[Amps] Line Isolators for RF feedback

[Ian White, G3SEK]

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.

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.

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.

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)
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 the 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.

Steve 'GSQ pushes a lot of power through ferrite, and may have something 
to add here.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek