[RFI] CAT 3 Cable?

[Tom Rauch]

> You might  be surprised to learn that multiple turns around large ferrite
> cores yield choke impedances of more than 1K ohms over a couple of
octaves.

I actuallysuspect most people with any background understand how multiple
turns through a bead are better. The problem is applying the devices, not
the "surprising" revelation N^2 often works.

The important points I am making are:

1.) Most people grab a core without knowing what it is or how to use it and
slap it on a wire and expect large changes. It is an impedance RATIO problem
and in many cases a few hundred or thousand ohms is nothing, let along ten
or twenty ohms from slapping a bead over a wire.

2.) RF chokes are readily available that are easy to mount and have k-ohm's
of impedance for minimal cost and work. Capacitors are cheap and commonly
available.

3.) Multiple turns through a core can be a problem. In the first place, you
really should use seperate cores for each conductor. Parallel windings of a
pair do NOT increase differential rejection. In th second place, you have to
get multiple turns through the core center, and that means the wire can't
hav a plug on it and the core has to be somewhat large.



> For the HF bands, Fair Rite #31 and #43 are the weapons of choice.

Not true at all. If you use a core, you want one that has the highest
possible impedance in the operating range. If you do not establish a
filtering system with a bypass capacitor so you have known impedances in the
system, you should use a core with very low Q. You want a core with a very
low Q so the impedance is largely resistive. Otherwise you can make the
problem worse by adding series XL near the RF-floating consumer equipment
terminating the line.

31 and 43 (44 has largely replaced 43 in generic beads) are OK at upper HF
or UHF, but they are the wrong materials for most HF suppression systems.
They are used simply because that's the highest permeability material that
manufacturers can easily form into the almost useless (for HF) snap on beads
everyone resorts to.

The optimum core in a "slap a bead on a wire" system (either multiple or
single turns) has a loss tangent curve crossing the permeability curve at
low HF. That generally is something like a 73,75,or 77 material. But it is
also generally true that a couple RF chokes and a cheap .001 to .005 uF disk
cap and a couple off-the-shelf Mouser components will do an infinitely
better job at HF. You really want independent series impedances in each
conductor to suppress differential mode at the same time it gets rid of
common mode problems, and a simple cheap bypass cap (or better yet one on
each conductor of the line to ground) makes the system predictable and much
more effective than relying on the parasitic shunt capacitive reactance of
the consumer device as part of the filter system.

73 Tom