#326: How to Measure the impedance of "unknown" coax using a NanoVNA

MNI TNX ES VY 73 DE TA4IV 👍❤👏

🌟🌷🌟

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.

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.

could you give us this again to those of us who aren't mathematicians?

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.

goot

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).

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?

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?

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?

Zo=SQRT(Z(short circuit) x Z(open circuit)) is a better way to measure impedance of any transmission line with any length.

Simply a great teacher !!! TNX & 73s.

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?

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?

Why do you have to sweep through to do the readings as opposed to just reading the frequency at the prime axis crossing?

Can we measure the impedance of NFC tag using nanoVNA?

This will not work on a 50 Ohm coax.

Thanks a lot...

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?

How would you measure 2 parallel conductors? Would you just ground one wire?

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

Would love to see the a comparison between Nano and Rhode Schwarz results to find accuracy (price of equipment) versus frequency

Kewl! CE ... CE...

Having problems with circuits? Try circuit solver Spot circuit solver on the playstore!

Your videos are always a pleasure to watch. That being said, what's that notedpad you're showing in your videos from a time back, some sort of an erasable one or something?

very good artifice, very simple, thanks for the class, py1bys

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.

Lots of cheap coax adaptors out there. How can I measure to make sure the “adaptor” is 50ohm? Love your videos.

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...

Great teaching techniques...👍👍honestly I have learnt a lot from your valuable explanations, 🙏🙏

Does this method work if I have to measure the impedance of the antenna wire of an EFHW ??

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"?

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

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!)

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

really educational and something even i (a non technical person) could undeestand or apply in some way.
.
Thank you so much, sir!

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 !

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 wonderful as usual from you. But how can you measure it using Osilliscope and a function generator?

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.

Those NanoVNAs are awesome little tools.

Every university should use this video for microwave courses...

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

Your videos are great. You are concise and knowledgeable. Thank you sir.

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What fun! Thank you for this!

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?