On Mon, 06 Apr 2009 17:03:50 -0400, Jack Smith wrote: >You might wish to experiment with Steward mix 35 or 40 toroids. Mix 35 >has a relative permeability of 5000 and mix 40 is 10,000. Fair-Rite's 31 >material has a relative permeability of 1500.
Relative permeabilility varies with frequency -- it is NOT a constant (unless you use a parallel equivalent circuit). The numbers you are quoting are low frequency values. >Steward's 35 and 40 material are MnZn ferrites and will have dimensional >resonance in the HF frequency range, with the exact frequency depending >on the core dimension. I suspect that dimensional resonance won't be an >issue for a current choke at your frequency range, however. Fair-Rite >claims the 31 material, although also a MnZn ferrite, does not have >dimensional resonance. Fair-Rite has always said that -- they said it to me six years ago when I first convinced them to make #31 toroids! Perhaps they said it because they viewed it as a negative. Or perhaps they wanted to throw off their competitors. :) But our data for multiturn chokes disproves that! Fair-Rite #31 DOES have a mild dimensional resonance in the 2-5 MHz range. That is what gives the pronounced broadening of the impedance curve when you wind enough turns to move the circuit resonance below 5 MHz! That is, the dimensional resonance and the circuit resonance combine to act as very low Q stagger tuned circuits, extending the usefulness of the choke by something on the order of 3/4 octave as compared to materials like #43. That means that a single choke on #31 can cover 160, 80, and 40 effectively, and still be decent on 30M. With fewer turns it can cover 80-20. And so on. >Type 43 is a NiZn ferrite and does not have >dimensional resonance in the HF frequency band. In general, NiZn ferrites exhibit no measureable dimensional resonance because it occurs at VHF, where it is damped by the loss in the core. To answer Julius's question directly. See my "Choke Cookbook" for specific recommendations on winding chokes for the HF bands. http://audiosystemsgroup.com/RFI-Ham.pdf BTW -- I just got an email from Jay Terleski, WX0B, telling me that he is now stocking #31 toroids, bought out Fair-Rite's stock (900 pieces), and has more on order. He didn't quote prices. Jay also told me that he is now using #31 material for the high power transmitting chokes that he builds. I suggest that you ask for a quote on at least 20 pieces. As active as you are, you'll put them to good use in a few months! 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
