Imagine that you have two toroids of the same size, say, 2cm diameter.
Now let's compare three coils: One 15 turn coil wound on the iron
toroid, one 15 turn coild wound on the ferrite toroid, and the third
wound just on thin air, but with the same size and shape.

The air coil might have an inductance of roughly 1uH, and at 7MHz the Q
factor might be on the order of 300, if the wire is thick enough. There
is no magnetic loss in air, so the loss of this coil comes solely from
the copper.

The coil on the iron toroid will have much higher inductance. Not 10
times more, because some of the flux is still in the surrounding air,
but close enough. Maybe 9 times as much. So this coil will have about
9uH in this example. Now for the Q factor: The wire resistance is still
the same, so the loss from the wire is also the same, but the inductance
is 9 times higher, so if there was no core loss, the Q factor qould also
be 9 times higher! Unfortunately there IS core loss. But the loss of
"red" material at 7MHz is so low, that the Q of this coil will probably
end up a little higher than that of the air core coil, perhaps in the
neighborhood of 400.

Now let's take the coil on the ferrite toroid. Its inductance will be
very high, let's say perhaps 700 times as high as the air coil (not 850
times as high, because there is still some flux in the air). And the
wire resistance will still be the same, so it will become negligible at
this high inductance. BUT the loss in the ferrite core is NOT negligible
at all! The ferrite has a relatively high loss, and 43 material at 7MHz
might provide as much loss resistance as it provides inductance! So, the
resulting coil might have an extremely low Q, as low as 1 or 2!

For this reason, type 43 material is totally unsuitable for resonant
circuits at 7MHz, while "red" material is perfectly suitable, as are air
coils.

You might then think that "red" material should be used in all cases,
because it has such low loss? No, not at all!

Let's take the case of a non-resonant transformer operating at 7MHz.
Let's say, you need a 4:1 stepdown, from 200 Ohm to 50 Ohm. Wind 15
bifiliar turns on the ferrite toroid, connect them for 4:1, and the
inductance across the whole thing will be a whopping 2000 to 3000uH! So
the magnetizing current will be extremely small, the distorsion
extremely low, etc. And what about the high loss of the ferrite? No
problem. Even if the unloaded Q factor of this transformer may be just 1
or 2, that means a loss resistance of the same or twice the inductive
reactance. At 7MHz, due to the high inductance, this is in the range of
100 kiloohms! This loss resistance, in parallel with the 200 Ohm source
impedance, produces an extremely low and totally negligible loss. So,
the transformer on the ferrite core will be highly efficient, so much so
that it's almost a perfect transformer!

If instead you use the iron core, the problem will be getting enough
inductance. Some people say you want a reactance at least 10 times the
circuit impedance. In this case that would call for 2 kiloohm reactance,
or about 20uH. In our example, this might require about 22 bifiliar
turns. That's quite possible to do, but the resulting transformer will
have twice the wire loss, while the core loss will be as negligible as
it was for the ferrite transformer, and the big difference is that a
transformer that requires 10% as much magnetizing current as the whole
signal current, isn't a anything near to perfect! And if you try to
obtain enough inductance on the iron core by simply using more turns,
you quickly run into serious trouble from the high loss in the long,
thin wire!

Clearly, for such a transformer the ferrite material is a better choice,
while for a resonant circuit the ferrite material is unuseable, while
the iron is quite good.

Now if you go up in frequency, the loss of 43 material gets so high,
that a coil wound on it performas almost like a pure resistance! The Q
might be 0.1 or even lower! In this case, even the high inductance
obtained cannot save us, and the material becomes unuseable. That's why
above perhaps 10MHz it's usually better to use 61 type ferrite instead
of 43.

Something similar happens to the red iron cores. As you go above 10MHz
or so, the losses rise a lot, and it's better to switch to the yellow
material, which has just a slightly lower permeability, but much lower
loss in the higher HF range.

The above just scratches the surface. There are many other factors. For
example, powdered iron has a rather stable permeability, while ferrite
does not. This is one more reason that makes ferrite unsuitable for
resonant circuits, unless it is airgapped to stabilize the overall
permeability. But for transformers, the permability changes don't
matter, because anyway the inductance is way high enough.

Note that these aren't really questions of power level! Bot ferrite and
powdered iron caqn be used at low and high power. The difference is much
more in how the core is used. To state it very simply: If you need an
inductive reactance that is in the same order of magnitude as the
circuit impedances, such as in filters, tanks, resonant circuits, etc,
use powdered iron, or air-wound coils. And if you need an inductive
reactance that's way higher than the circuit impedances, such as in
non-resonant transformers, use ferrite. And after this basic decision,
compare material specs within the chosen category, to select the exact
material. Because both ferrite and powdered iron exist in a wide range
of different characteristics.

> What would happen if a mixer were made from a 43 core?

This is commonly done! Diode ring mixers for example are very often
built using ferrite cores. But 43 material is a good choice only for the
MF to lower HF range. If you want to get into higher frequencies, a
different ferrite material is surely better. Over 20 years ago I built
several direct conversion radios for 40 meters, using 43-type ferrite in
the diode mixers.

> What would happen is an output transformer were made from 'red' cores?

It can be done. But if it were designed to work well (low magnetizing
current, wide bandwidth, low total loss) it would end up much larger and
more expensive than a roughly equivalent transformer made on a ferrite
core. And still, the performance of the ferrite transformer would be better!

But if you want to build the low pass filter after you amp using ferrite
cores, you are out of luck! Unless you use such a large air gap, that
you can hardly talk of a ferrite core anymore! If you use 43 type
ferrite, you would need a very long and thin core to even be able to
make a coil with the low inductance needed, and then the equivalent loss
resistance would be about as low as the circuit impedance, resulting in
an absolutely huge power loss, and the ferrite exploding if the power is
high enough!

I hope this helps clear things up!

Manfred
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