Topband: core inductance and current - some measurements.

[n0tt1 at juno.com]

On Mon, 17 Nov 2003 12:58:59 -0500 "Tom Rauch" <w8ji at contesting.com>
writes:
> > The very popular Fair-Rite 43 is not immune from
> > magnetization, as I have tested it.  I suspect
> > 73 is at least as bad.  The issue comes up for
> > me because I use DC in my system to switch relays.
> > If the relay DC passed through the transformer due
> > to a short or wiring error, I can't just fix the
> > problem and ignore the possible core damage.
> 
> I just tested three core styles. One was a small bead recommended 
> for choke
> baluns, the other two were 43 material and 73 material binocular 
> cores used
> in my transformers.
_..._

> I'm sure every style of core would test differently, even if using 
> the same
> basic material. But overall I have to say I just don't see a problem 
> in
> normal applications for these cores, at least not in the impedance 
> curves
> from 300kHz to 100MHz. I've never noticed one in real life either, 
> not to
> say it couldn't happen.


I have a remote switch that individually controls 6 relays through a 
~1/4 mile long coax line that is used to switch receiving antennas. 
For RF chokes in this device I use 73 mix binocular cores (the same 
type of core that Tom,W8JI uses), and I wanted to take a fresh look 
to see how much the inductance might change with lower 
currents on these cores.  (I had measured the cores before using 
using them in the switcher by another means, but I figured it never 
hurts to test another way.)

I reduced the turns count for this test because the inductance readings 
for more turns is out of range using a MFJ269.

The measurements were made with the '269 by connecting a 0.1mfd
capacitor in series with the choke under test, with one lead of the
choke being grounded and the free end of the capacitor connected
to the instrument.  A variable voltage source with a current meter was 
connected at the junction of the capacitor and choke through a 2.5 mhy 
pi-wound ("standard") RF choke.  The frequency was 1.818 mhz and the
"inductance" mode was being used on the '269.

The indicated inductances with just *3-1/3 turns* on the 73 mix Binocular

core were:

0 ma    90.04 uhy
15 ma 90.04 uhy
29 ma 89.71 uhy
58 ma 97.11 uhy
110 ma 75.41 uhy

My remote relays draw around 40ma and there is no
significant change in inductance with a steady ~40ma current 
through the choke...so for Topband use, those cores, IMHO, 
are perfectly OK to use for chokes for lower-current applications
like relays.  Looking at the  above readings though, one can see that 
the inductance starts to drop off with currents in the neighborhood 
of 100 ma.

73,
Charlie
N0TT

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