Build Simple Effective Common Mode Choke (CMC) aka. Line Choke for RFI

[@noelmoore4474]

Just setting up my first HF station using an EFHW. There was evidence of RF in the shack and so I tried this. Quite apart from its effect on transmitting, It has cut the noise on 40m from S5 to S1. It works!

[@rallypoint1]

I have to say this was the best and easiest explanation of how to make a toroid choke with part explanations.👍🏼

[@RadioTech7388]

HEAR HEAR

[@n8nkqrp595]

Steve, out of boredom and more so, to help other's boredom, I've begun sharing what I call 'desperation' antennas, etc. All things you can get into operation in 5 minutes, etc. I've had shocking results with a homebrew multi-tap unun and wires sometimes only 2' high. I"m trying to entertain, and moreover, get people to screw around with antenna projects and experiments. It's SO FUN and cheap! My antennas outperform the commercial ones and cost under $10. lol. I point people to you as much as possible because you are the Master of All Baluns & UNUNs! 73 OM and thank you!

[@RobertFrampton-e3m]

I made one of these and it works awesome with my EFHW wire antenna using 100w

[@briancroner3334]

Good video. I've used a T240-43 for common mode choke wrapped the same way you have here and have had great success with it with my end fed 71' random wire antenna.

[@Snarky_Radio]

crystal clear explanation. The only way you could make this easier... it to build one for me. Thanks Steve.

[@Thambiah-Pillai]

9turns with FT240-31 works very well for me, thank you for youe video

[@greggabry3128]

Thanks Steve have been learning lot's and experimenting to make my antennas and rfi go away!

[@crazyham]

Excellent Video, Much Appreciated

[@Matt-qq8dh]

Thank you Steve for this great video on common mode chokes and how to build one. If I wind 9 turns of RG58 around a 240-43 core,, would that work? I'm running out of room to wind the cable. I'll operate on 40M through 10M.Thanks again.

[@k2cjbradio]

Thanks Steve! Great reminder of simple-yet critical-things to do. 73 de Chris K2CJB

[@tomasz5274]

Thanks for great series of EFHW videos! Could you please elaborate how far the choc should be from EFHW feeding transformer? I didn’t got it right from video. Many thanks!

[@markthompson9870]

Highly informative. I have one of these made up already (12 turns of RG58 on FT240-43), but I believe I'm still getting some common mode current at 30MHz. So I've been thinking of adding another choke made up with a a toroid of FT240-61 (again 12 turns RG58) in series. Maybe that would work to eliminate my problem on 30MHz?

[@n4lq]

Sounds good but maybe too many turns. Keep it 6 or less turns for 30mhz. If your new choke didn't help then I doubt if another one would matter. Maybe it's not common mode noise.

[@randlecarr3257]

Awesome tip

[@tekspeditionplanet9109]

Steve, this is exactly how I make my CMCs. I've been criticized for having such a tight loop with coax. After researching, I found that manufacturers' specs call for a minimum loop diameter of close to 2 inches for multiple loops of RG-58. Even greater for larger coax like RG-8 or RG-213. I think their specs are based on compression/deformation of the dielectric, which may change the distance between the center element and the outer conductor/shield. Since these tight winds over a 240 size toroid are clearly less than specs, what are your thoughts and do you do anything to compensate? Great video.

[@n4lq]

Normally I use RG-400 which can handle the tight wrap. Also since only the pass through the center matters, you can allow the outside loop to stick out as far as you want. It looks funny but works just as well. Just be sure the pass through the center is against the inside of the core. Outside does nothing anyway.

[@michael-n5zr]

RG400 has a 1in radius spec due to it being Teflon.

[@jacobboth1914]

Hi Steve. #31 IS CONDUCTIVE ! I have several different toroids from Fair-Rite with #31 material and depending on the model they show a resistance between 100 kOhm and about 1,5 MOhm. Jacob PE1LLA

[@mrtechie6810]

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?

[@n4lq]

You have confused electrical chokes used for power lines with chokes used for removing RF from the shield of coax. We certainly do NOT want to cancel differential signals! Differential currents are what flows INSIDE a coax or between two parallel wires acting as a feed line. Common mode currents are equal in phase between these conductors are considered a problem that needs removing. What you described is a choke used in electrical work and this is a totally different concept. Basically a common mode choke in the RF world is just a broad band inductor that presents a high impedance to RF. Since we don't want RF on the coax's shield, we simply wrap the coax around the core to form an inductor that has a high impedance for any currents present. The number of turns and core material determine the total reactance to this unwanted shield current. For AC electrical work we want all RF removed from BOTH wires and just the 60 hz AC power passing on through. For antenna systems this would be just the opposite of what we want. The name "common mode" has a different meaning for electrical engineers compared to RF engineers, a common problem.

[@mrtechie6810]

@@n4lq thank you. How confusing! So what distinguishes it from a (not "common mode") choke?

[@n4lq]

@@mrtechie6810 It's a choke..... Choking common mode current is how it is used, thus the name. Chokes are just inductors. Inductors oppose changes in current. Inductors have "inductive reactance" which is the measure of the resistance it has at a given frequency. For the electrician type, a low reactance choke would suffice to block radio frequencies from entering the device but have little effect at 60 hz AC frequencies. As the frequency increases the reactance also increases. The choke in the video would explode if 60 hz AC were passed through it due to excessive reactance.

[@rogerp5816]

Why is it that some of these chokes are made like you show and others wind half the turns, go down through the hole and wind the remaining turn in the opposite direction?

[@n4lq]

They do thst to keep the ends further apart. Just leave a gap and you'll be fine.

[@NeWt0z]

According to some published material the only advantage of that is to have the input and output wires on opposite sides of the choke. This might be easier for some. It's only for convenience.

[@dreupen]

This was my question too. Since it is just as easy to wind this way, it makes better sense to me. I made mine out of 240-43 because I have 43's on hand. I plan to add a 31 core also. I love making my own stuff. Thanks Steve!!

[@mrtechie6810]

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?

[@hobbiehobbs]

Very much appreciate you making these great videos. 💪🇺🇸💪

[@arniep740]

Thanks for this video. You can also stack cores for more power-handling capability. 73 de Arnie W8DU

[@RobertCartier]

You mention to install the choke .05 wavelengths from the matching transformer. Can you elaborate

[@n4lq]

That comment applies only to the EFHW antenna.. .05 wavelength of coax is needed between the transformer and the choke to provide sufficient counterpoise for the antenna.

[@chucksw1]

How long should I get the cable, for 12 turns on 2 in Series????

[@marklowe7431]

9 turns on 2x 240-43 is good for 2kw over the whole 1-30. Nothing like headroom and no saturation.

[@cthoadmin7458]

Hi Steve, this is just what I need! I built one of your EFHW matching units with 3 stacked ft240 cores, and it works great, giving very good matches, but at higher power I find my microphone “bites” on transmit. Guess that’s the rf coming back. I guess I could put one of these chokes between the coax going to the matching box and the rig. Thanks for taking the time to makes these videos Steve.

[@richarde735]

check your grounding and be certain that your ground is not only connected to a ground rod but that’s also commonly bonded or attached to the electrical ground. you want to avoid multiple paths to ground. hope that helps! www.repeater-builder.com/tech-info/bonding/amateur-radio-bonding.html

[@samgrieg]

Could 43 mix used in lieu of 31?

[@joelmartin2826]

Only if you plan not working below 20 meters. So 30, 40, 60, 80, 160 won’t see much benefit from a common mode choke using mix 43. Google Jim Brown, K9YC for more precise info-KQ1N/6-73!

[@k8aik8ai]

Joel, you may want to check out G3TXQ's (sk) chart on his web page. His measured data might say different.

[@samgrieg]

@@joelmartin2826My interpretation of G3TXQs results is that using 43 mix with 12 turns yields more effective choking on lower HF than 31 mix?

[@connectvk5543]

@@joelmartin2826 you might want to recheck that as it is incorrect. Mix 43 for 20m and above is the one you should be using.

[@rays9033]

If adding a second toroid (spaced apart from the first toroid) to the same jumper cable, should the second toroid be wrapped in the same direction or opposite direction....or does it not matter? I just wanted to make sure the second toroid does not cancel the effects of the first toroid.

[@n4lq]

Doesn't matter.

[@rays9033]

@@n4lq thank you!!!

[@OldSal-d8h]

Thank you de N9NY

[@rays9033]

Would using a 4" toroid (FT400-31) with a higher permeability be more effective than the 2.4" FT240-31 torroid?

[@BrianHall]

How does the toroid choke compare to the snap-on ones? I bought a bunch of Mix 31 snap-on ferrite cores, but I still have tons of noise from my Wellbrook ALA1530 active loop antenna.

[@HammerToneAmps]

Type 31 cores have gotten scarce all of sudden .. you've got a lot of disciples Steve!

[@k8aik8ai]

Check out KF7P Metalwerx for cheap cores

[@javiersalazar5020]

Hi I have powet line noide on 40 and 80m. Will this help me reduce the noise a bit on these 2 bands? I use a half wabe sipole for 80m and being this band the one with more noise 59+10. Thanks Javier TI2JS

[@1fanger888]

Thanks, KC3BXZ Joe 73

[@cameronzotter3620]

Thanks Steve! Question- Where should I put this choke? Ideally, I would like to put it inside, about 1 foot from my Yaesu 450D. Is that advisable?

[@n4lq]

Tough question. First, is it needed? Do you have a problem? If not then no choke is needed. Chokes work only if there are RF currents for them to choke. These RF currents vary in strength along the feedline are are frequency dependent. This makes it tough to pinpoint. In the case of an EFHW, don't put the choke near the transformer unless the transformer is grounded via a very short ground wire to a good ground system. Keeping it back at least .05 wavelengths from the transformer usually works. Your idea of putting at the transmitter is fine but be sure why you want it and try to observe its effect. If it doesn't do the job then try relocating it. Experiment is needed and it is different in each instnace.

[@cameronzotter3620]

@@n4lq Thanks for the info!

[@acestudioscouk-Ace-G0ACE]

This was aposite, I'm shack building after a 30yr break from the hobby!

[@vne2328]

Thanks for expanding my vocabulary to include the word apposite.

[@acestudioscouk-Ace-G0ACE]

@@vne2328 LOL! Even if I did spell in with a typo!

[@dback4430]

Hi steve , how many cores for legal limit , if you can stack them ?

[@n4lq]

The transmitter power shouldn't matter with just one core as long as you don't have some crazy SWR. I do recommend using RG-400 Teflon coax and not this RG8x mini coax especially for high power. Stacking cores is not as effective as is putting them in series.

[@dback4430]

Thank you for that. I was also wondering if you could wind them with enameled wire like a balun. I have loads of that left from your end fed design.

[@n4lq]

@@dback4430 Yes but with a skew in SWR due to the wire pair not being 50 ohms.

[@christophersylvain4085]

@@n4lq stacking cores will distribute the ferrite heating into all the cores used. cores in series are adding inductance.

[@richarde735]

Steve, you mentioned that it was acceptable to use more than one RF choke in series. I just made a choke the other day using two 240-43 with 9 turns of RG142 Teflon coax. According to the chart two 43 mix cores and 9 turns will provide 8k of attenuation on 20, 17 and 15m but drops way off on 80 and 40m. having said that, two 31 mix cores with 17 turns will provide 8k of attenuation on 80 and 40. how far should they be from the 49:1 transformer as well as away from each other? I have one other question... on the EFHW 80-10 or the 40-10 version, how well do they work on 17m and would I get any benefit or negative reaction if I had a 67’ or 130’ EFHW with one additional wire cut for 17m - in other words a Fan EFHW?

[@n4lq]

The rule is....You must put the chokes at least .05 wavelengths away from the transformer on the lowest frequency used regardless of the mix. That means about 13 or 14 feet minimum. You don't need much spacing between cores. Just a fraction of an inch or so. Forget the "fan" idea. Just adding a few inches of wire onto the transformer's output will skew the swr for the whole antenna. A 17m addition would wreck everything. Things are totally different than a dipole due to the high impedance feed.

[@SunilAruldas]

@@n4lq the .05 wavelength rule looks very important, maybe it should be added in the description? Great video

[@richardosborne4269]

What coaxail cable have you got there? What is the alternative which I cane use? thks

[@nicholasskific4795]

Mr. Ellington, I watched your video on making an EFHW and was wondering if the same 12 AWG wire to create the transformer can be used for the actual antenna portion?

[@n4lq]

Yes. I use 12 ga.

[@n4lq]

Rg8x

[@nicholasskific4795]

Steve Ellington thanks, so that is rg8x as the actual antenna

[@n4lq]

@@nicholasskific4795 No. Antenna is just wire

[@rallypoint1]

For mobile I have a choke on the outside near antenna feed. Would it be ok to put another in line BUT inside the vehicle?

[@n4lq]

Sure.

[@rallypoint1]

Steve Ellington awesome...thanks for the quick reply!!👍🏼

[@clau_ham_lu4]

Dear Steve, I am from Argentina, I have seen your very good previous video about Multiband & EFHW Antenna. My tower (12Mts) is supported 5.5 meters from the ground, on the roof of my house. The earthing javelin is obviously in the ground so my Multiband & EFHW Antenna earthing cable, in my opinion, will be very long. Do you have any suggestion in this regard, do you think it will work? The general opinion states that the ground connection is very long, which will not be effective. Please, I wish to have your opinion on this. Thank you so much Claudio Saliwonezyk LU4EN

[@n4lq]

Grounding the EFHW is for lightning protection, not for enhancing SWR or transmission. You may ground the transformer directly to the tower via the ground / earth connection or you may ground the outer shield of the coax at the building entry point to the earth rod. Always ensure that all of your earth systems are bonded / connected via heavy wire.

[@larrymccoig2714]

Nice helpful video! Well done, just thought I would mention that when you switched from the Fair-Rite website to DigiKey the part numbers are different. I believe the part number on the Fair-Rite website is for a type 31 but the part number on the DigiKey is for a type 43 toroid....unless I am confused (which is always possible) -AD0YW

[@NHSbandUnofficial]

I'm just curious how long your jumper is? 3 feet?

[@n4lq]

I believe it's a 10ft jumper but it doesn't matter as long as it fits your need.

[@NHSbandUnofficial]

@@n4lq I understand I just don't want to order a cable too short. Thanks for replying!

[@n4lq]

@@NHSbandUnofficial Here is the best coax. RG-400 is very small, can be wrapped tightly around the core and can handle any amount of power. rover.ebay.com/rover/1/711-53200-19255-0/1?ff3=4&toolid=11800&pub=5575522562&campid=5338568994&mpre=https%3A%2F%2Fwww.ebay.com%2Fitm%2FRG400-UHF-Male-to-UHF-Female-259-Double-Shield-Coax-Cable-PICK-LENGTH-RF-US-Lot-%2F163975768296%3Fvar%3D%26hash%3Ditem0

[@NHSbandUnofficial]

@@n4lq Great, thanks! What would you think the minimum length of RG400 it would take for 12 turns?

[@n4lq]

@@NHSbandUnofficial minimum about 2 feet so get 3 feet to be sure

[@MichaelLloyd]

Digikey is now over $7.00 each and backordered. 73 NE5U

[@rallypoint1]

Yup they are $7 but back in stock. Cheaper in price and shipping than Palomar.

[@MichaelLloyd]

@@rallypoint1 I backordered them. Now I need to build the choke.

[@rallypoint1]

Michael Lloyd Awesome!! I ordered mine too. Mouser was cheaper but out of stock.

[@marcinmichigan2772]

how long of a piece of coax do you recommend Steve? de K8MH

[@DE2TRF]

mega cools video top 73 de DE2TRF

[@jesseobrien5864]

How many turns for 11 meter base station cb sir? Thanks

[@UpsidedownArmadillo]

Doesn't work. I bought a Fairrite FT240-31. I was only able to get about 10 wraps on it with RG-8x and 3' of it. I wasn't able to get them as tight as you. I know it's a Common mode problem because I did the center conductor only test with the rig. S2 with center only, S7 with shield attached. Still an S7 with this choke. I'm pretty disappointed.

[@n4lq]

I don't understand how that "test" can work. Without the shield connected you don't have any antenna. Naturally everything including noise will be weaker. So do you know where this noise is coming from?

[@UpsidedownArmadillo]

@@n4lq I'm following this "theory" from Palomar Engineers. (short read) palomar-engineers.com/wp-content/uploads/Palomar-Engineers-Common-Mode-Noise-on-Coax-User-Diagnosis-Test-1.pdf There's a vid on youtube as well where the guy does it and his noise jumps thru the roof. I'm putting this choke right behind the radio. And of course I have noise from all kinds of things in the house. Wall warts, you name it. But I was hoping to get the most of it with this.

[@n4lq]

@@UpsidedownArmadillo Got it. I"m not sure how conclusive that test is. The most effective method is to locate the noise source and either replace it or install a choke on it. Use a portable radio to locate the noise. Google "K9YC cookbook". He has lots of info.

[@eknaap8800]

@@UpsidedownArmadillo The coax running to your radio (and choke) will still radiate. The best place for a choke is near the shack entrance of your coax; the choke wil attenuate from that point on.

[@philjensen5151]

@@UpsidedownArmadillo You need to identify and choke your noise generators. With ferrite (like this) or snap-on chokes. I bought about $200 worth of snap-ons and FT140 and FT240 cores and equipped every single appliance in my home, and dropped my noise floor below -110dbm. Start with your DC power adapters, doorbell transformers and garage door openers.