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

Electronics for the Inquisitive Experimenter

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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!

Опубликовано:

20 сен 2024

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Комментарии : 100

@perrseb5772 14 дней назад

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

@eie_for_you 14 дней назад

You are very welcome, my friend! 🙂

@aquatekt1402 2 года назад

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

@eie_for_you 2 года назад

You are very welcome! :-)

@agstechnicalsupport Год назад

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

@eie_for_you Год назад

You are welcome!

@espenskog8745 11 месяцев назад

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!

@eie_for_you 11 месяцев назад

WOW! I'm so glad that you are blessed by the videos! That's my goal every time. I will do everything I can to keep them coming! :-)

@peterv3122 4 месяца назад

What a great lesson. Thank you!

@eie_for_you 4 месяца назад

Thank you! 🙂

@glaros0 Год назад

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.

@eie_for_you Год назад

In retrospect, yes, you are absolutely right! This resonance is what is leveraged when creating an antenna trap out of coax. It is certainly all about increasing the inductance of the outside of the shield so the inductive reactance is higher. I realized all of this some time later, but simply uploading a newer version of a video over an existing one is not allowed by the RU-vid folks.

@z3r0c00l2 Год назад

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

@eie_for_you Год назад

WOW! I'm so glad that this helped! 🙂

@toddanonymous5295 2 года назад

Excellent Ralph. I like your custom test jig.

@eie_for_you 2 года назад

I want to improve it some by getting the two connectors a bit farther apart and housing it in a metal box somehow. This should improve the "isolation" calibration step.

@tglenn3121 5 месяцев назад

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

@eie_for_you 5 месяцев назад

Thank you and you are very welcome! I'm so glad that you found it helpful. 🙂

@WECB640 Год назад

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

@kn6eze Год назад

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.

@eie_for_you Год назад

Thanks, man!

@Sempiter Год назад

Very nice video 👍

@eie_for_you Год назад

Thanks, man!

@mudithamadawa7510 Год назад

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

@eie_for_you Год назад

You are very welcome, my friend! 🙂

@DucatiMTS1200 2 года назад

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 .

@eie_for_you 2 года назад

Glad it was helpful! :-)

@buzhug35 5 месяцев назад

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

@eie_for_you 5 месяцев назад

You are very welcome! 🙂

@CamilleCullen-ow6qj 4 месяца назад

Great video, many thanks!! Robert K5TPC

@eie_for_you 4 месяца назад

Thanks! ... and you are very welcome! 🙂

@K6TJO 2 года назад

Superb lesson! Thanks

@eie_for_you 2 года назад

Glad you liked it! :-)

@XBKLYN 7 месяцев назад

Great vid...thank you !

@eie_for_you 7 месяцев назад

Thanks! 🙂

@richardchandler9027 Год назад

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.

@eie_for_you Год назад

I'm glad you found the video helpful! 🙂As I experimented with it on the bench, the law of diminishing returns was clearly at work. It never became detrimental to what I was trying to accomplish. It just didn't make enough of a difference to bother.

@TheHeraldOfChange 2 года назад

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.

@eie_for_you 2 года назад

I wish I could help you, but your description leaves me with 20 hundred questions. I am glad you found the video helpful and interesting. I have one coming out in a couple weeks on the 555 Timer I.C.

@BusDriverRFI 2 года назад

Interesting tests and all but you say that the inductance which is added only affects the outside or the shield. Then you proceed to measure the outer shield inductance and indeed the reactance is increased. However you don't measure the inductance change of the inner conductor to show that it is unaffected. Tests I have seen show the inner conductor actually increases it's inductance as well. But you don't show one way or the other. You just use conjecture to assure that this is true.

@subramanianr7206 Год назад

@@BusDriverRFI hi, what you say is true but not exactly though. You assume the inductance of the centre conductor also increases; yes you are right. But you have forgotten one thing. The inner surface of the coax cable shield is also there. The desired "Differential Mode Current" flows on the inner surface of the outer conductor and on the outer surface of the inner conductor. The Common Mode Current travels on the outer surface of the coax shield. When you coil up the cable it results in the increase in inductance in all the "three" conductors, namely 1.the outer surface, 2. the inner surface (of the shield) and 3. the outer surface of the inner conductor. Increase in inductance in the conductors, namely 2&3 are getting cancelled because the magnetic flux fields created inside the cable are of same magnitude but of opposite polarity. Only the net inductance and hence the reactance that's left is on the outer surface of the coax shield braid. And this is the reactance we prefer to be there on the coax shield to oppose the CMC (Common Mode Current) I think this answers your question. De VU2RZA

@BusDriverRFI Год назад

@@subramanianr7206 I don't have a question. I'm pointing out errors.

@EricFullwood 2 месяца назад

Nice

@eie_for_you 2 месяца назад

Thanks! 🙂

@steventhomas729 2 года назад

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

@eie_for_you 2 года назад

Because the snap on ferrites affect only the RF currents running on the OUTSIDE of the shield and the business portion (innards) is relatively unaffected by them, I do not see that there is a limit. With that said, I think you will discover that there is the "law of diminishing returns" operating here. Each additional ferrite you attach provides a degree less benefit than the previous one. Again, thank you for the encouragement! This is why I do what I do. :-)

@ornithopterindia 10 месяцев назад

👍

@29Megs Год назад

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.

@eie_for_you Год назад

Actually ... hmmm ... I thought I had, at least to myself anyway. The loss through the coax (usual signal path) was pretty much nonexistent ... no more than you would get with a similar length of the same coax that I used. Furthermore, the impedance match was also virtually unchanged as well. On the other hand, connecting things up to the outside of the braid, the loss was significant. While *we* (includes me) have a hard time wrapping our head around how current can flow differently on opposite sides of the same conductor, that's just the magic of RF. The higher the frequency, the less of the actual metal is used to conduct the current! (the skin gets thinner and thinner). Mind numbingly weird, eh?

@mrtechie6810 Год назад

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?

@eie_for_you Год назад

As I said in the first video in the series on this subject (ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-yGYY7Nr5ba4.html), yes, referring to the current running down the outside of coax as "common mode current" is, indeed, a technical misnomer. It is a term that was applied to the phenomenon of RF current flowing down the outside of the coax however incorrectly. So, you are 100% correct on both counts. For an explanation of all of this, take in the first video in the series.

@engineeringsolutionseee3362 Год назад

❤❤❤❤❤

@eie_for_you Год назад

Thanks, man! 🙂

@stanholmes4293 Год назад

Another question. How do you know that the choke is getting rid of all your common mode current? Most users of the T2LT antenna use coiled coax choke. You showed this provides a narrow bandwidth but high attenuation. If you use the coax around the toriod providing a wide bandwidth put lesser attenuation. How can you test that common mode current is close to zero.

@eie_for_you Год назад

It can be measured using the *right* clip-on RF ammeter. It has to be measured at various places in the coax, as it appears as a standing wave. The "signal leakage" (yes, real life coax isn't perfect) from a coax can be mistaken for common mode current. Here are a couple articles that I have found: owenduffy.net/blog/?p=458 www.qsl.net/on7dy/Documentation/Common%20mode%20chokes/A%20simple%20common%20mode%20currents%20meter.pdf I'm sure there are others.

@SherwoodR631 2 года назад

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

@eie_for_you 2 года назад

I am glad that you found the video helpful! That's my goal. :-) As I understand it, "flyback" is an issue when dealing with DC through some sort of an inductor. This is mainly a concern with larger inductors. In this application, we are talking much smaller inductances and, with RF being an AC signal, I cannot see it as an issue here.

@cesargalvan3750 2 года назад

Excellent trilogy of videos, now That I understand better the concepts, it is posible to design a balun and an rf choke in one box?, so we place only 1 part and less parts, for instance balun 1:1 for 1-30 MHz. ? Saludos XE2GRC

@eie_for_you 2 года назад

Thank you! Yes, as I understand it, a carefully and thoughtfully designed and constructed BALUN can also act as a common mode choke. I also know that not all BALUNs do a good job at it. I've done quite a few measurements on off the shelf BALUNs for both impedance matching and common mode rejection. I have not been as impressed as I expected to be. Maybe my exectations are too high??? I have long intended to dive heavily into the world of BALUNs and creating my own, but have not done so yet. I would LOVE to know what you come up with.

@daveborchard2019 Год назад

I don’t understand why a common mode choke is needed. In a ham radio / antenna what is the benefit? Does it reduce interference? Does it reduce RF from feeding back towards the radio? Or is it something else? Why is it a good idea to use a common mode choke?

@eie_for_you Год назад

This common mode current comes back into the shack and causes things to be RF "hot". In severe cases you might even get R.F. burns off of stuff in the shack (I've had that happen ... back in my high school days when I didn't know what was happening). With RF in the shack, it can cause issues with other equipment.

@ncooper8438 10 месяцев назад

Do these chokes reduce the effects of nearby noise threats on the wire and at the same time reduce emitted radiation from the wire/screen ?

@eie_for_you 10 месяцев назад

Good question! Yes on both counts. If we are concerned with the effects of nearby noise sources, then the choke needs to be as close to the end of the coax as possible (nearest the point where the signal is used ... e.g. the receiver/shack end of the cable). 🙂

@dikludas Год назад

🙏🏼🙏🏼🙏🏼

@eie_for_you Год назад

🙂

@edwardstavin6672 8 месяцев назад

In your video i think you mentioned you were doing this with rg8 coax. Can you think how this could be done with LMR400 as that happens to be transmission line in one case i have? It won't be possible to get that through the toroid and i think the min bend radius is 4 inches?

@eie_for_you 8 месяцев назад

Actually, I think you miss heard me. I was doing it with RG58 coax. I agree, RG8 or LMR400 with the minimum bend radius of 4 inches ... you just will not find a toroid of sufficient size to do this. For this, you would either have to use a BUNCH of properly selected, snap-on ferrite cores or the old "roll of coax" common mode current choke methods. 🙂

@stanholmes4293 Год назад

When testing the choke with the VNA, do you only connect the braid of the coax?

@eie_for_you Год назад

Yes, that is the idea. What's more, the idea is the OUTSIDE of the braid because that is what you are looking to affect with the choke.

@wadepatton2433 6 месяцев назад

Is that not 10 turns where you show "8-turns" crossed? I learned to count every pass though the middle and I see nine wraps with one crossover. That's ten. Not that accuracy in turn count appears to matter much in this application, but in other applications the number of turns can be critical to best performance--of course as well as the mix. Plenty of testing has been done on the mixes I use for the transformers we make.

@eie_for_you 6 месяцев назад

Yes, I did use the term "turns" very loosely. I suppose to be more technically accurate I should have said, "times around" as opposed to "turns." You are also right, the accuracy of "turn count" doesn't matter that much. The law of diminishing returns works against us as we add more and more turns. 🙂

@wadepatton2433 6 месяцев назад

@@eie_for_you I hate for interested hams to get confused with basics when they have little background in electronics and the pic and the caption were yelling to me. Counting turns means a lot on the transformers I've built and one of the common errors is turns count. The image that sticks out in my head from studying these things is one of a wire passing straight through a core without any bend at all-which is captioned "This is one turn." Every time through counts. That's what they taught us in the books, or we learned for the test and then later applied to antenna transformer building. Carry on. 73.

@eie_for_you 6 месяцев назад

@@wadepatton2433Yes, counting turns properly does mean a LOT in creating transformers. I agree 100% with you. You are also right, there technically ARE 10 turns on that common mode choke in the illustration. Everything you said is true about counting turns. In the presentation of this material I was thinking in terms of the more generic terms of "turns," as I said before. Not defending the imprecise terminology, just explaining it. In this context, you are the first person to find issue with this out of the thousands of views it has had. Unfortunately, YT does not allow content creators to upload a revised video to replace an existing video. They can only upload a completely new video and delete the old, losing all of the metrics of the original. What I WILL do is add a note in the description to explain the terminology so as to possibly avoid confusion.🙂

@wadepatton2433 6 месяцев назад

@@eie_for_youSounds good. I still have a bunch to learn when it comes to electronics, but once I learn it-it sticks. I'll look at your other stuff-might learn something.

@eie_for_you 6 месяцев назад

@@wadepatton2433Electronics is such a HUGE field, we *never* stop learning!🙂

@rikardlalic7275 10 месяцев назад

Thank you so much for these lessons. I have one question about your Amiudon ferite core self made choke? It has two halves, 2 x 4 wraps. But as far as I can see, the two wire wraps were wraped in opposite directions, in the sense of the turns. Would not magnetic fields actually cancel each other that way?

@eie_for_you 10 месяцев назад

Very observant! And, you are right, they *are* wrapped in opposite directions. It turns out that this way of wrapping them makes the choke more effective. It isn't that this is fighting itself in terms of its ability to do its job. Instead, it is the effects of the existence of the unwanted current is fighting its own existence like mixing two signals that are 180 degrees out of phase of each other. Does this make sense? 🙂

@rikardlalic7275 10 месяцев назад

@@eie_for_you It does, thank you. 🙂

@eie_for_you 10 месяцев назад

@@rikardlalic7275 Great! 🙂

@mjktrash Месяц назад

Where to you put the toroid choke? (which end of the coax, radio, or antenna, or both like some suggest)

@eie_for_you Месяц назад

The best place is at the source or close to it. This might be a bit of a difficult issue if you are using something like a dipole only because of the weight of the assembly. If you are using a tower, then you could put it at the base of the tower. If the common mode issue is significant, then a second one before the coax enters the house/building. I say this because I'd like to keep it out of the house and away from the radio. 🙂

@stanholmes4293 Год назад

Hi How would you go about designing a choke for a particular frequency 145MHz min -20db

@eie_for_you Год назад

I have not designed a choke to operate up to these frequencies (yet), but if I were to do so, I'd start by looking at what ferrite materials operate in this range. I *think* 31, 43 or 44 are up there (e.g, fair-rite.com/product/toroids-5943011121/). Remembering, too, that RG58-type coax has pretty high loss, you would probably want to make this a unit to insert somewhere in the feedline being mindful that the characteristic impedance of the coax used in the choke is the same as the feedline it is inserted into. I'd also lean toward N-type or BNC connectors because they have **MUCH** better characteristics at these frequencies that the ubiquitous PL-259/SO-239 (UHF) connectors. (COMPARED: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HjbUK7yjpjA.html). Then experiment.

@stanholmes4293 Год назад

@@eie_for_youThanks for the response and I have watched your connector youtube. Very informative.. To get some idea of number of turns what formula would you use or is it based on the length of coax?

@eie_for_you Год назад

@@stanholmes4293 If we are talking about the toroid-based choke, unfortunately, I have no nice predictive formula for you; I've always just experimented to find what gives me what I want. I have noticed that the law of diminishing returns applies. If you are talking the "roll of coax" type, there are some on-line calculators available. I haven't seen the formulas behind these calculators. How you create your roll of coax *DOES* make a difference, I have seen that.

@eie_for_you Год назад

@@stanholmes4293I was just thinking to add one more comment here ... if you are looking at the "roll of coax" type, there are two aspects to this: [1] the number of turns and [2] the diameter. But, more than this, too, there is how the coax is rolled which will affect performance. Because you are looking to increase the inductance of the outside of the shield, closely wound turns with coax laying snugly against coax (neat and tidy) will provide better performance than a randomly rolled coil of coax.

@steventhomas729 2 года назад

Wow! Those three videos were very informative and helpful. By the way, is there a maximum number of "snap-on" mix 31 ferrites that should not be exceeded on 50 ohm coax cable? I am using "Palomar" mix 31 snap-on ferrites in an attempt to solve a problem in my radio shack with my hf transceiver. It appears as if my problem was less pronounced when I added more snap-on ferrites to my coax near the antenna's fed point and near to the connectors on the back of my radio. Now my problem has been solved. But, I was wondering if there was a limit to how many ferrites you can put on a length of coax, or turns through the

@eie_for_you 2 года назад

I am really glad that you found them helpful! It was a lot of fun doing all of the research and experimentation for them, too.

@edisukriansyah5230 Год назад

Demo measure choke using nano Vna ?

@eie_for_you Год назад

Yes, you could use a nano VNA. I probably used my higher end VNA when I did this, but the nano VNA would have done the job, too.

@leewilliam3417 8 месяцев назад

Mmmmm 😊

@eie_for_you 8 месяцев назад

🙂

@tlebryk Год назад

So, just a coil of coax does NOT create a broadband choke, based on actual measurements. Correct type of ferrites are the way to go!

@eie_for_you Год назад

Yes, this is true. And, yes, choosing your ferrites carefully for effectiveness is critical to making it an effective choke. With that said, coiling your coax like what is often done *does* increase the inductance of the outside of the shield (much like winding it around a toroid core, but to a much lesser extent). Increasing the inductance does cause the reactance to increase as we increase frequency. So, to some extent, it does decrease the possibility of this unwanted current. I would say that, unless you are targeting a specific frequency, you are much better off either using ferrite beads or winding the coax around the toroid as I show in the video. So you are right on! 🙂

@Gabaab 4 месяца назад

seems no one can agree on what toroid mix is "best" everything from mix 31, 43, 52, 61 gets tossed around all over the place.

@eie_for_you 4 месяца назад

Sometimes it all comes down to personal preferences. 31 is a MZn ferrite designed specifically for EMI suppression applications from as low as 1 MHz up to 500 MHz. 43 is a ferrite core good for wideband transformers up to 50 MHz They don't really give any application specific information about 52 material. It is fairly new ferrite material. 61 is also a ferrite core for high Q inductors from 0.2 to 15 MHz and broadband transformers up to 200 MHz. Here is a link to a great resource on the subject: www.amidoncorp.com/product_images/Amidon-Tech-Data-Flyer-v19.pdf ---AND--- www.amidoncorp.com/specs/ ---AND--- fair-rite.com/materials/ Hope this helped ... some. 🙂