When you wire two chokes of different value in series, you almost always get a series-resonant "hole" in the attenuation somewhere between the parallel-resonant frequencies of the two chokes. It's not hard to see why that's true if you model each choke as an ideal inductor in parallel with a capacitor. So you need to design the combination so that the series-resonant frequency falls somewhere unimportant.
Al N1AL On Thu, 2008-07-31 at 09:14, Jack Smith wrote: > Jim: > > Thank you for the reference. > > Indeed, the self-resonant frequency of the typical small (FT50 size) > chokes I've wound are in the 5-10 MHz range, but at 100 MHz some > (depending upon the core material) still show enough Z to be useful. > For truly wideband 10 KHz - 100 MHz choke action, it's necessary to > series two wound with different core material and turns, e.g., 35 turns > on Steward 40 material for the 2.5 mH, followed by, e.g., 10 turns on > FairRite 43 material for > 50 MHz. > > You have to pay particular attention to the u' and u'' values of the > ferrite material and how they change with frequency as this is a case > where core loss at high frequencies can be good. > > Jack K8ZOA > > > Jim Brown wrote: > > On Wed, 30 Jul 2008 20:33:53 -0400, Jack Smith wrote: > > > > > >> Yes, I have several ferrite core 2.5 mH chokes here, including the > >> Hammond one you mention. There's a significant difference in high > >> frequency performance of the pi wound on ceramic form versus the smaller > >> pi-wound over ferrite and that's one of the things I'm hoping to > >> illustrate in the article. > >> > > > > The RFI and Ferrite tutorial on my website includes clear explanations of > > the nature of ferrite materials from a circuit point of view. You may find > > it helpful in explaining why your very correct in your analysis. You are > > welcome to cite it as a reference. > > > > http://audiosystemsgroup.com/RFI-Ham.pdf > > > > However -- ANY 2.5 mH choke has a good chance of looking capacitive at 50 > > MHz. > > > > 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 > > > > > _______________________________________________ > 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 _______________________________________________ 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
