Common Mode Currents, What Are They and Should We Worry? Jim's Got Some Splayning To Do Desi, W6LG

What are common mode currents and should we worry about them? What are differential currents? If you are getting confused, stay tuned! Jim will explain with the help of the March 2024 QST and Larry WA0QZY
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[@MM0IMC]

GM3SEK has written a good article on this subject and I used his recommended oval shaped ferrites as a choke.

[@ham-radio]

I have heard of his book and know that he is a superb author on several subjects. Do you think the shape matters much? Thanks, Jim

[@MM0IMC]

I think it's more a case that you can get more turns through it. Fair-Rite 2643167851, 43-mix material is the one I'm talking about. I've emailed a copy of GM3SEK's article...

[@dans115]

Looking good sir! Thanks for pushing information so we all learn.

[@ham-radio]

You bet and I learn too. I do a lot of studying prior to recording. I want to limit the number of mistakes. With technical issues, mistakes are easy to make and not realized it until the video has been published. I have had that happen a few time. Thanks, Jim

[@radiotech181]

Thank you Jim for your expert knowledge. By the way you are looking well !!! 73 & GL.

[@BusDriverRFI]

I watched this again. 1. We have the transmitter current flowing through the inner conductor and the outer conductor. We have common mode currents flowing on the outside of the outer conductor where we know the RF current normally flows. Where then do the differential mode currents flow? 2. I do antenna modeling on EZNEC. Lewallen admits that there's no reference to CMC in the help directory? He has an article written about CMC that discusses and contrasts the importance of CMC. If it's important and bad, why are these currents not utilized on the modeling software? 3. Some people say CMC is extra currents and if that's the case, they are energy generated for free from nothing. Perpetual motion. Others say it's from the power we have used from the transmitter. In that case, why do we choke those currents rather than matching them to the antenna so they aren't attenuated out and lost as heat? If the CMCs are excess energy added from nothing, we wouldn't want to add that to the currents to the antenna because we could get energy runaway and melt down the universe. But if these are added currents in addition to the currents going from the transmitter, someone could win a Nobel prize for energy on that. Make millions on the patent alone. N9XR

[@user-tj8wv7ri7d]

Thanks for the explanation. I had read the article, and now after your ""splaning", even I understand now. You're looking good OM. 73 de AA7MO

[@ham-radio]

My answer to your comment went somewhere. Not sure where it ended up. Desi used to call the house. One time at dinner, I answered the phone. Desi thought I was Bill Heath and launched into a tirade full of obscenties. I said to my father, "it's for you." Lucy and Desi would have an annual picnic for employees and family. They were both nice. Lucy always smoked. 73, Jim Heath

[@jamescady723]

I thought I was crazy for choking at the antenna feed point and at the shack entry point. Being relatively new, I had issues with my 130' sloper, but it works for me! So I guess, I'm not so crazy after all. 🤣 The more I read about antennas, the more fascinating ham radio becomes. Thanks, Jim.

[@sfrahm1]

Good to see ya up & kicking Jim. Yes absolutely, I always use common mode chokes as well, no matter what type of antenna. Usually doing it at both ends of the coax to help keep noise in the shack from getting back up into the antenna. According to the fair-rite 31 material data sheet, 31 material is not a powdered iron core, it is a MZn (Manganese-Zinc) Soft Ferrite Ceramic made up of a mix of iron oxide, manganese and zinc. 73 Kind Sir!

[@ham-radio]

There is so much for me to learn. The short answer is that it works no matter the exact contents. But many will likely benefit from your description. Thanks for the help. Itg is much appreciated. 73, Jim

[@amateurshooter6054]

Thanks Jim

[@ham-radio]

Thanks. How's the weather been up there? The wind here was up to 70mph. Luckily, I did not have damage to anything. 73, Jim

[@mewrongway]

Awesome info Jim! 👍👍👍

[@ham-radio]

Thanks 👍

[@TEN-TIMES-HARDER]

Love the way you expound ecen if its not new! They work big time!!! Palomar sells choke kits, i like to support the little guy...

[@ham-radio]

Yes, the folks at Palomar are very knowledgeable. I have bought a few things from them over the years. I remember the prior owner from any years ago. You can call them, ask a question, and then place a correct order for you station. I am not being paid to say that. It just happens to be the truth. 73, Jim

[@johngarry7098]

Don’t know what you did. But your audio on this video is excellent! Thanks for fixing it.

[@ham-radio]

Thanks. I listened with headphones and I can hear a digital mixing in the background. I think I know the source and I will try to fix it. There are so many things that can go wrong with the video and the audio. I never seem to get it all good at the same time. Thanks for the information about the audio this time. 73, Jim

[@CamilleCullen-ow6qj]

Hi Jim. I have always used 10 ferrite beads at the (coax) feed point of an antenna based on your prior analysis that more than 10 does no good. As a result, I have never had a lick of a problem even from end fed antennas using up to 200 Watts. Many thanks for all your great videos!! Robert K5TPC

[@k9eihamradio]

Jim. Common mode currents can also be absorbed by the earth, like when a coax is ran along the ground. Also, a Mix 31 toroid core is the most cost effective choke compared to beads. 12 wraps of coax through a core will provide somewhere (frequency dependent) around 4000-5000 ohms of choking impedance. Not bad for $10.

[@ham-radio]

I have some buried coax to a vertical on the back fence. And, yes, it is a way to stop CMC. Ditto coiling coax to eliminate CMC. There are probably other ways that I have not even thought about. It's an interesting subject that CMC's travel on the outside of the shield. 73, Jim W6LG

[@ricksshop]

"Ladder line, window line, parallel feeds, don't have the issues a coax does." I would like to hear more on that subject. By the time I got into radio, good quality coax was available and didn't see the point in twin lead. In particular, when should an amateur consider twin lead over coax? Thank you for what you do.

[@g0fvt]

FWIW open wire feeder CAN have extremely low loss even with a mismatched load. A long dipole fed in the centre with open wire is a great multiband antenna.

[@ricksshop]

I have tall trees about 100 ft from my shack, thinking about running 300 ohm twin lead to the top feeding a folded dipole made from the same twin lead. Run a 6:1 or 4:1 balun at the shack wall. Thoughts?@@g0fvt

[@bigeify]

Also with a tuner, you can use the antenna on multi bands. I have a 135 foot doublet fed with ladder line to a 1:1 balun into coax to a tuner. With this set up I can run 80 through 10 meters. I can even load on parts of 160 meters. You won't do that with a coax fed antenna.

[@kingduckford]

I remember first starting with CB, and with cheap 43 mix ferrite chokes off of Amazon (beyond being the cheapest, they are the best mix for 10-11 meters), and found that some beads on each end of the feedline lowered the noise floor by two to three S units. After this, I became quite convinced of the potential of the common mode choke. I have purchased both beads, as well as "pipe bomb" chokes by Palomar Engineering. Proof of the pudding and all that. They work, they clean up noise, and the better shielded your system and shack are, the better.

[@g0fvt]

Hi Jim, I always enjoy your videos and I do dissect your content, not just to be contentious! It is good to look at variants of the scenario, most people only consider the effects of common mode current while transmitting. It is good to consider the effects on receive too. In my case as an example I had significant noise on the chassis of the radio, of course this is able to whizz up the outer of the shield to the antenna feedpoint where it can then then transition to the inside of the shield. A common mode choke at the shack end of the feeder reduced this noise by 4 s-points. Naively I believe that just one choke right at the antenna would leave me with noise being radiated by the feeder very close to the antenna. Equally while on transmit I believe that despite an effective common mode choke right at the antenna that the feeder itself in a high RF field will produce common mode current. Overall I believe that it is good practice to have common mode chokes at both ends of the feeder (I am sure there are optimal placings due to current nodes). FWIW in my case I also have a common mode choke and filter in the electricity supply feed to everything in the shack, adding this produced a similar improvement to the received noise to the common mode choke on the feeder at the shack end. Sadly using both did not reap much in further improvement. Sorry for the poor grammar, typed on the fly! 73

[@ham-radio]

I can't type on a cellphone. I am in bed recoveriing from a problem. I am feeling pretty weak this morning. You make good points and I hope that others will read what you wrote. I am tempted to make a followup video discussing this subject a little more. It is very interesting and often missed. Thanks for your help. 73, Jim

[@g0fvt]

@@ham-radio sorry Jim, I know you have been going through the mill lately. It is pretty heroic that you are making great videos with so much going on. I am grateful for the reply but please look after number 1 first. 73

[@robincross4625]

Everything you say is correct for 50 Ohm feed-point antennas. I retired from Broadcast. For antennas without 50 Ohm feed-points or using parallel wire transmission line things are a little different. There are other ways to keep common more currents off of those.

[@ham-radio]

Thanks for the info! Maybe we can talk about that difference at some time. I had assumed that parallel feedlines did not have common mode current issues since there is not that "thrid" path back to the shack. Thanks for the help Robin. 73, Jim

[@paulm0hpd319]

​@@ham-radioparallel feedlines can suffer from cmc same as coax from an antenna imbalance, the currents become out of phase that don't cancel each other out causing the feedline to radiate

[@JayN4GO]

Funny you mention this.I was looking at a force 12 yagi and most call for a enclosed ferrite cmc. The beads with shrink appeal more with harsh Wx. I should’ve done this with the Mosley instead of the ugly balun. Hoping to go up a section soon and looking at guy anchors. Good to see you Jim

[@ham-radio]

Thanks Jay. Nice to hear from you. Hope things are well with you guys. We've had lots of rain. Wind gusts last week were the strongest in years. Some peak gusts were measured at 70mph. I was lucky Jay and had no antenna damage. CUL de W6LG

[@JayN4GO]

@@ham-radio recovering from surgery here. I have a hard time sitting down for hours on in. I can’t imagine how you feel. Wind always makes me a bit nervous with antennas. I have my tower on the corner side of the lot so no neighbors are in danger. Our local code guy is very lenient about towers but I don’t want to be that one ya know. It’s only up 30ft and clears the peak of the house by 2 feet. I see that rohn calls for 260$ a piece guy anchors for all models of their tower. I’ve seen some strange guy points around here. 4x4 post 3’ deep with an eye bolt for example. I’ve been watching about all of the rain you guys are getting. This is year 2 of constant rain ? I hope your children are spending a lot of time with you. Fight the good fight om.

[@philmaxwell1858]

I now have 8 ferrite beads at the feed point to my antenna. I wouldn't want to behave in a common mode!

[@ham-radio]

Thanks Phil. I became pretty ill and will be talking to one of my doctors this morning. I think you and I can add some more beads at the bulkhead feedthrough connector in the wall. Then we can add some at the back of the transceiver. And Phil, the next time we talk remind me about the flux you had sent. Yesterday was not a fun day. I am currently in bed recovering. Tomorrow will likely involve yet another transfusion; 19. 73, Jim W6LG

[@fubarsnafu4994]

Thanks again. I've pass a few of your videos around cause I enjoy the approach. To many how to videos. Need something to chew on from time to time and common current I'm all ears.

[@oobihdahboobeeboppah]

Thanks Jim. The effort and cost to put chokes at the feed point(s) and the back of the radio gear is really the most cost effective way to resolve a pile of issues and give us peace of mind at the very least. A 1:1 balun (choke) at the feed point of a dipole will also give us a more normalized radiation pattern than if the choke were not there. Insertion loss of a choke is so small that having it in place pays dividends in many ways.

[@ham-radio]

Indeed. Your advice is just what we all need to do, because, it helps with all kinds of issues. Thanks, Jim

[@g0vlf454]

As ever, providing important information in simple terms.

[@chevyguy6510]

Jim you are MY Elmer for sure! Thank you for making these videos for us.-KD9TRA

[@ham-radio]

I am thinking about a second video on the subject because there are some things that I missed and some things that got muddled a bit in my sometime rambling explanation. 73, Jim

[@greasydot]

Thanks Jim for reiterating the common mode chokes. I have been rebuilding and installing then on every antenna. Metal mic grilles and coffee lips gets exciting sometimes with RF. 😂😂😂

[@ham-radio]

I can turn off the dishwasher! My XYL not too pleased with that happens. 73, Jim

[@greasydot]

​@@ham-radio 🤣🤣🤣🤣 I bet not. My XYL wanted a new dish washer so I brought home a 20 year old green eyed red head and almost ended in a divorce.

[@socialite1283]

"Balanced" is where there are two signal wires (a + hot wire and a - cold wire that contains a signal that is the inverse of the hot wire) and one earth/shield. The negative signal is flipped and added to the positive signal, and thereby cancelling the "common mode" (meaning the same in both wires) noise that was picked up along the line. That is a balanced line. This is in common use in the audio and television industries. Balanced connections would be useful for signals prior to hitting the power output transistors.

[@ham-radio]

You are describing audio. Audio that might be in a mic line with XLR connectors. That is not what I am discussing a coaxial cable that can have a few watts to 1500 watts that is intended to be radiated at frequencies of millions of cycles per second with an impedance of 50 ohms. Did I understand you correctly? Regards, Jim

[@socialite1283]

@@ham-radio Yes - I am describing balanced line signals for the purpose of removing common mode interference

[@MrMeasureTwice]

What I learned in the first 3-4 minutes was if I don't use coax, and use ladder line I don't need to watch the rest of this video. 😁

[@n0vty873]

I don't have all of your experience., but I agree that the feed point should always be choked. I prefer to choke the coax at both ends and to keep my coax as neutral as possible.

[@ham-radio]

Agreed, do it at both ends. Do it wherever you suspect it will help. I will be adding more at my station as part of an upgrade. Thanks for the help. 73, Jim

[@ths3900]

Did we know about this in the 60s and 70? When did we start understanding how to fix it?

[@ham-radio]

I sure did not. Even SWR was not known to me in the first year. I did not have an SWR meter until I acquired a Heathkit SWR meter. 73, Jim Heath

[@marks2254]

Thanks Jim, that was very helpful. I put some ferrite beads on my feedline (inside of my entry panel). Glad to be made aware that was the wrong place. My meter told me that my SWR was about 1.1 at 7.200MHz before I put the beads on and it went to over 2 at the same frequency after I put them on. And I was unable to improve it from that higher number by adjusting antenna wire lengths. When I removed the beads it returned to about 1.1 and my TV resumed turning off and on when I keyed the mic. Why would the SWR change that much and not respond to changing the antenna wire length? I'm running an off center fed dipole trimmed for 7.200 MHz and the peak is at 23 feet.

[@ham-radio]

The length of the coax may make the SWR look good; at that length. Having a choke of some kind at the feedpoint and at the beginning of the feedline is good. I have beads at the transceiver, bulkhead coax connector in the wall and at the antenna. Keep in mind that the length of the coax can make changes in the perceived SWR. Perceived may be a bad adjective. Thanks, Jim W6LG

[@marks2254]

Thanks Jim

[@user-tl5tt5yw3c]

With a antenna in the back yard ,not far from feedline no matter what one does won't they get rf into the shack ?thanks for video

[@ham-radio]

In my station, no. My beam is above me and I am running 1500 watts output with an ERP of 5kw or so. I do everything I can to keep RF out of "stuff". 73, Jim

[@RajaramSrinivasan59]

Dear Elmer Jim! Thanks for this really good explanation. I wanted to know what happens if the snap-on ferrite bead is a bit loose on eg, my RG213. What would happen to it's ability to stop common mode current? 73

[@kg5pte]

Feed point at the radio or antenna ?

[@randystarace4166]

Hi Jim, tell me what happens to a dipole that you have a common mode choke on and you want to use a frequency other then the resonant one. Will the CMC eliminate the CMC currents due to the none resonance of that frequency you tuned to or will the coax radiate.??? Randy KK7TV

[@ham-radio]

The ferrite beads are resistors, lump impedances that are good from 3 to 30Mhz. At some frequencies, that lump impedance is better than other frequencies. But in general, they are fine thorough the HF bands. Randy, I used the 31mix. The answer to your question is yes. 73, Jim

[@tripplefives1402]

Treat the choke like a low pass filter of the inductor-resister variety. Lower frequencies require bigger values for inductance. Typically a choke will just have the largest inductance that is feasible so it should block all RF. If you tuned your antenna without the choke on the feed shielding it will need to be retuned a bit because the coax was acting as part of the antenna and radiating and changing the capacitance value of the base of the antenna.

[@tripplefives1402]

@@ham-radio Well, they behave like inductors. Think of the beads like the ferrite core of a wound inductor and the shield passing through them as a single turn of wire. It forms a low pass filter where you have the inductance in the bead and the resistance in the shielding setting up an inductor-resister style low pass filter. At high frequencies even a little bit of inductance goes a long way.

[@ham-radio]

Thanks. That is a better answer than mine. 73, Jim

[@jonmcadams5401]

Thank you Jim! As a footnote some (non-resonant in the Amateur bands) HF antennas with 9:1 ununs use the outer braid of the coax feed line as a counterpoise that is intended to radiate. For those types of non resonant antennas, a common mode choke can be installed along the length of the coax, rather than at the feed point to decrease noise and greatly reduce RF in the shack. Palomar has details about doing this for various lengths of coax and recommended non resonant HF antenna lengths. I hope this is useful for those who might want to use a non-resonant end fed antenna to transmit on multiple HF bands including WARC bands.

[@markruby2434]

Jim, Why do antenna manufacturers say you don't need a choke or balun at the feed point of a double bazooka antenna?

[@ham-radio]

I have no idea. Which manufacturer? 73, Jim W6LG

[@markruby2434]

It was the company Bob Heil got his from. They made the best double bazookas. Since the antenna itself is coax I thought maybe it is balanced overall because antenna and transmission line are balanced with each other

[@markruby2434]

another question, why do more hams not simply use ladder line if it is a balanced set up?

[@markruby2434]

I found it IAC antennas no longer making antennas @@ham-radio

[@markruby2434]

The Double Bazooka Dipole is a very efficient single band antenna which is very quiet, and does not require the use of a balun. @@ham-radio

[@faymetcall7286]

Thank you Jim for an excellent topic & briefing. My only questions are: despite the potential of adverse SWR creating common mode current, is a choke solution necessary for transmitting power of less than 25-watts indicated through a mobile transceiver, connected to an OEM Diamond Antenna, mobile magnetic mount VHF/UHF antenna? How about for 65-watts tx pwr into the same antenna placed upon my automobile's trunk lid?

[@paulm0hpd319]

Swr doesn't create cmc

[@ham-radio]

Hi Paul, perhaps you can tell us why SWR does not contribute to CMC. 73, Jim

[@paulm0hpd319]

@@ham-radio already have done on other comments also easy to research in arrl publications and reflections by Walt Maxwell

[@elanehayduk7255]

🌺 *Promosm*

[@DeeegerD]

Get to the point Jim. Smart and impatient guy 😅

[@ham-radio]

It is a difficult concept and needs even more explanation. Thanks for the advice. 73, Jim W6LG

[@BusDriverRFI]

We can't grasp the story because he's wrong about just about everything.

[@ham-radio]

Who's wrong? 73, Jim

[@BusDriverRFI]

@@ham-radio I don't get the QST anymore but I listened to what you said and I really find it hard to believe he ever took a circuit analysis class from the garbage he spews. If he took one, especially back then, he probably forgot everything. I assure you that the concept of "common mode currents returning back on the outside of the coax" wasn't made-up yet. If he ever took a class on Common Mode Rejection Ratio" or CMRR, he would realize that. It's sad so many hams put out garbage without checking their own work. Like Buffalo Springfield said, "Nobody's right if everybody's wrong".

[@paulm0hpd319]

Common mode current has nothing to do with resonance or swr ,a resonant antenna with 1.1 swr can suffer from Common mode current, an antenna imbalance is the cause of cmc ,i am usually a fan of your videos but disappointed with this one

[@BusDriverRFI]

With a 1:1 SWR, how can you get any commode currents?

[@paulm0hpd319]

@@BusDriverRFI antenna imbalance, incorrectly installed antenna, for example a dipole that's not outstretched properly or close to a structure etc jim does mention this ,cmc can mess with swr but isn't the result of swr, reflected power from an impedance mismatch joins the forward power which doesn't travel on the outside braid of coax ,a choke only chokes the outside of the braid it doesn't correct an impedance mismatch

[@BusDriverRFI]

If you have a 1:1 VSWR, all of the currents will be dissipated in the antenna. There's no current remaining. Where does it come from?

[@paulm0hpd319]

@@BusDriverRFI the antenna

[@BusDriverRFI]

@@paulm0hpd319 so you're saying that you are getting currents generated from a passive device. That's amazing.