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.