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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
