Feedline Ferrites: what noise do they suppress and _not_ suppress?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI6EQ, Feb 1, 2017.

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  1. WB4SPT

    WB4SPT Ham Member QRZ Page

    Very simply, it comes down to keeping the coax shield current the same as the center conductor current. If NOT, then you may see issues on transmit or receive.
    Any study of coax will bring one to the conclusion that it works as a shield, ONLY if the shield current is the same as the center conductor current.
    In Tx, the bad effect is "rf in the shack"; at least RF currents in the wrong place, meaning on the metal chassis of things.
    In Rx, the bad effect is coax "noise" pickup from culprit sources. So, the coax "shielding effectiveness" is compromised by the issue of currents not being the same (and cancelling). This means, then, that running coax that doesn't control CM currents, near things like indoor LED drivers, uP's, SMPS, etc will cause coax pickup and therefore will send this junk into the receiver, along with the intended (normal mode) signal. If you have a short run of coax, not near any bad emitters, then you will not see Rx benefit of the ferrite. Most of us DO, unfortunately have hostile sources of noise, in the home, that could work there way into the coax.
    The antenna, as it picks up signals is essentially unrelated to this analysis. Actually, it is related, by the fact that the Z of one side of the dipole is different than the other side, so therefore can support unbalanced currents. That's what the choke fixes. It makes it more like an open circuit to CM currents, looking towards the antenna. An open circuit cannot support ANY CM current, so they go away. This is why a high choking Z is good.
    KD6RF likes this.
  2. N1OOQ

    N1OOQ Ham Member QRZ Page

    Note that ferrite is not just ferrite. There are several different materials, and not all of them do useful things at hf.

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