Re: [Amps] Line Isolators for RF feedback

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

R. Measures wrote:

> > > 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?

The major manufacturer - see www.fair-rite.com


> Permag Pacific is a manufacturer, and their catalog says  Nothing about 
> different frequency ratings for one-turn (bead-balun) applications and 
> multi-turn applications.

I haven't seen their data, but in the Fair-Rite data you will find the 
relevant information under the heading of "EMI Suppression". This is 
because the balun and interference suppression applications share the 
same objective: to create a high impedance along the wire/cable passing 
through the bead.

Figure 2 in Fair-Rite's App Note on 'How to Choose Ferrite Components
for EMI Suppression' shows how resistive loss in a typical nickel-zinc 
ferrite (#43 grade) starts to climb rapidly above 1MHz, which is why it 
is not suitable for tuned circuits above that frequency.

But it *is* still suitable for EMI suppression and balun applications at 
higher frequencies, provided that you use enough beads to create a large 
impedance.

W8JI's balun page explains the limits on heat dissipation quite clearly 
(www.w8ji.com).  There is also good background information in the 
'Choosing the correct balun' page at http://www.dxengineering.com

> > 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.

Obviously, not always...


RF engineering doesn't EVER reduce to simple "one-liners".

(except for that one :-)


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