wrote:
From: Wes Stewart on Fri 12 Aug 2005 10:09

I see no theoretical reason why ferrite loaded chokes cannot be used
as long as all of the design caveats are observed.


I can only add that the Micrometals *free* toroid calculator
incorporates approximate self-resonant frequencies in their
program's calculations. Excellent program for toroidal inductor
design...can't beat the price! :-) www.micrometals.com
. . .

Micrometals manufactures powdered iron, not ferrite, cores. The two
materials are quite different. Among the larger manufacturers of
ferrites are Fair-Rite, Magnetics, Indiana General, Siemens, and Ferroxcube.

When making an RFC using a core of any magnetic material, you need to
insure that the core won't saturate from the flux generated by the DC
current. This is done by including flux density in the calculations, and
comparing it with the flux density which the material will support
before saturating. Powdered iron cores will tolerate a much greater flux
density than ferrites without saturating -- but the permeability is much
lower, also, which necessitates more turns and therefore higher flux
density for a given inductance and DC current. The presence of an air
gap in the magnetic path reduces the flux density for a given
inductance, so saturation isn't nearly as much of a problem with a
solenoidal inductor than with a toroid. However, once again, there's a
trade -- the air gap reduces the effective core permeability.

If you don't want to calculate the flux density, you can evaluate an
inductor by measuring the inductance or RF impedance while the normal
amount of DC current is flowing through it. The current has to come from
a source with considerably higher impedance than the inductor's, say
from a high voltage source with series resistor, or a transistor
connected as a current source. The measurement device is connected
through capacitors. Some saturation isn't a problem as long as the
impedance remains high enough for the purpose at hand -- as the
saturation level increases the impedance will drop.

Low frequency ferrite -- with an air gap -- can be a good choice for an
RFC because at HF the impedance is resistive or, at worst, looks like an
extremely low Q inductance. This results in an impedance that stays
relatively constant for a very wide bandwidth (typically decades), and
is just about completely free of resonance effects. But again, flux
density has to be considered whenever it has to pass DC current.

Roy Lewallen, W7EL