1:35 more to the point... 9950 ohms is a reflection coefficient of .99000, 10k ohms is a reflection coefficient of .99005. That's a relative difference of 0.005%. If your VNA and test setup has, say, a 0.1% relative accuracy, then 10k ohms might read somewhere between 9k and 11k. If you have a 1% relative accuracy, then 10k ohms might read somewhere between 5k and infinity.
It is hard to say 1% accuracy applies to reflection coefficient and that it might be linear over the range. If it applies at the p=.99 then you are correct. Larry, W0QE
@@w0qe Yeah, admittedly I'm still simplifying things a bit, but it goes to show why it's such a hard problem. I also like to build chokes and measure them :)
Hi Larry: Could you perhaps do a short video where you actually set up the nanoVNA, measure a common mode choke, and then show all the numbers? I think for us newbies it would be very useful to see it actually performed in addition to the description. Thanks.
Hepcat, I loaned out my NanaVNA and with the Covid-19 stuff I haven't tried to get it back yet. However the focus of my channel is not equipment reviews but education to allow others to be able to understand and make measurements. For example in this video the take away should be that an 0805 size resistor is really the resistance value in parallel with a capacitance of .05pF. Get a few resistors and become accustomed to making measurements seeing what you can do. If your question is about how to measure the common mode impedance of a choke that is physically large I discussed the effect of clip leads on measurements in video #77 ruclips.net/video/xLGSblc-Fec/видео.html which might help. Also videos #80, #81, and #82 deal with baluns which are common mode chokes. Does this help? Larry, W0QE
I'm wondering, in which case do you even need to measure such high impedances at RF frequencies at almost any degree of accuracy? When is such information practically vaulable?
Ivkan, For me the most common need to measure high RF impedances is when I measure the common mode impedance of an RF balun. The common mode impedance can easily be in excess of 10k ohms where resonance occurs and knowing the that the value is at least reasonably accurate is all that is really important. Larry, W0QE
@@w0qe So, I presume that the purpose of such measurement would be to calculate the "CMRR" of balun to balanced-line system? I never thought of that :D I usually thought that an impedance of~1k would be enough for such purposes :D
@@dedamarsovac In higher power transmitting applications where the common mode voltage across the balun is high and if the common mode impedance is modestly resistive the power dissipation of the balun becomes too high. It is hard to keep the common mode impedance nearly completely reactive so often just making the total impedance in the > 5k ohm range is the easiest way to not have the balun overheat. Does this make sense? Larry, W0QE
@@w0qe It does make sense. It actually hides another "measurement method" : just apply a lot of RF power in common mode across the balun and measure the temperature using a thermal camera :D Just kidding, not gonna do that no :D I never worked with such high power RF things, I only worked with fairly low power RF stuff on PCBs, designing for various gadgets having GPS, Wifi, BLE, LoRA, Sigfox, etc. Not a watt of RF Power :D And there's so much to learn I'm still not up to a challenge of designing a proper LTE (wideband) antenna to be embedded in a small device having any reasonable efficiency.
As for the measurement part, I always liked to use tricks to make any kind of high precision/accuracy/range measurements, often to the point of "in real life, does it even matter", which I often find hard to answer :D Be it nanoohms, teraohms, femtofarads, or just being a volt nut :D That's pretty much what actually bothered me and why I asked the question :D
Edi, I don't understand your question. At UHF frequencies you need a VNA capable of measuring at the frequency you want to measure at and the only difference vs measuring at lower frequencies is that you need to be more careful with your measurements due to the shorter wavelength. Larry, W0QE
What a great demo, both for a test jig that I've never seen before and for all the ways SimSmith plotting can be used.Dan, AC6LA
Thanks Dan,
SimSmith is really quite amazing!
Larry, W0QE
Great explanation! I hope to apply some of this to making measurements on end fed antennas..
Wow, that's in fact kind of "eye-opener" on those effects! Thank you! Great video
Thanks!
Larry
Which VNA did you use for these measurements? Great videos!
I used my HP-8753 and my AIM 2180 for s21 measurements and my AIM 4170 for the s11 measurements.
Larry, W0QE
1:35 more to the point... 9950 ohms is a reflection coefficient of .99000, 10k ohms is a reflection coefficient of .99005. That's a relative difference of 0.005%. If your VNA and test setup has, say, a 0.1% relative accuracy, then 10k ohms might read somewhere between 9k and 11k. If you have a 1% relative accuracy, then 10k ohms might read somewhere between 5k and infinity.
It is hard to say 1% accuracy applies to reflection coefficient and that it might be linear over the range. If it applies at the p=.99 then you are correct.
Larry, W0QE
@@w0qe Yeah, admittedly I'm still simplifying things a bit, but it goes to show why it's such a hard problem. I also like to build chokes and measure them :)
I'm glad to see the video.
Thanks KS,
Larry
Hi Larry:
Could you perhaps do a short video where you actually set up the nanoVNA, measure a common mode choke, and then show all the numbers? I think for us newbies it would be very useful to see it actually performed in addition to the description. Thanks.
Hepcat,
I loaned out my NanaVNA and with the Covid-19 stuff I haven't tried to get it back yet. However the focus of my channel is not equipment reviews but education to allow others to be able to understand and make measurements. For example in this video the take away should be that an 0805 size resistor is really the resistance value in parallel with a capacitance of .05pF. Get a few resistors and become accustomed to making measurements seeing what you can do. If your question is about how to measure the common mode impedance of a choke that is physically large I discussed the effect of clip leads on measurements in video #77 ruclips.net/video/xLGSblc-Fec/видео.html which might help. Also videos #80, #81, and #82 deal with baluns which are common mode chokes. Does this help?
Larry, W0QE
@@w0qe
Yes, that does help. Thanks. I have parts on the way.
It seems most of us calibrate VNA with 50 ohm, which is not the best option for Hi-Z measurements.
I'm wondering, in which case do you even need to measure such high impedances at RF frequencies at almost any degree of accuracy?
When is such information practically vaulable?
Ivkan,
For me the most common need to measure high RF impedances is when I measure the common mode impedance of an RF balun. The common mode impedance can easily be in excess of 10k ohms where resonance occurs and knowing the that the value is at least reasonably accurate is all that is really important.
Larry, W0QE
@@w0qe So, I presume that the purpose of such measurement would be to calculate the "CMRR" of balun to balanced-line system?
I never thought of that :D
I usually thought that an impedance of~1k would be enough for such purposes :D
@@dedamarsovac
In higher power transmitting applications where the common mode voltage across the balun is high and if the common mode impedance is modestly resistive the power dissipation of the balun becomes too high. It is hard to keep the common mode impedance nearly completely reactive so often just making the total impedance in the > 5k ohm range is the easiest way to not have the balun overheat.
Does this make sense?
Larry, W0QE
@@w0qe It does make sense.
It actually hides another "measurement method" : just apply a lot of RF power in common mode across the balun and measure the temperature using a thermal camera :D
Just kidding, not gonna do that no :D
I never worked with such high power RF things, I only worked with fairly low power RF stuff on PCBs, designing for various gadgets having GPS, Wifi, BLE, LoRA, Sigfox, etc.
Not a watt of RF Power :D
And there's so much to learn
I'm still not up to a challenge of designing a proper LTE (wideband) antenna to be embedded in a small device having any reasonable efficiency.
As for the measurement part, I always liked to use tricks to make any kind of high precision/accuracy/range measurements, often to the point of "in real life, does it even matter", which I often find hard to answer :D
Be it nanoohms, teraohms, femtofarads, or just being a volt nut :D
That's pretty much what actually bothered me and why I asked the question :D
How to measure antena frek UHF ?
Edi,
I don't understand your question. At UHF frequencies you need a VNA capable of measuring at the frequency you want to measure at and the only difference vs measuring at lower frequencies is that you need to be more careful with your measurements due to the shorter wavelength.
Larry, W0QE