Common Mode Current, How do these Chokes work? (013c)

In this video I help you understand how Common-Mode Chokes work, showing some that you can easily build for yourself.
I also show you how to evaluate the performance of a Common-Mode Choke using a very simple test jig.
IMPORTANT NOTE:
Please note that I use the generic term of "turns" to describe the number of times that I have gone around the toroidal core in this video. In a technical sense, every time the coax passes through the core, it is considered a "turn." Thus, the choke that is pictured in the thumbnail has a total of 10 turns.
===================================================
00:05 Introduction Common-Mode Chokes: How do they Work?
00:16 What is the idea behind a Common-Mode Choke?
02:03 CHOKE DESIGN #1: Simply Increasing the Inductance of the outer skin of our Coax
02:27 METHOD #1: Wrap the coax around a ferrite toroid or rod
03:08 THE EXPERIMENT: One Turn at a Time
04:17 OVERALL PERFORMANCE
04:55 METHOD #2: Slide Lots of Ferrite Beads on your Coax
07:08 CHOKE DESIGN #2: Using the Parallel Resonance of your coil of coax
11:24 EVALUATING THE PERFORMANCE OF A COMMON-MODE CHOKE
14:13 Final Comments and Toodle-oots!

[perrseb]

Thank you for your crystal clear explanations! That’s the best ones I found about CMC and chokes

[aquatekt]

The clearest expansion of common mode currents I have listened to. Thanks

[espenskog]

I have seen a few of the videos about common mode current now from a few channels, and it is starting to sink it. It being 20+ years since I studies HF at college, it is so nice to have your videos for kick starting old those old brain cells with old school-memory. Really enjoy how you laid it all out very well. What is extra nice is to add those videos you did from your test rig and the vna-output. Seeing it "live" is very nice. A good source for learning. Keep it up!

[agstechnicalsupport]

Very instructive video on chokes and common mode noise control with practical techniques. Thank you for posting !

[glaros]

Useful video, many thanks. The 'dips' in the S21 response at 10:41, are in effect LC resonances, from an inductor the choke turns into a capacitor (due to parasitics) after the resonance and so on, it peaks, then it becomes an inductor again, but its common mode suppression is then less effective (as seen from the response), etc. For this reason, I think it is a good approach to keep the resonance as far as away from the useful operating band as possible, 73.

[WECB]

Well done sir. Easy to understand. I hope you'll comment in a future video that the reason why we use ferrite is because it is a "mix" of particles and each particle type behaves a bit differently to the RF. This mix is what lowers the Q of the circuit and allows a nice broadband attenuation and not the sharp high Q circuit of the "ugly balun" or the steel bolt. I look forward to future videos. (just subscribed) 73

[toddanonymous]

Excellent Ralph. I like your custom test jig.

[zrcl]

Amazing ... That helped a lot with my mobile hf setup

[kneze]

Excellent presentation. I'm a new'ish ham of 3 years and a Guanella 1:1 current balun has eliminated the painful shocks I used to receive when operating 20W portable with a random wire antenna or even a resonant antenna on a non-resonant band.

[DucatiMTS]

Well your video was very helpful as I can now successfully test a CMC choke. Thanks for your time and effort in producing this video .

[mudithamadawa]

Thank you sir for the great video. Love it too.❤️

[tglenn]

Excellent and informative video. I'm not good at tech stuff, but I find this helpful. Thanks!!

[TheHeraldOfChange]

I enjoyed watching this video and found the subject quite interesting. Although it hasn't really helped me better understand my personal research project: I wanted to put a 5mH Impedance in series with the positive to my 4 Ohm Exciter, to create a first order two-way crossover/ low pass filter. The toroidal coil I bought ended up being a 5mH 1:1 (18 turns) balun/common mode choke, and I have "no idea" on how to use it in my circuit, or if it will do the job I wanted it to do. So, I keep looking for more information. Cheers, thanks for the video.

[richardchandler]

Great presentation. A year ago I would have been lost . Now it make sense. A bit over my head. Just an ordinary general. But still great information. Can I ask at what point do more and less turns become useful, detrimental or just a waste. I have built several 240-31s with 18 turns on RG 400. My present common mode current is about 6 mAs on 40M. Still have more testing to do. Thanks so much.

[buzhug]

Thanks a lot for this video about a complex subject!
Best regards from France
F4JWQ

[steventhomas]

Continuation....Is there a limit to the number of turns you should use.

Lastly, thank you for posting such an informative and easy to understand video about common mode current! 73, de Steve

[CamilleCullen-owqj]

Great video, many thanks!! Robert K5TPC

[Megs]

Perhaps you’ve already demonstrated this but if not, I’d like to see a loss measurement of the coax shield differential RF current through that same choke. Some claim that it’s only the “skin” of the shield that is affected by the choke -I’m skeptical. Also, even if that’s true, there must be differential current through that same shield skin, no?
Thanks.

[SherwoodR]

very interesting - thank you for the great video. Does a ferrite / torrid common mode choke suffer from Flyback when the TX stops?

[mrtechie]

From a quick search, a common mode choke has multiple windings, with "equal turns wound in *opposite directions*" so that the magnetic flux cancels for differential signals.
E.g. each conductor in a pair is wound in the opposite direction of its mate. And the pair are magnetically coupled so their fluxes cancel for common mode signals.

"A common mode choke is an electrical filter that blocks high frequency noise *common to two or more data or power lines* while allowing the desired DC or low-frequency signal to pass. It gets its name because it blocks or “chokes” high-frequency signals while low-frequency signals pass through an electrical circuit by passing direct currents (DC) and alternating currents (AC)."
I envision this for a balanced wire antenna, where the feed conductors feed are wound in opposing directions. Not sure what happens if coax is wrapped in opposite directions. Might still be just a choke.
Isn't the goal here to prevent RF flowing over the *outside* of the coax?