[Amps] Line Isolators for RF feedback

[R.Measures]

On Aug 7, 2004, at 1:18 AM, Ian White, G3SEK wrote:

> R. Measures wrote:
>>> I'll stick with my bead balun -- it fixed all the RF problems I was 
>>> having, so unless it bursts
>>> into flames, I'll keep it!!
>>
>> Joe -- the only problems that I have heard of is the ferrite beads 
>> getting so hot they cracked and dropped off.  This is hardly 
>> surprising since the ferrite material that is typically used in bead 
>> baluns is rated by the manufacturer (not by the seller, mind you) at 
>> 1MHz maximum,
>
> That "frequency rating" is only for resonant circuits, and does not 
> apply to baluns. For balun applications you need the "wideband" rating 
> - this *starts* at 1MHz typ and extends up to 50-100MHz according to 
> the grade of ferrite.

This is according to the seller or the manufacturer?  How does the 
ferrite material know the difference between tuned-circuit RF and balun 
RF?.  Permag Pacific is a manufacturer, and their catalog says  Nothing 
  about different frequency ratings for one-turn (bead-balun) 
applications and multi-turn applications.
>
> Over its "wideband" region, a ferrite bead becomes increasingly 
> resistive as well as inductive. The resistive losses in each bead will 
> be I-squared*R and these will of course heat up the bead.

... and crack, and fall off.
>
> However, the "I" that we're talking about is the surface current on 
> the coax, that the balun is trying to suppress. If you use enough of 
> the right beads, you will create so much R in series with the current 
> path that I will be reduced to a very low value, so the heating effect 
> in each bead will be very small.

So the rule is add beads until they don't smoke.
>
> The only times you can expect serious heating in a bead balun are if 
> at least two of the following factors apply:
> 1. Defective balun (not enough beads, wrong core material, broken 
> beads, incorrect construction)
> 2. Diabolically unbalanced antenna (so fix the antenna already)

Ian --  How would you fix a half-wave dipole so that it becomes more 
balanced?  It seems to me that the problem is with the feedline being 
unbalanced -- i. e., building an interface (balun) between the 
naturally balanced antenna and its unbalanced coax feedline.

> 3. High power.
>
>>>
>>> And there was a group of us talking about this very subject the 
>>> other day.  How would one
>>> determine how much power could be run through a bead balun if I were 
>>> to build my own?  (Assuming
>>> enough ferrite to cover about 12 inches of RG-400).
>
> It depends how much surface current you're trying to suppress, and 
> what power you're using, so Rich is right:
>
>>
>> -  Simple, jack up P until a bead cracks and drops to the floor.  A 
>> friend accomplished this with 2500w.  The bad news about ferrites is 
>> that they start generating harmonics long before they self-destruct.
>
> Harmonics are generated by saturation of the magnetic material; and in 
> general, beads don't saturate in this application. RF loss and 
> saturation are two different things.

True, Ian, but they live next door to each other.
>
> Steve 'GSQ pushes a lot of power through ferrite, and may have 
> something to add here.
>
>
> -- 
> 73 from Ian G3SEK      ...

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org