Topband: core inductance and current - some measurements.

[Robert Kavanagh]

At 08:21 PM 11/17/03, N0TT wrote:

>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.

It is not just the current that must be considered.  This effect also 
depends upon the number of turns and the dimensions of the core.  11 mA 
through a 33  turn coil on the same core would have the same effect as 110 
mA in a 3-1/3 turn coil.

See the hysteresis curve for 73 mix in the Fair-Rite catalog for more 
information. Also see page 56 in QEX for May/June 2003 for an example of 
how the inductance of a coil wound on type 43 material changes when d.c. 
flows in a winding.

Bob, VE3OSZ