> Subject: Re: TopBand: Toroids For Ununs
To: <topband@contesting.com>
> Date: Tue, 14 Oct 97 18:22:07 +0000
> I know. He got a lot of flack over that and the CQ articles were after
> the book.
> He grudgingly admitted that powdered iron might be a better choice in
> highly reactive situations such as a nominal 4:1 balun for some open wire
> line antennas.
It can also be a problem with 1:1 SWR's in coaxial lines, it's a
problem of using the WRONG ferrite core. Saturation is almost never
the problem, the problem is usually heat caused by a small percentage
of power loss overheating the core. We just are trained to call it
"saturation", when is almost always isn't.
Core losses are related to flux density in the core, and core flux is
caused by any net common mode current in the windings. Contrary to
what is implied, un-uns HAVE this problem just as other transformers
do. They are not a magic elixir, and are no better than many other
winding techniques. They are actually worse than other winding styles
in many applications, such as when baluns drive imperfect loads.
The biggest mistake is looking at the initial permeability instead of
the permeability at the operating frequency, the second is ignoring
the loss tangent (or the Q) of the core.
Consider permeability and Q at the operating frequency, not at some
other frequency. Remember as initial perm goes up, Q and permeability
go DOWN at higher frequencies. You can almost bet if the core has a
initial perm of over a few hundred, you will have heating problems at
high power because the core's Q will be too low (loss tangent/ u too
high at the operating freq).
I'd take Carl's suggestion and stay away from cores with high loss
tangents at the operating frequency (especially if you are running
high power).
73, Tom W8JI
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