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Snap on Ferrites

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K6GBW, Jan 17, 2021.

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  1. K6GBW

    K6GBW Ham Member QRZ Page

    This might be a silly question but I haven't been able to find a definitive answer. My coax is picking up a bit of noise from some nearby power lines. I placed 10 type 31 snap on ferrite beads on my coax about five feet before it comes into the shack. The noise floor dropped about 2 S units and now I can hear weaker stations that I couldn't before. My question is will more ferrite beads make any improvement? Can you actually have too many and cause a problem?

  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    A proper CM choke made by winding about 10 to 15 Turns of small coax through a FT240-type 31 ferrite ring-core would work better.

    Snap-ons are great for diagnosing a solution to RFI, but they are not as effective as a well-constructed choke...

    WB2UAQ, AK5B and PU2OZT like this.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    So, directly answering your question...YES, adding more snap on ferrites will most likely reduce the interference.
    AK5B likes this.
  4. K6GBW

    K6GBW Ham Member QRZ Page

    Thanks for that. I used the snap ons because I just happen to have a bunch that I was given. I can pick up a 240-31 though if it will work better. Any benefit to placing chokes nearer to the antenna? My coax runs underneath the soffit all along the back of the house. It's parallel to some power lines about forty feet away so I suspect I'm picking up some noise from them.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    What kind of antenna?
    Answers will be different based on your answer.
  6. K6GBW

    K6GBW Ham Member QRZ Page

    KA0HCP. Thank you, I have about twenty of these things that I was given. I placed ten of these on the coax just outside the shack. They did make a noticeable difference. I was considering placing the others on the coax nearer to the antenna but I didn't want to over do it. I'll probably pick up one of the torrid rings and place the snap ons nearer the antenna and then use the remaining snap ons on power cords and such to see if that has any effect.

    Thanks guys, much appreciated.
  7. K6GBW

    K6GBW Ham Member QRZ Page

    The antenna is a Bushcomm SWC-100. It's a bit of an odd antenna but I use it on non-ham frequencies as well as ham so it makes frequency changes easier. I can use it on 60-10 meters without a tuner and just a touch of tuner on 80. I use the radio for local/regional stuff mostly and spend 90% of my time on 80-60-40 and only occasionally go higher. The coax is about 65 feet of RG8X. The antenna is suspended from a small tower at 30 feet.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Not clear if there is actually a balun inside the box or not. Since the antenna is "symmetric", then it shouldn't require much CM blocking. My guess is that this is a very "inefficient" antenna, sort of like the B&W T2FD.

    My suggestion is that it will benefit most from having the coax shield bonded to an 8ft ground rod before the coax enters the house. Be aware this has Electrical Building Code implications, but then so does bringing an un-grounded coax into the house.

    After grounding the coax shield, next try some snap-ons between the new ground rod and the rig. I would also try moving those snap-ons to the antenna side of the ground rod.

    Not a silly question, at all... Keeping RFI originating in your own house from being conducted up the coax to your HF ham antenna is science-based... contrary to what you hear from old-farts on this forum...

    A wound-coax CM choke as discussed by k9YC is equivalent to dozens of snap-ons... The blocking impedance is proportional to (# of turns through the core) squared. Snap-ons have only a single turn per each... 10 squared = 100. 1 squared = 1!
    Last edited: Jan 17, 2021
  9. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    KU3X likes this.
  10. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

    Can you actually have too many and cause a problem?...... NO THE MORE THE MERRIER,



    73 - K1LKP

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