[TowerTalk] Fair rite materials for choke baluns

[jimlux]

On 7/6/16 9:44 AM, Grant Saviers wrote:
> Why would an RF choke ferrite core be "ruined" if saturated?  And if
> saturated, why can't it be demagnetized?

Most magnetic materials have some residual magnetism (except things like 
soft iron).  If you exceed a threshold H field, then it "takes a set". 
I believe that's the "tip of the curve" on the B/H plot

Yes, you can, sometimes, demagnetize.  when they demagnetize steel 
tools, for instance, they impose an alternating field that is greater 
than the field required to magnetize, and then you gradually reduce the 
field. Since the field everywhere in the metal isn't exactly the same, 
that has the effect of randomizing the residual magnetism on all the 
little magnetic domains (e.g. when the field drops too low for a 
particular domain, it stays where it was last left), so the net is that 
the thing is "demagnetized".



>
> Wouldn't the modern rig envelope amplitude decay of a "dit" or of voice,
> tend to demagnetize the core out of saturation?  Since RF chokes aren't
> square loop ("hard") ferrites, wouldn't normal amplitudes tend also to
> return to lower remanent magnetization as minor loops are traversed?

I think that for a lot of materials, there's really (small signal) 
curves within  (large signal) curve

31 material has about 0.5 Oersted spread between the two arms of the 
loop with the tip of the loop at 5 Oersted. (400 Amp Turn/meter), that's 
about 10 times the saturation field (about 0.5 Oersted estimating off 
the plot where the curve really starts to bend)

The data sheet says coercive force of 0.28 Oersted (at 25C)

>
> OTOH, it would be interesting to add a DC current winding to a choke and
> run some experiments.  Why would the results would be any different than
> with a laminated metal core choke designed for a power supply?

Probably quite similar, difference is in relative magnitude
>
> However, from what little I know about ferrites, it is possible with
> overheating to irreversibly damage them.

There's several factors there: one is the curie point - raise above it, 
and the material becomes not-magnetizeable (e.g. the domains are 
scrambled), and not all materials become magnetic again as they cool. 
There also might be some chemical or structural changes - in steel, for 
instance, the crystal structure can change over temperature.   For 
notionally non-magnetic stainless steel, this is how it can become 
magnetic, because it goes through a transition to a different form of 
steel and crystals.  I'm not a metallurgist, so the above is about the 
limit of my knowledge.