[RFI] Ferrite Loops

[Tom Rauch]

I received several identical e-mails about using a 259 to
check cores.

Rather than type identical responses or do a bunch of
cutting and pasting in my unfriendly mail software, what I
do here is:

I make sure my analyzer is calibrated correctly, but even
close will do OK. I just use a single short loop of wire
through the core from the ground post to the center pin. If
the impedance with a single pass through the core is too low
to read in the desired frequency range, it probably is a
useless core anyway for suppression.

What you really want is a very high R compared to X for
suppression in the desired frequency range.

You'll find length of the core (hole depth) is a linear
function. Impedance is proportional to length. Thickness of
walls and diameter has a smaller effect. The larger the
diameter the thicker the walls need to be for the same
impedance, but there are limits.

Good suppression materials like 73 or 77 have R=X at 2MHz or
lower.

Multiple passses through the core are more effective than
adding bead length along the wire, and you can pretty much
forget worries about shunt capacitance from winding styles
below upper VHF.

http://www.w8ji.com/toroid_balun_winding.htm

Pardon the poor pictures, but clearly the "split widing"
isn't worthwhile at HF. The question is why is it everywhere
in books? Is it like the wive's tale about moving a balun to
the input of a tuner, something we accepted without testing?

If you depend on high X, remember it can actually HURT
suppression. It is like adding inductive reactance in series
with the line, and it can actually INCREASE current if the
common mode impedance of the line is capacitive!!!

Series resistance will always decrease line current, the
exception being if added isolation occurs at a point where
the rest of the line becomes resonant. As was pointed out,
there are sweet and sour spots for adding suppression.

73 Tom