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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. AE0Q

    AE0Q Ham Member QRZ Page

    You should study the charts by G3TXQ (as noted and shown in the original video), 43 mix can be correct for a CMC, it depends on the frequency range you want:

    http://www.karinya.net/g3txq/chokes/
     
    G8HGN, VK2VLT, WK0DX and 3 others like this.
  2. MW1CFN

    MW1CFN Ham Member QRZ Page

    The reason I found RG58 'bad' isn't because of losses - where you're of course correct that they are inconseuqntial for the short lengths involved. It's because a lot of it sold in the UK and perhaps more widely, has aluminium outer conductor, which turns to dust in no time at all in a rainy environment, and worse if anywhere close to the sea; taping/enclosing only slows the process, not stops it (I speak of experience!)

    1kW? I hardly ever go above 15W, and often use only 5W. The sea gives me 10-20+dB 'free gain', depending on path and time of day.
     
    M1WML, KR3DX and (deleted member) like this.
  3. K9UR

    K9UR Premium Subscriber QRZ Page


    Fair enough. Aluminum jacketed RG58 is not the correct spec for that cable designation.

    Here we use belden or commacope branded cable with copper braid. Chinese made cable is avoided. The Belden is not expensive and is good quality. I would guess 95% of public safety installations in vehicles has said coax for the last 30+ years.
     
    M1WML and MW1CFN like this.
  4. K9UR

    K9UR Premium Subscriber QRZ Page

    You’re using Mix 52 for a choke 1:1 or for a 9:1 /49:1 type transformer ? ? ?

    mix52 works well as a 1:1 current choke in the 100-200 MHz…. But it works poorly at HF to stop common mode currents. It’s an ideal material for those using J-poles on 2M and up to stop Feedline radiation but not ideal at HF. In fact it hardly works compared to mix 31

    52 works fine as a HF transformer…

    in general:

    HF auto-transformer (3.5-30mhz) : mix 43 and 52

    1:1 current Choke. (3.5-30 mhz) . Mix 31

    160 meters (medium wave) is a different story and preference is mix 75 for common mode chokes (1:1 current balun) and type 43 for transformers although other mixes like 65 and 200 can also work.
     
    M1WML likes this.
  5. WB6FQZ

    WB6FQZ Ham Member QRZ Page

    What's wrong with this? If you don't agree then move on!
    WB6FQZ said that.
     
    M1WML, VK7HH and (deleted member) like this.
  6. KZ5R

    KZ5R Subscriber QRZ Page

    Sorry did not say it was a Mix 52 but close to it so I used it for what should be a very close comparison of a wound 52's insertion loss. We use 31's for all chokes except for specific types where we know our own mix will be more temperature tolerant and still provide excellent choking. You do know we are not exactly new at this...right?
     
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  7. K9UR

    K9UR Premium Subscriber QRZ Page

    What mix do you suggest for 160M for best choking characteristics ?
     
    M1WML likes this.
  8. AE0Q

    AE0Q Ham Member QRZ Page

    Lots of generalizations above. Are you ignoring all the data on the G3TXQ site?
    http://www.karinya.net/g3txq/chokes/
     
    G8HGN and M1WML like this.
  9. KZ5R

    KZ5R Subscriber QRZ Page


    For 160m I would use a minimum of 2 stacked 31's with three being the best. However, you need to modify the number of turns for a stack of three.
     
    M1WML likes this.
  10. WP2ASS

    WP2ASS XML Subscriber QRZ Page

    1. Putting the choke in an enclosure is bad practice. The common mode currents will cause heating. Without having the choke in the free air you run the risk of heating the choke to the currie point of the ferrite.

    2. For antennas like end feds you really need to use more cores than a single one or add a couple chokes in series, very close together.

    3. You do not put the choke down the feedline to use the outer portion of the braid as a counterpoise. This is because coax is actually 3 conductors. The center conductor, the inside and the outside of the coax. If you use your coax with the choke spaced away from the feed point (as a counterpoise) then any noise brought onto the outside of the coax will coupe into the inside of the coax at the feed point. Then the common mode choke will not help you with that noise ingress.

    You're much better off putting the choke as near the feed point of the antenna as possible and adding a counterpoise. Even better, adding a second choke at the point your feedline comes into the shack or directly after the last item in line (tuner / swr meter / etc).

    Hope this helps.

    --Shane
    KD6VXI
     
    M1WML likes this.
  11. VK7HH

    VK7HH Ham Member QRZ Page

    Thanks. I can say I now have this on the antenna and I see no evidence of RFI (yet) and my noise levels are surprisingly low, in fact I went around the house to turn stuff on to try and cause RFI!
     
    M1WML, KO2Q and (deleted member) like this.
  12. VK7HH

    VK7HH Ham Member QRZ Page

    Thank you Ed, I did exactly as you described and hope that it helps other hams out there in the future. 73
     
    M1WML and (deleted member) like this.
  13. VK7HH

    VK7HH Ham Member QRZ Page

    I'll do a follow up video in the near future
     
    KC7ZXY and M1WML like this.
  14. VK7HH

    VK7HH Ham Member QRZ Page

    That's referred to here (in Australia) as cellfoil. I never use it either, most LMR cables have it, such as LMR200. The RG-58 I used was proper Benelec stuff with single close woven braid shielding and polyethylene dielectric.
     
    KC7ZXY, M1WML and MW1CFN like this.
  15. KZ5R

    KZ5R Subscriber QRZ Page


    Shane

    Sorry but I feel your number 3 regarding choke placement could not be more inaccurate. I would urge you to do more research on this as what you are suggesting is completely counter productive.
     
    AE0Q, K9UR, M1WML and 1 other person like this.

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