nanoVNA - Testing the Common Mode Attenuation of a DG0SA 1:1 Current Balun by VE6WGM

This balun was built for use in an 80 meter dipole antenna. I wanted to ensure that it is actually behaving as a balun transformer.. matching the balanced load of the dipole to the unbalanced coax feedline. A true balanced system naturally has an excellent CMRR. In this case, if the balun is doing it’s job, most of the energy being supplied by the radio and feedline will stay on the antenna even if there is a bit of mismatch on the antenna. This ensures no unwanted RF appears in the shield of the coax and the antenna’s radiation pattern remains that of a dipole.
I make no claims at being an RF engineer, so if you happen to notice any faults or inaccuracies in what I have presented here, please feel free to correct me. There’s one thing with Amateur Radio.. just when one thinks they have something figured out, along comes some new information and former ‘truths’ must then be re-examined and new ‘truths’ formed :-)
March 2021 - I now realize that I did not take into account the loss of signal introduced by the resistor power divider used to apply common mode signal to the input of the choke, and in the combiner used to measure the output of the choke. I likely will be approaching the subject of testing CMC’s again in another video... :-)

[@jarmstrong2843]

Good video. What you did is not really different from what I have seen on other videos. So, I think you are safe providing good information.

[@claytonmoore9530]

Just what I was looking for! Thanks from a newly licensed (2 days ago) Advanced. 73, VE1MJF

[@nelsnelsen6741]

Thank you, nice presentation.

[@VE3WZW]

Brilliant video, shared on my twitter and subscribed with thumbs. 40 years in this hobby and the joy of learning. Thank sir.

[@ve6wo]

Thank you, and you’re welcome. I’m glad my videos are useful :-)

[@salvatoreavvento2703]

Great! Could you please explain how to set the nanovna to measure CMRR? Thanks

[@Aleziss]

Again I am really amazed how you can calibrate and measure with open wire leads... my nanovna-h readings goes everywhere... even getting my hands close to the screen or wires make the reading change...

[@ve6wo]

I’m not doing anything special that I know of.. just plug the BNC coax with alligator leads into the VNA and use them :-)

[@Aleziss]

@@ve6wo you can't go really high on frequency though with those type of connections... I was trying to check choke attenuation from 3 to 54MHz and those leads worked but it was extremely sensitive to the proximity of my hands or the placement they were positiononed on the bench...

[@ve6wo]

That is true 100%

[@g0fvt]

FWIW although your methodology is different to the way I measured mine the results appear to be broadly similar, close to 35dB at best. I have read your March 2021 update above, your plot is unlikely to be much affected, after all you calibrated with the resistors in place. (ie your 50kHz measurement showed next to no loss).

[@kgrigio]

Great video and I am attempting to follow it, but I can not figure out the calibration portion. How do you go about setting up the calibration and get to the point of taking the measurement?

[@ve6wo]

Please try this document and see if you’re able to make your way from there :-) www.nemarc.org/Absolute_Beginner_Guide_NanoVNA.pdf

[@tomdchef]

Hi Greg, Is it possible to show an in dept video on the setup of the jig. Great video. I have made a few cmc's and would love to test them.

[@ve6wo]

It’s actually very simple. The grounds of the alligator leads connect together. Port 1’s red lead is connected to a power divider that feeds common mode signal to the input of the CMC. The output of the choke connects to a combiner and the attenuation is read by port 2’s red alligator clip.

[@ve6wo]

Also take a look at the schematic shown in the video and use that to hook your test setup together. I’m actually working out in camp right now, so it’s going to be a bit before I could attempt a video.. so this is the best I can do for now. Sorry :-)

[@RobertResearchRadios]

Excellent Video! The commercial RF people that have never played at HF will of course usually find some objection to a DIY low budget setup, but it doesn't invalidate your results most of the time. Have you considered using this approach to compare the CMRR of this 1:1 current balun vs. a typical air core coaxial choke "Ugly Balun" wound around a large diameter (~3" ID) piece of PVC? It would be interesting to see if the ugly balun performs similarly to, or even better than this ferrite core balun. I'd also be interested in the insertion loss of the 2 baluns and how they compare. I might even have to give this a try myself. I've been playing at L band lately, but it could be fun to take another crack at some HF stuff.

[@ve6wo]

I do not use ugly baluns, but it would be interesting to compare the effectiveness of the ugly balun as an unbalanced to balanced transformer as well as testing CMRR. I am headed back to work now after the COVID shutdown, so I’m not going to have much time to experiment anymore, but if you happen to do the experiment, please make a video and post the link here :-)

[@HammerToneAmps]

Ugly baluns are a curse upon the ham radio community. I don't know why we see them so often recommended, when they work so poorly compared to a good ferrite cored choke.

[@markedwards3284]

Gregg, Thanks for the nice video. It is exactly what I have been looking for but I have some questions. Could you explain a little more how you determined this circuit is measuring CMRR and not differential current. Also how did you determine the resistor value? Four 25 ohm resistors in series-parallel produce 25 ohms. Shouldn’t they be 50 ohms each to produce 50 ohms in series parallel? Straighten me out. Thanks again, Mark. W0QL

[@javiercristiano6690]

Instablaster.

[@ve6wo]

Hi Mark. I have been considering your questions for some time now. I would first like to point out that I am not an RF engineer, so I do not have the training to be able to analyze this circuit as I was taught to do in technical school for DC circuits, thus, I am not able to assertively state an opinion on this. I am using the published work of DG0SA (now SK) to set up and perform these measurements. That being said, I am also curious to know the answer, so I recently set up an experiment on my test bench to try to determine how the circuit behaves.. I asked the question, does the balun see the resistors as being in series or in parallel? I took and fed an unbalanced 1 Mhz signal into my DG0SA 1:1 current balun using my nanoVNA and monitored the SWR (S11). The output of the transformer was connected to 2 x 12 ohm resistors. SWR was noted. I changed the resistors to 2 x 27 ohm resistors, SWR was noted. I changed the resistors to 2 x 51 ohm resistors, and SWR was noted... Results: 2 x 12 ohm VSWR = 2.1:1. 2 x 27 ohms resistors VSWR = 1.067:1. 2 x 51 ohm VSWR = 2.004:1. My conclusion based on this observed behaviour is that the balun sees the resistors as being in series, and the input and output sides of the balun can be seen as isolated circuits for the purposes of this particular thought experiment. Now that I've thrown my neck out there to be chopped... hahaha! If there is anyone who reads this and actually knows the answer to this question, please feel free to explain to us :-)

[@zeljkoadzic1692]

"'Hi What purpose shall your 4 x 25 Ohm Resistors have? 73 Jens DF5HC""

[@ve6wo]

They have two purposes. 1. On the input side of the balun the resistors are used to supply common mode signal to the balun. 2. On the output side they are used to maintain the 50 ohm characteristic impedance of the system and to provide a point to measure how much attenuation the common mode signal is undergoing due to the CMRR of the balun.

[@zeljkoadzic1692]

@@ve6wocopy paste## Adzic Zeljko OK thanks. You just need to consider them in the Common mode rejection. If you measure without them (shorted input and output) you get the direct common mode rejection. 73 Jens DF5HC

[@joeshmoe7899]

It simulates 50 ohm cable with common mode current.

[@rickeaston3228]

You wasted your time and parts wiring it like you did. You could have just driven one winding and measured S12 of it and accomplished the same thing without all that wiring.

[@ve6wo]

I guess the idea here is to simulate as closely as possible a balun as it would function in its actual application. I will try your idea sometime and see how it works.