[TowerTalk] RFI Beads & Sleeves

[Joe Giacobello]

Tom, do you want the impedance to be largely resistive so that it's not 
so frequency dependent and the ferrite is effective over a wider 
frequency range?

Just curious, because I just bought a bag of inexpensive ferrites on 
Ebay, and they didn't start showing a resistive component until about 7 
MHz.  Name brand ferrites specified for amateur applications (e.g., 
Fair-Rite) show a resistive component at 2 MHz and below and have a 
higher reactive component until about 11.5 MHz, where the cheap ones 
become more reactive.  However, their resistive component was still 
substantially below the Fair-Rite.

I posted the data on the TenTec forum, but the tabulation was verkecht.  
If you're interested, I can send it to you privately.

73, Joe

Tom Rauch wrote:

>>I am going to order some used components from Surplus
>>    
>>
>Sales of Nebraska. I
>  
>
>>thought I might order some beads to use to help me with
>>    
>>
>some RFI problems I
>  
>
>>am experiencing. For example, the A/D for in computer
>>    
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>assisted rotator
>  
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>>controller has some RFI problems on the higher HF bands.
>>    
>>
>
>Bypassing and grounding is generally much more effective
>than throwing a bead at a problem.
>
>  
>
>>Can someone recommend which type of beads I should have
>>    
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>handy to assist with
>  
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>>RFI problems as they arise.
>>    
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>
>You generally want the highest possible **resistance** and a
>core that allows multiple turns. Impedance goes up nearly by
>the SQUARE of the turns increase. Two turns through one bead
>is generally about twice the impedance of one turn through
>two of the same beads.
>
>
>Look carefully at the impedance vs frequency, and IGNORE the
>"frequency range" in descriptions. Pick the highest
>impedance in the operating frequency range where you have
>the problem. Once again, bypassing with capacitors is often
>much more effective than slapping a bead on the problem
>(unless the equipment already has good bypasses). A bead is
>useless when the common mode impedance is significantly
>higher than the bead impedance.
>
>  
>
>>Here is the URL to the web page with the information on
>>    
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>the beads and
>  
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>>sleeves that Surplus Sales of Nebraska sells.
>>http://www.surplussales.com/FerBeaSle/FerBeaSle-1.html
>>    
>>
>
>Why not go to a regular suppression core vendor? At least be
>sure you look at the real data for the cores. You generally
>want hundreds or more ohms of impedance, and you want that
>impedance to be largely resistive. That precludes using 43
>and similar materials in suppression applications. You want
>something with a well-defined broad peak at HF for HF
>suppression, not a material who's impedance keeps climbing
>with frequency well up into VHF.
>
>73 Tom
>
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