[TowerTalk] RFI Beads & Sleeves

[Jim Brown]

On Tue, 01 Feb 2005 10:07:04 -0500, Joe Giacobello wrote:

>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.

it is not a matter of "cheap" or "name brand," but rather the purpose for 
which the particular ferrite bead or sleeve is designed. Go to www.fair-
rite.com, download and study their pdf catalog. It is a WEALTH of 
information about ferrites of all sorts. The ferrites you got are obviously 
a "mix" and shape designed for suppression use at VHF and/or UHF. 

To answer the other question, YES, YES, YES -- ferrites used for 
suppression should be as resistive as possible and have as little reactance 
as possible in the frequency range where you want suppression, and for 
precisely the reason Tom talked about -- you don't want the reactance to 
partially or completely tune out the reactance of the wire you're putting 
it on, or, if it does, you want there to be lots of R in the series 
equivalent circuit. 

Jim Brown  K9YC