable ? Thanks !
Yes. I demonstrate measurement of a piece of 75 ohm coax here:
ua-cam.com/video/8c13PPA51zs/v-deo.htmlsi=DpD8uSrqmqZBgJGP
An adapter could be cancelled out with the selection of "n" for added nano seconds in DISPLAY --> SCALE --> E-DELAY
At 8:36 you state that you have "nothing" connected. You have the OPEN cap connected - which is different than nothing, right? ;) A great video!
Hehe! Yeah, nope... I actually do have ‘nothing’ there. I know, I know.. the “open” cal standard is different from “nothing”... it’s sloppy, yes. The reason I can get away with this is due to the relatively low frequencies I’m using.
Have heard you need to earth cable/antenna if connected to he vna, needing discharged, as there may be a charge in the equipment sometimes that can damage the VNA.
I’ve never worried about this and have never had an issue… however, if there is some kind of circumstance present at your location that brings up a legitimate concern that you may have static electricity on the line, then you’re best to discharge the center conductor and shield to ground before hooking up to the nanoVNA. Common sense should prevail here, I think.
Measure loss S21 at 300Mhz and use that number from the catalog it gives you better resolution for shorter pieces.
Ok.
If anyone hasnt said, when you change the freq to CW it good practice to recalibrate. If done, You might have found to 0.01db error
Yes, you’re supposed to recalibrate whenever you change spans. I tried it and it made zero noticeable difference to my readings. I guess it depends on how exact you want the readings to be, so there might be a time and place for it, however, it seemed to be unnecessary in my case :-)
@@ve6wo
Thanks for sharing
Did you have to upgrade it somehow to 2GHz or does it arrived already extended pls?
No, I have not modified my nanoVNA. It comes with an upper limit of 1 GHz out of the box which is far beyond the upper frequency range that I’m interested in (30 Mhz).
Sir, when u check insertion loss, u didn't calibrate port 2 , why?
Im trying to find a way without doin math and getting VF, using the nano to cut an unknown coax to a half wave length of a certain freq. Example cutting a 1/2 wave coax for 4:1 balun.
I have not seen any video for it.
It could be a 50ohm or 75ohm 4:1 balun.
That’s a great video idea! I would love to do one on that subject, however, I’m out in camp working for the summer, so I can’t make one anytime soon. Will look for an opportunity to make one.
hi how to measure the kength of a coaxial cable ?
Vey usefull video. But i have a question. When i have an antenna with transmission line which give to me a good swr factor and z, (aprox 50Ωμ), but i have a small value of capacitance, (200pF for example), to smith graph. Is this value of capacitance affect the tramsmission signal or not?
This reading would point me towards looking at the antenna. It is likely a bit too short.
If you have a good match you will have efficient power transfer and a slightly capacitive reading will have an effect, but it will be virtually immeasurable.
If you wish to have a purely resistive load, you can try either adding inductance or lengthening the antenna, but as I said, I would expect that you will not be able to measure the difference in the signal being radiated.
I made the question above as i' m thinking how this capacity works. If it works like a capacitor connected to the output of transmitter, this will cause a part of rf goes to the ground, but maybe my thought is wrong.
IWould love to see this Z0 test done on balanced feedline. like some window or open wire line..
Maybe at some point in the future I will acquire a suitable small signal balun transformer to couple the nanoVNA’s S11 port to a balanced feedline for testing.
Comparing s11 to s21, s11 measures the round trip loss but s21 measure one way loss. is the s11 loss twice the s21 loss ?
Yes, the S11 loss measurement is round trip, so the loss is double because the signal has travelled twice the length of the cable.
I'm try to learn how to use my NanoVNA-F V2. I know how to use it in relationship to testing a 915 MHz antenna. What I'm trying to learn is how else I can use it in relationship to my hobby of helium Hotspot Setup and configuration. I'm hoping then as I learn I can actually teach other people in the helium Network the basics. The biggest problem that I have in learning how to use the Nano is most of the people that have videos talking to hate that is way over my head. Is there a place somebody can direct me that will take me step-by-step how to use the Nano in relationship to my hobby and goal?
The phase going from -180 to +180 doesn't mean anything - the phase is linear (has a linear slope to it), it's just if you keep plotting it without resetting it at some point, you end up with phase angles above 360* which ceases to mean anything, or is hard to work with. So the rule of thumb is you graph phase from -180* to +180*, and if it goes below -180 or above +180, you wrap over to the other side. Also, the phase being positive or negative has nothing to do with it being capacitive or inductive - it's not on the same scale as a smith chart. The phase plot has absolutely nothing to do with capactiance or inductance, only delay.
I have to disagree, somewhat. Although the phase rotating from -180 to +180 doesn’t really have any particular significance as far as I know, as you pointed out… the phase angle (as displayed on the VNA and on the smith chart) is most certainly related to the resistance and reactance in the device under test, which could present as capacitive or inductive depending on what the stimulus frequency is.
Although the phase angle on a smith chart is generally explained to represent the distance in wavelengths along the transmission line, when you look a little deeper, you’ll find that this phase angle is actually the phase of the reflection coefficient, as measured at the point where the VNA was calibrated.. the calibration plane. Negative phase values represent capacitive reactance as seen at the calibration plane, positive values represent inductive reactance as seen at the calibration plane.
Please watch this video where I explain the mathematical relationship…
ua-cam.com/video/-gGJHhJ2lUs/v-deo.htmlsi=MkYxI0Tc5-SnjL4i
@@ve6wo Still my overall point is if you look at your phase diagram, and simply remove the jumps from -180 to +180 (which is only a notational convention to keep the scale of the phase graph contained), you would have a straight line continuing infinitely with the same slope. A "linear phase" shift shows you that basically nothing odd is going on with the DUT (the cable in this case) - it's just getting electrically longer in terms of wavelengths, as the frequency increases, which makes perfect sense as the wavelength decreases as the frequency goes up, so the cable looks longer when measured in # of wavelengths. Also, a smith chart doesn't display phase information - it only displays complex impedance at a given frequency. Maybe there's a mathematical relationship there between complex impedance and phase (or the derivative of complex impedance and phase maybe), but I haven't thought through that. I will watch your other video, but I'm skeptical...
@@gorak9000 That’s fair :-)
Let me know what you think after you watch the other video.
Is it possible to measure output power of ht (radio)
Nope
One point you omit is "what accuracy do you need?"
I usually call the cable good if I read 50 ohms +/- 1 ohm. If it reads above or below that then I start looking more closely for issues, beginning with a thorough physical inspection.
If you’re looking at the loss of the cable, it’s good if you can test the cable when it’s new and record the value. Each year, test the loss and mark it down in a log. When you start to see a significant change then you’ll know to start looking.
When I was maintaining commercial mountain top repeater sites, this is how we did it.
Actually, that’s true for all the measurements. SWR of feedline and antenna, coax impedance, and coax loss.
If a significant change is noted anywhere over time then you’ll know.
In the case where you have a random cable with no history on it, then measure the specs on it, compare to what it theoretically should be if new.. and use your best judgement.
@@ve6wo Its difficult to detect increasing loss in a chunk of installed tower mounted coax with VNA instruments without some climbing, and cutting. Except for lightning strikes ruining the antenna, or coax, field strength measurements or lack of radio range seems to be the measurement of choice. We all know that increasing coax loss just makes SWR measurements better. I had once tried to research if the swept frequency return loss between antenna match points could somehow indicate coax loss. It would seem that the match point troughs would not be so deep. I've tried using attenuators to simulate coax loss, but could not reach any good conclusions.
@@wb7ond Yes, the technique shown in the video requires the end of the coax to be disconnected from the antenna. If you're not into climbing a tower to do annual cable checks at your station, then this method is not for you.
I would think the loss in a 75 Ohm cable is the loss, I don't understand what matching the load or a mismatch has to do with anything. Ron W4BIN
Calibration, with same connected cable, isn't how is supposed to be. All this connected adapters, cables - must be removed before calibration. After, you need to set electrical delay (port extension), to each of those connected adapters/cables
Well... Shoot. All this is likely everything I need to know/do so I can analyze my mobile coax/antennas.. But.. Waaaaaaayyyyy too much math for me to deal with.. Way over my head.. 😱😢😢
.. At least.. Finally found a video that actually shows how to use more features then I have a clue to understand..
.. I think... All was illustrated just fine. Just that me am math never got along..
... But thanks for the video..
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Still not sure how I am gonna do what I need to do.. Thou this will do it..
you have the same accent as mr carlson, are you from the same area? :D
You mean this Mr. Carlson?
ua-cam.com/video/2xrsicfiN-0/v-deo.html
Hahaha! That’s pretty funny.
@@ve6wo nah mate i mean this guy, who is also into electronics and repair and radio stuff.
ua-cam.com/users/MrCarlsonsLab
Wow! We do sound a lot alike! I have no idea where Mr. Carlson is from, but I’m located in Western Canada. I subscribed to his channel. lots to watch there, very cool stuff!
How do we use the NanoVNA to measure velocity factor?
I have never used the nanoVNA to measure the vf of a cable, however, I did find a video on it here..
ua-cam.com/video/aWvPB299U60/v-deo.html
Insertion loss and return loss are not the same.
Nope they’re not. Did I make a mistake somewhere in the video?
That measurement of yours for Z0 makes me very squeamish. While you do get the correct result for Z0, you are not getting the correct values for L and C. Those parameters are supposed to represent the characteristic inductance/capacitance of the cable, which will NOT be accurate when you do the 90-degree thing. The only reason this works is because the ratio between them is cancelling out the error. If, however, you tried this method to calculate phase velocity, you will absolutely get the wrong answer, since vp = 1/sqrt(LC).
I think a far simpler method for measuring L and C is to just reduce the frequency to something very low (e.g., 100 kHz) and then measure reactance of the cable directly. If you do this, you will find that the measured C is very different than the one you just did at the 90-degree phase shift. It will also be far more reliable, since all you are seeing is the net reactance of the cable.
Incidentally, if you try the low-frequency method to measure L by attaching a short circuit, you will get a higher value than expected. This is because the inductance of a transmission line is HIGHER at lower frequencies. The "impedance" of a cable is only valid at relatively high frequency (usually a couple MHz), since the skin-effect forces all the currents to the surface of the conductor. This reduces the inductance, which brings the impedance down to the specified 50-ohm. Fortunately, there is a simple method to correct for this and get accurate results.
Hi James :-)
Thanks for sharing your thoughts. I encourage you to put together your thoughts in a video and to present them using the nanoVNA to demonstrate your idea of what is the correct and more accurate way to measure coax cable Zo. I would like to see what others come up with, and I know there are a good percentage of amateur radio operators interested in this subject.
It turns out, James did make a video!
Here it is :-)
ua-cam.com/video/p0oOz0uhn4E/v-deo.htmlsi=FmmkvCIN7Yi_Wt-7
Good information! But geez, you're giving me a headache with the changing light levels, out-of-focus video, weird level changes on the audio.. camera moving around. I hope you'll get a handle on your production techniques for future vids.
Yes, it’s the typical problems that newbs to the video production world go through when first starting out :-)
I was aware of these things, however, I figured sharing the information even if not perfectly presented was still more important than not sharing the videos because they aren’t produced well.
Thanks for the comments.. I may upgrade my abilities sometime in the near future :-)
@@ve6wo -- thanks, Gregg, for taking the criticism constructively. 73, Doug, AD4AL.
A very good explenation, but the sound is for a non Canadian/English listener terrible. very73 de DL9WD
I don’t quite understand, sorry :-(
I found his speaking very clear and understandable I'm from Europe (Ukraine).
???Now the sound is clear@@volodymyrzakolodyazhny7740