Good point. That's why ferrite beads make good parasitic suppressors - at VHF frequencies they act more like resistors than inductors.
Where I ran into this problem was trying to come up with a plate choke for a kilowatt amplifier that would work from 1.8 to 19.7 MHz. The high inductance needed for the 160 meter band pretty much guaranteed poor performance at 10 meters. I couldn't make the choke lossy because it would burn up at those power levels. The solution was to use two chokes in series and select values that resulted in a series resonance that fell safely in between amateur bands. Al N1AL On Thu, 2008-07-31 at 10:34, Jack Smith wrote: > Indeed such is the case, and it is quite pronounced with high Q inductors. > > You may wish, however, to model and/or measure two series chokes when > both are wound on lossy ferrite material. The response looks quite > different. The resistance in parallel with the inductors radically > modifies the response when the material causes the inductor Q to be < 1. > > I'll E-mail you some early measurements and a quick simulation plots. > > Jack K8ZOA > > Alan Bloom wrote: > > 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
