G4NSJ - Common Mode Chokes used with coax and HF antenna aerials

Common mode chokes are very easy to construct and they are extremely useful when using coax fed HF dipole antennas. They help to block RF from traveling back down the outside of the coax. See the article below by Steve Hunt...
gm3sek.files.wordpress.com/20...
www.radio-workshop.co.uk/g4ns...

[@downhillgeezer6560]

Ray - your explanation of RF flowing along the inner skin and also along the outer skin of the coax braid is absolutely brilliant ! Although I was aware RF flowed along the surface ( skin ) of a conductor, it never occurred to me there could be 2 DIFFERENT RF currents on the braid, inner and outer. I thought common mode chokes worked by some sort of black magic, but now I finally understand how they operate. Thank you !!
"I'm not much good at explaining things" - Bollocks ! ;-)

[@g4nsj]

Thanks, I'm pleased that the video was useful. More coming soon! Cheers, Ray.

[@g0fvt]

Ray, the late G3TXQ did some great work. I made many measurements with common mode chokes I made, I did find that with many turns the common mode impedance at higher frequencies was poor, presumably due to capacitance between turns. My new one maintains a high common mode impedance over a wider frequency range. Beware of random cores, many are iron dust, the types to use are the 31 and 43 mixes as you mentioned. I found a reduction of about 2 s-points in noise in an A/B comparison on 40m and 80m. If I can find your email address I will send some pictures and plots. What people forget is that the shack ground is usually noisy and without the common mode choke it can reach the antenna where it crosses from the outside of the cable. 73

[@g4nsj]

I didn’t realise he was a silent key, such a shame. Great info, thanks. My email…. [e-mail]

[@g0fvt]

@@g4nsj Yes, sadly he passed a few years ago. I launched an email at you, nice Listers btw!

[@woodskid4ever]

Love your simple and straightforward delivery, Sir Ray! Happy to find your light-hearted channel. Subscribed.

[@g4nsj]

Thanks, glad you found the video useful. Cheers, Ray.

[@TheREALJosephTurner]

This video came just at the right time for me. I'm finishing up my new 18x28 hobby building, and I'm getting ready to start putting up the dipoles. Might as well build a few of these and have them ready to go! Love your videos.

[@g4nsj]

Excellent! Good luck with the project. Cheers, Ray.

[@marketingmixture]

Ray - thanks again. You have a fantastic way of explaining complex subjects in an understandable “nuts and bolts way”. Dave Shaw G4NOW

[@g4nsj]

Thanks, Dave. Glad you like the video. More coming soon. Cheers, Ray.

[@AdamSWL]

Another project I must undertake.
Thanks Ray! Your videos are always enjoyable to watch.
73's!

[@g4nsj]

Thanks, Adam. Glad you liked it. Cheers, Ray.

[@tdumnxy]

Thanks for picking up this subject. I like your "common mode" approach, a good video. Such a lot of voodoo and people doing without understanding out there. Also, well done for mentioning the issue of crossing the coax from one side to the other of the toroid. I don't do that but plenty of people insist that you should. I hit upon the method of passing coax from outside through the air brick under the eaves. RG8x fits easily and you just need to make a small hole in the inside cover (I made a slot so the sliding cover still works). Thanks again old man. 2E0FXZ

[@g4nsj]

Thanks, Stephen. It's nice to read your comments. Cheers, Ray.

[@user-uv4xe3cq2y]

Well done, Ray! Thanks. I'll be visiting more often.

[@g4nsj]

Thanks, Ted.

[@TwoDollarBill000]

More videos please, we really love them.👍

[@g4nsj]

More on the way!

[@ludo9234]

Thank you very much for this information. Carry on the good work.

[@g4nsj]

Thanks! 👍

[@glennstevenson6242]

Good on you mate!

[@barrywendon5168]

TNX Ray another good video 73 de G6JMX

[@g4nsj]

Thanks Barry.

[@joerowland7350]

I have used pvc pipe stuff whith steel wool n the coax threw it just. Under the antenna for 30 years
I really enjoy your videos

[@g4nsj]

Hi Joe, what a good idea. I’ll give it a try. Cheers, Ray.

[@joerowland7350]

Yes sir give it a go n I use steel conduit bared in ground up it floor
Whith the steel wool feed it in my radio room n old silent key that wus broadcast engineer should me that 30 plus years ago

[@g4nsj]

@@joerowland7350 I'll certainly try that, thanks!

[@garyclinton8805]

Liked this one ray and you did explain it very well cheers.Gary the tortoise in Dublin ha ha best of luck

[@g4nsj]

Thanks Gary, nice to hear from you. The tortoise in Dublin... love it!

[@enginedave]

So if we are only concerned with CM on the outside of the shield, would that mean that a toroid with coax wrapped on it is a better choke than a wire wound toroid? (Wire wound meaning the coax is cut and separate wires from the shield and from the center conductor is wound bifilar around the toroid)

[@g4nsj]

Good question. I say yes, a torrid with coax round it is better.

[@RGC198]

Hi Ray, interesting video. Just wondering, have you ever attempted signal phasing with MW or SW? This allows the possibility of receiving a DX signal over a closer signal on the same frequency. You would need two antennas, aiming one into the direction of the wanted DX signal and the other aimed at the unwanted signal, with a tunable attenuater connected to reduce it. I know, this definitely works with FM, but so far, I have never attempted it with MW or SW. Two indoor loop antennas may even be enough. In theory, it should work, though I don't know of anyone who has actually done this yet. Anyway, take care. Rob.

[@g4nsj]

Hi Rob,
No, I haven’t tried that but it sounds very interesting. I think I’ll give that a try on medium wave. As you suggested, using two loop antennas. It’s a great idea. Thanks for that. Cheers, Ray.

[@RGC198]

@@g4nsj Please let me know if you have any success with it. Maybe even do a video??

[@ianbarton9359]

At the start of the video I assumed this choke would be up at the antenna end of the co-ax to prevent feedline radiation interfering with the antenna radiation. Towards the end of the video you said it would be mounted at the back of the Tx/Rx. So now I am confused!!! Where is the best place to mount it? Is it beneficial to have one right at the antenna and another right at the input of the Tx/Rx???

[@g4nsj]

Hi Ian, if it's located at the radio end, it stops signals being picked up by the coax and feeding them into the radio and associated equipment. The outer of the coax acts as an aerial in its own right. The choke stops these signals at the radio end.

[@ianbarton9359]

@@g4nsj so which end do you think is the best to install it. I am able to put it at the antenna end or the radio end?

[@g4nsj]

@@ianbarton9359 Hi Ian, I think the radio end of the coax is best. That's where mine is. Or, put one each end.

[@ianharling9569]

Hi Ray.
Another fascinating video 👍.
73.G7HFS/PA3IKH

[@g4nsj]

Thanks, Ian. Glad you liked it. 😃

[@Desertwolf426]

I don't by the 3 conductor premise. If that were actually true it would be a 4 conductor: center conductor and its skin effect + the shield and its skin effect.. unless there is something I am missing... change my mind I guess 😉.
P.S. love your work 😁

[@g4nsj]

Good point. Thanks for that., Cheers, Ray.

[@cesarastiz9691]

@@g4nsj Dear Ray, I recommend a video from John Portune, W6NBC, "Ugly" of "Coax" Baluns. John gives a very clear explanation on skin effect and the 2 to 3 wire transition subjects on coax cable. 73 de lu3hip.

[@raysrants]

@@cesarastiz9691 Thanks, that sounds interesting.

[@miker8379]

I laughed till I cried on the square root

[@g4nsj]

Excellent!

[@clems6989]

Common mode is one of the most misunderstoid concepts in the entire hobby..

[@paulm0hpd319]

Swr and grounding also totally misunderstood