[TowerTalk] Ferrites 31 vs. 77 material

[jimlux]

On 11/2/19 10:18 AM, Jim Thomson wrote:
> Date: Sat, 2 Nov 2019 13:25:19 +0000 (UTC)
> From: Roger Parsons <ve3zi at yahoo.com>
> To: "Tower and HF Antenna Construction Topics."
> <towertalk at contesting.com>, "jim at audiosystemsgroup.com"
> <jim at audiosystemsgroup.com>
> Subject: [TowerTalk] Ferrites 31 vs. 77 material
> 
> <Very funny.
> <The posts from both N6RK and AB7E support my statement that variability in ferrite parameters has been known for a very long time. You're the one who asserted that it has just been discovered and that therefore Steve G3TXQ could not have known about it.
> 
> <73 RogerVE3ZI
> 
> ##  I  believe it  was  N3RR  that  bought  700,   (seven  hundred)   type  31  cores  from  one  supplier,  all  from  the  same  lot number..2 years  ago.  He  used  a  simple  1 turn  link to test them..and then  graded them.  They were all  over the  map,  + and – 22%.   Thats a whopping  44%   spread.  No  2 ferrites  the  same!     ALL  made  in China......so what do  you expect ?  So  much  for  fairite moving  their  factory  to  China.   QC  down  the  tubes  ever  since.
> 


+/- 20% is a reasonable tolerance for this kind of component -

from Dexter Epcos:
"Even with the best grinding methods known today, a certain degree of 
roughness on ground surfaces cannot be avoided, so that the usual term 
“without air gap” or “ungapped” does not imply no air gap at all. The AL 
values quoted allow for a certain amount of roughness of the ground 
faces. The tolerance of the AL value for ungapped cores is –20 to +30% 
or –30 to +40%. Closer tolerances are not available for several reasons. 
The spread in the AL values of ungapped cores practically equal the 
spread in ring core permeability (±20% … ±30%), and the AL value largely 
depends on the grinding quality of the matching surfaces."


https://www.ferroxcube.com/en-global/download/download/11

the "filter" materials (material 3C11, page 77) show mu has a +/- 20% 
tolerance (yeah, it's for a lower frequency, but I didn't want to go 
hunting for one for HF...)

There's also a strong temperature dependence on some of these 
materials.. mu might go from 1500 at 0C to 2000 at 50C.