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Reduce RFI with a Common Mode Choke

Discussion in 'Amateur Radio News' started by VK7HH, Sep 29, 2021.

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  1. VK7HH

    VK7HH Ham Member QRZ Page

    Common Mode Current on your coax can really mess up causing RFI and interference in the shack. In addition, you could also be receiving unwanted interference on the coax outer shield.

    In this video I show how I built myself an easy CMC choke using a toroid and RG-58 coax.

    K9EI, KA8UGB, PS8ET and 16 others like this.
  2. KR3DX

    KR3DX Ham Member QRZ Page

    Why the "split winding"? Can someone refute what is presented here?:
    I'm not trying to be critical or demeaning, it's just that if you want to present something as fact, make sure that it is factual.
    W1YW, SM6CJB, K0WJ and 1 other person like this.
  3. KA4ZZZ

    KA4ZZZ Ham Member QRZ Page

    So whats the issue
    N9NY, WK0DX, AD6FR and 3 others like this.
  4. KA4ZZZ

    KA4ZZZ Ham Member QRZ Page

    Whats not factual.
    SV1LJA, W1YW, WK0DX and 3 others like this.
  5. W9NUP

    W9NUP Ham Member QRZ Page

    What about minimum bend radius for the coax?
    AK5B, KG0BK, K0WJ and 5 others like this.
  6. W1MDM

    W1MDM Premium Subscriber QRZ Page

    does it work?
    AK5B, PU2OZT and M1WML like this.
  7. AI5DH

    AI5DH Ham Member

    Why would you use RG-58 and an enclosure for? So amateur-like.
    AK5B, K5MPH, K0WJ and 2 others like this.
  8. K5YVY

    K5YVY XML Subscriber QRZ Page

    This is amateur radio... and who don't like a project box? :)
    KG7WPQ, WK0DX, KC7ZXY and 15 others like this.
  9. 2E0TWD

    2E0TWD Platinum Subscriber Platinum Subscriber QRZ Page

    Appreciate this, thanks.
    KA8UGB, N9NY, KG7QJB and 6 others like this.
  10. K5QBF

    K5QBF XML Subscriber QRZ Page

    Moving the windings apart is intended to reduce capacitance between input and output or so I am told.
    VK3SQ, AD6FR, M1WML and 2 others like this.
  11. M0VUE

    M0VUE XML Subscriber QRZ Page

    Hi Hayden,

    Thanks for the video. Unfortunately I notices some of the QRZ people are being a little negative above. LOL.

    Anyway, couple of small observation in relation to my QTH and antennas. I have 12 turn single core 240-43 CMC's on both of my antennas which are both EFHW.

    For my 80m EFHW the antenna CMC is located in my shack just before the linear, this is because the EFHW on 80m uses a broad band transformer based on a 49:1 2x240-43 which uses the 25' of coax as a counterpoise. The counterpoise is terminates with the CMC. I have no RF in the shack and on receive the noise level is down 4-5S points with the CMC in place.

    On my second antenna is a EFHW for 20M using an T2-200 core and LC match and a .05l counterpoise (Ref AA5TB). On this antenna I can put the CMC at the bottom of the antenna or in the shack with no affect on the antennas performance. Its actually in the shack before the linear again to eliminate noise.

    My CMC do not have split windings, I could do it I guess, but my fat fingers don't help things out.

    The reason why I mention my setup above is, if I was using a dipole, I would put the CMC below the antenna feed point to stop the coax becoming part of the antenna and one in the shack to stop the noise coming in through the out coax to the radio. I'm guessing you have a dipole?

    Just though I'd let you know, keep up the work on the YouTube channel.


    M1WML likes this.
  12. K9UR

    K9UR XML Subscriber QRZ Page

    I have used same or similar method It works great. I don’t bother with the project boxes and just wind the choke into the feed line - no need for extra connections either. It definitely benefits the noise situation. Also I now use mix 31. Not mix 43. The mix 31 seems to be better for choking RF on 80-10. On 160M I am experimenting with mix 75.

    great job. Keep going !
    KC7ZXY, M1WML, KR3DX and 3 others like this.
  13. VK7HH

    VK7HH Ham Member QRZ Page

    Thanks for the reply Chris.
    I have a video on my channel for the negative commenters

    A few have commented that they get great results with Mix 31, so might be a good thing to try myself.

    Yes I had an EFHW (detailed in another video) and I did the same thing, the coax was the counterpoise and I put the CMC in the coax line 0.05 wl from the feedpoint at the lowest frequency, which was about 4.2m or so.

    The antenna I’ll be using this on is the DX Commander vertical, so I’ll pop the CMC close to the feedpoint (when the rain stops outside and I can get the antenna up).

    thanks for the kind comments regarding the channel and videos, I really appreciate it and hope that some find value out of it.
    WA9JOQ, KJ7RDV and M1WML like this.
  14. VK7HH

    VK7HH Ham Member QRZ Page

    Thanks mate, Mix 43 was what I had spare in the box, but I think I might start looking at Mix 31 for v2.0!

    M1WML and K9UR like this.
  15. W4GOV

    W4GOV Premium Subscriber QRZ Page

    I have made my own and purchased some from the major online retailers for CMC, Baluns, etc. I use RG400 and MIX 31 on mine as do most of the manufacturers. They also favor split windings. Regardless of the type of winding, I personally use a CMC on my beam and my EFHW antennas (133' and 260') with great success. I typically have seen 4-5db difference in noise as well as no more RF in the house on sensitive items. It's good to see people posting videos of how they are doing things. This is the new way of both showing what you've learned and getting feedback on how to improve.

    Keep up the good work and have fun!
    AK5B, KK6RJR, AD8FB and 2 others like this.

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