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

[Tom Rauch]

I just measured two cores of 77 material Jeff sent me. I
wasn't in any hurry since Mike had posted data, but since I
had the analyzer up and calibrated on the correct fixture
today I ran them. Looks to me like 77 mtl is still useful
for suppression but over the top.

It's obvious the material is on the downslope of impedance
well below 1 MHz. One characteristic cores have when
operated beyond peak impedance is bunching and speading the
turns has minimal effect on impedance. Core diameter also
has a minor effect on impedance.

In this case the impedance of 140-77 and 240-77 were
reasonably similar regardless of winding style or core
diameter as long as I used the same number of turns. The
impedance contained a "capacitive" component, which appears
to be mostly related to effects inside the core material and
only a minor extent to parasitic reactances in the winding
and connections.

There really weren't any problems from leakage "across" the
winding, nor were resonance effects between 1 and 30 MHz
observed (as would be expected if the impedance was -j all
through the range).
I limited frequency range to 1-30 MHz.
Using 15 turns of RG-178 coax the impedance and common mode
attenuation with a 50 ohm source and load impedance were:

FT-140-77

1.8MHz  3.2K -j831     -43.87dB
3.6MHz  1.98K -j1.6k   -36.13dB
7 MHz  1.36k -j1.1k      -30.44dB

FT-240-77

1.8MHz 2.88k -j780     -43.29dB
3.6MHz 2.11k -j1.04k   -35.6dB
7MHz 1.57k -j969         -30.27dB

Spreading or bunching turns made minimal difference.

Keep in mind the attenuation was using my network analyzer
as a source and load, and I measured S12 or S21 parameters.

These impedances are certainly OK for isolating unwanted
currents in systems with reasonably low common mode
impedances at each end of  the insertion point. They won't
do very much for isolating the feedline from the antenna
when the core is placed at or near the floating-above-ground
antenna.

I have dozens (if not hundreds) of data printouts of other
core materials, and hopefully sometime in the future I'll
find time to put them up on the web. It does not look good
for the immediate future, however. Someday someone needs to
do a simple article in plain English about how these things
fit into systems.