NanoVNA and checking a common mode choke


Last week a new toy arrived a Nanovna and I have been playing around with it and learning the ins and outs of the unit. This unit allows me to check antenna SWR, attenuation and coax switch isolation to name a few things. I am not going to get into the details of the Nanovna as there is lots of information for those interested available on the internet. My first project was to check the attenuation on one of the common mode chokes I made. It was made from RG316 coax, FT 240-31 mix toroid and had 10 turns on it. I never put this common mode choke through any testing as I did not have anything to test it. I was going by some charts, the type of toroid mix and the number of turns needed for 40m to 10m. 

Now that I had a Nanovna I could perform an attenuation test on the common mode choke I was using and see some actual numbers. I have been reading and -30dB or more across 40m-10m is great for a common mode choke. I calibrated the Nanovna and performed an attenuation test and to my surprise, the common mode choke I made failed! The common mode choke I made was above -30dB from 40m to 10m. It was not time to fix the problem and bring my readings below -30dB. 

The red line is the results


I tried reducing the windings around the core increasing them and doing a Nanovna sweep each time to see the results. My final result was 14 windings and that gave me the ideal results. The Nanovna indicated -35dB on a full band sweep (6.5MHz to 30MHz) on individual band sweeps the readings were 10m -44dB, 15m -34.5dB, 20m -35dB and 40m -35dB. I am much more pleased with these readings. 

Full band sweep

The software to display the readings is called Nanovna saver, it's free and works great. I am very pleased with the results from the Nanovna and I was able to check and fix my common mode choke. I will now move on to the next project and that is the isolation between antenna 1 and 2 on my LDG AT200pro2 but more on that in another post.

Mike Weir, VE9KK, is a regular contributor to and writes from New Brunswick, Canada. Contact him at [email protected].

4 Responses to “NanoVNA and checking a common mode choke”

  • Gary NG9T:


    Thanks for sharing your test. I recently bought a new MINIVNA from an SK’s estate and am looking forward to learning more about it. There are plenty of YouTube videos so the basics aren’t hard to do.
    I’ve got a QRPLABS QMX kit to build and I wonder if it can help me get the toroids wound just right? Hmmm. Something to look into.
    73 de NG9T

  • Mike VE9KK:

    Good morning Gary and very nice to hear from you. Thanks for stopping by and taking time to read the post. Yes these are very nice gadgets for sure. I took my time learning about it (still more to learn). I found the YouTube videos very helpful as I learn better by seeing and doing as to reading about it. Have fun with the VNA.

  • Tim KD0UUU:

    Has anyone verified the accuracy of the NanoVNA vs a “real” test equipment to ensure the readings are trustworthy? I am very interested in buying NanoVNA or Mini1300 but unsure which would be better overall for ham use or how trustworthy they are. Like they say you can make a screen show anything you want but can you trust that it is telling you the truth?

  • Mike VE9KK:

    Good morning Tim and very nice to hear from you. Thanks for taking the time to read the post and for leaving a comment. The best answer I can give you is join one or 2 of the Nanovna groups. Yes they are Nanovna groups but I do believe you will get an honest answer from some very well qualified people there. The two groups I belong to are Nanovna-F and Nanovna-users. On both groups they are a great bunch of people and have helped and answered many questions for me.
    Thanks and have a good week,

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