[Amps] 160M PI network Toroidal Coil

[Manfred Mornhinweg]

Peter,

> The numbers I've seen for flux density suggest that at 160m, you
> should be at no more than 100 gauss or 10 mT for reasonable losses
> and linearity.......and preferably a bit less.

Yes, that's about correct. The exact value varies a bit with core 
material, size, and so on, but will be close to 10mT.

This tends to require long, relatively slender cores, or very low 
permability ones.

> So even if you cool the toroid, I'm still a bit doubtful about
> linearity.

So am I. But the emphasis is on "doubtful". I would have to try it, as I 
don't have good data about that material's linearity. Real saturation 
anyway only starts at much higher flux density levels. But hysteresis 
effects have a good chance to cause significant nonlinearity.

> Plus there's the temperature coefficient to consider, too.

As long as the loaded Q is low enough, that shouldn't become a real 
problem in a tank circuit. It's much worse when you are trying to make a 
VFO coil on an powdered iron core, as is often suggested in ARRL 
literature! In my youth I built several radios with "automatic built-in 
scanner" that way, until I figured out that nice big air core coils are 
far more stable!

Nowadays of course I use DDS chips instead of VFOs, so that problem has 
been solved!

> Fortunately, dust iron has a fairly high Curie temperature, but that
> needs to be given some consideration - I suspect Amidon don't have a
> figure for it.

I think that the Curie temperature is probably high enough, so that 
insulation failure of the wire, or binder degradation of the core, are 
the limiting factors.

> For some reason, I've never been a big fan of toroidal dust iron
> inductors....

They have their uses, where they provide some worthwhile advantage over 
other solutions. But they are certainly not an essential type of 
component. If all iron dust cores disappeared from this world, I could 
still design all my circuits, with only minor limitations, using either 
air core coils or ferrite cores, depending on the case. Instead if 
ferrite was taken away from this world, life would be much harder! Some
kinds of circuits we are used to just couldn't be built.

Manfred

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