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Author Topic: Ferrite beads, whats important Rs or Xs?  (Read 2597 times)
ND9B
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Posts: 52




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« on: August 25, 2014, 04:41:20 PM »

I have some miscellaneous split ferrite beads I wish to use as RFI chokes on some long speaker cables. I checked them with my antenna MFJ antenna analyzer to see what they do on the bands I work. What is the important parameter here, Rs or Xs? (I'm thinking that Xs only changes the resonate frequency of a line, Rs is what kills the RF.) So do I want high Rs, high Xs, or both?

Bob D.
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WX7G
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Posts: 6146




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« Reply #1 on: August 26, 2014, 03:16:51 AM »

For use on speaker leads R + X is best. If it were all X a high-Q circuit is formed and RFI may peak in one band. If it were all R, and high, a high-Q circuit is formed. I'd look for highest X with R somewhere around 100 ohms. Having said all that, experimentation in the final installation can lead you to the best combination of ferrite cores and the number of turns.
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N3QE
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« Reply #2 on: August 26, 2014, 09:59:32 AM »

I have some miscellaneous split ferrite beads I wish to use as RFI chokes on some long speaker cables. I checked them with my antenna MFJ antenna analyzer to see what they do on the bands I work. What is the important parameter here, Rs or Xs? (I'm thinking that Xs only changes the resonate frequency of a line, Rs is what kills the RF.) So do I want high Rs, high Xs, or both?

Either, but in common-mode choke applications if I had a choice I would lean towards a ferrite with significant R component. Fair-Rite mix 31 is a very good modern choice.

In any event, multiple turns of the speaker cable pair through a bead/toroid is the way to rack up impedance fast. If you can't get multiple turns through, you may need a long stack of beads.
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