Topband: FT-240-77 Toroid Impedance R + jX Components

[Michael Tope]

----- Original Message -----
From: "Chuck Hutton" <charlesh3 at msn.com>

> But that wasn't really intended to be my question. What I don't have a
> handle on is how to use the Fair-Rite u's and u"s data to predict your
> measured curves (at least roughly).

If you look at the portion of the mu curves where  u' is flat
and much larger than u", that is where you expect the core
to act like a high-mu low loss magnetic material. In that
region the inductance of the toroidal inductor sould be
relatively constant with frequency, and thus the resulting
impedance will be nearly a pure reactance that increases
linearly with frequency (e.g. XL = 2*pi*F*L). You can see
this behavior in my data below about 800 KHz. It is not a
coincidence that the u' is dominant and relatively flat over
this frequency range. Above that frequency, the mu of the
core starts to drop. At some point the rate of decrease in
mu overtakes rate of increase of frequency, so the effect
is a net drop off in impedance. Also, around 1.5 MHz the
u" component (loss component) of the permeability
overtakes the u' component, and in fact this is where you
start to see the real (lossy) part of the resulting impedance
start to dominate over the reactive part. This is just what
one would expect. Remember for a given # of turns the
reactive part (+jXL) part of the toroids impedance is
proportional to u'. The resistive component (e.g. loss term)
of the toroids impedance is proportional to u''.

> Equally perplexing to me is the difference between the curves on your web
> page and the Fair-Rite curves in Figure 22 on p. 181. The Fair-Rite curves
> have nice peaks that change frequency dramatically as the number of turns
is
> changed. The peak in your curves doesn't appear to move at all as the # of
> turns changes from 1 turn to 6.
>

Jim K9YC noticed the same thing, Chuck. In fact, when I
measured some type 43 curves here I got a similar downward
shift in the impedance peak as I added turns. I just assumed
that this downward shift was due to the distributed C from the turns.
The reason you don't see the same shift it in the 77 material is
that the peak resonance isn't due to the distributed C interacting
with the impedance of the toroid, but rather something going
on in the MgZn material itself (e.g. the peak in the 77 material
occurs at a frequency much to low to be effected significantly
by the distributed C between the turns).

> Thanks for the comments so far, and I hope you're not out of gas on this
> topic.

Well I wish I had more time to spend on this. I promised Jim I
would take a 2nd look at some other data that I have here, but
I have yet to get to it. Too many balls in the air :):)

73 de Mike, W4EF.................