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
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