[Elecraft] K3 6m pre-amp

[Jim Brown]

On Thu, 31 Jul 2008 09:42:16 -0700, Alan Bloom wrote:

>That's why ferrite beads make good parasitic suppressors -
>at VHF frequencies they act more like resistors than inductors.

EXACTLY!  

The equivalent circuit of a wire going through a ferrite bead is a 
low Q parallel resonant circuit. (For some ferrite materials, its 
two parallel resonant circuits in series.)  When we wind multiple 
turns through or around a core, we increase the capacitance between 
turns and multiply the inductance and loss by N squared, both of 
which serve to move the resonance down in frequency and increase the 
R at resonance. For all practical purposes, Q changes only to the 
extent that u' and u'' are changing with frequency. 

Thus, a material like Fair-Rite #43 which for most form factors has 
a resonance around 200 MHz can provide effective suppression at HF 
by winding multiple turns through it. The tutorial includes a 
development of the equivalent circuit (this work is original with 
me, and was first published in an AES Paper, also on my website). 

73,

Jim Brown K9YC