[Elecraft] OT: J and -31 mix toroids

[Jim Brown]

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