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[Amps] Broadband transformers, and ferrite plate chokes

Subject: [Amps] Broadband transformers, and ferrite plate chokes
From: Manfred Mornhinweg <>
Date: Sat, 08 Mar 2008 21:38:47 +0000
List-post: <">>
Hi Dan,

> Are you saying number 2, red, and number 6, yellow, material cores
> are not suitable for low power broadband transformers? Or is this a
> power and heating issue that is not relevant at low power?

I would not be so harsh as to stamp them "not suitable", but certainly
they are not nearly as well suited for that task as some ferrite cores are.

Heating is of course no problem at low power. But if the transformer is
lossy, that's still not good.

The big problem with using those red or yellow iron powder cores for
broadband transformers is that you cannot easily get enough inductance!
If you want a broadband transformer covering 160 to 10 meters, for
example for receiver interstage coupling, you want at least some 25uH of
inductance on a 50 Ohm winding, and if possible, about 40uH. Getting
25uH for example on a small yellow toroid like the T50-6 would need 79
turns, forcing you to use extremely thin, lossy wire. The same
inductance can be obtained from an FT50-61 ferrite toroid, winding only
19 turns, which is a lot more practical. The two toroids are the same
size, and the ferrite version costs only 10 cents more (75 vs 65 cents).

For an oscillator tank coil instead, the T50-6 can provide acceptable
stability in many cases, while the FT50-61 definitely cannot! So there
is an application for each material.


> If the plate choke was wound on a ferrite rod, or toroid, much less
> wire length would be needed.
> Why is this never seen in classical designs?
> On the other hand, a cathode choke wound on a ferrite rod IS a 
> classical, but never a plate choke. Why?
> And why is a ferrite rod always used for the cathode choke, but never
> a toroid?

Very good questions, and for the most part they have already gotten very
good and correct answers from other forum members who were faster than I!

But still, I did the exercise to find out how much use a ferrite core
would be for a plate choke.

In order to keep a ferrite core of 2cm diameter working at low enough
losses, about 100 turns of wire would be needed for a typical amp
running at 2.5kV RMS RF voltage. This many turns is close to giving
enough inductance for the application, without any core! So the
advantage of using a core is not very large in this case, and probably
most designers feel that the cost of such a relatively large core is not
justified  by the rather modest reduction in wire length. Also, most
designers don't know how to calculate saturation from DC in the magnetic
material, and fear problems in this regard, even if actually the
calculation is quite simple to do, and it turns out that with any
reasonable layout (and inductance value), we end up very far away from
the saturation zone.

The decision of whether to use a closed core like a toroid, or an open
one like a rod, depends mainly on how much inductance you want, and how
much flux density. The flux density depends on the applied RF voltage,
number of turns, frequency, and the core cross section. It does NOT
depend on permeability, core length, or weather it's open or closed! The
inductance instead also varies with effective permability, which depends
on material permeability and whether it's closed or open.

So, if you need a certain minimu number of turns to meet the flux
density spec, but then end up with far too much inductance on a toroid,
it's better to use a rod. And vice-versa, if you need as much inductance
as you can get, use a toroid and not a rod!

Ferrite has rather high permeability compared to powdered iron. So,
typically a coil could be wound on a powdered iron toroid, or on a
ferrite rod, but not on a ferrite toroid. If instead you need lots of
inductance, the ferrite toroid is the answer, while never the iron
powder toroid nor any sort of rod will be very good.


> More bad news -- the DC will also cause heating, very possibly
> overheating. When that happens you have no mu at all (until it
> cools).

That's a mistake. DC does not cause heating of a magnetic core, not even 
if it saturates the core!


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