Re: [IRCA] Cable connectors

[george magiros <gmagiros@xxxxxxxxx>]

I've been looking at Cat 5 cable too.   i've been really interested in the
shielded twisted pair variety (FTP or STP) to fight common mode noise.  I
found this quote from one of the authors of the high speed ethernet spec, Dr
Howard Johnson:

"In high-speed applications involving low-impedance circuitry, most of the
near-field
energy surrounding the conductors is in the magnetic field mode, and for
that problem,
only a magnetic shield will work. That’s what the double-grounded shield
provides.
Grounding both ends of the shield permits high-frequency currents to
circulate in the
shield, which will counteract the currents flowing in the signal conductors.
These
counteracting currents create magnetic fields that cancel the magnetic
fields emanating
from the signal conductors, providing a magnetic shielding effect.

i want to do an experiment using a length of straight wire, coax, twin lead,
twisted pair, shield twisted pair (plain and with a common mode choke),
checking each one using a receiver to see how much noise is on them and
comparing.  not sure if i should terminate them to 50 ohms or leave them
open at the end of the line.   should probably double ground them though.
maybe when i find work and have some money i'll do the experiment.

george
6462455704


On Mon, Sep 6, 2010 at 2:51 PM, Russ Edmunds <wb2bjh@xxxxxxxxx> wrote:

> My only contribution to this discussion is to say that I've tried RS
> clamp-on ferrite chokes in multiple applications from coax shielding for
> antennas to shielding the cables for the old CRT's for the computer, and
> never found that they did anything.
>
>
> Russ Edmunds
> Blue Bell, PA ( 360' ASL )
> [15 mi NNW of Philadelphia]
> 40:08:45N; 75:16:04W, Grid FN20id
> <wb2bjh@xxxxxxxxx>
> FM: Yamaha T-80 & Onkyo T-450RDS w/ APS9B @15'
> AM: Modified Sony ICF 2010 barefoot
>
>
> --- On Mon, 9/6/10, Chuck Hutton <charlesh3@xxxxxxx> wrote:
>
> > From: Chuck Hutton <charlesh3@xxxxxxx>
> > Subject: Re: [IRCA] Cable connectors
> > To: irca@xxxxxxxxxxxxxxxx
> > Date: Monday, September 6, 2010, 2:36 PM
> >
> > Rick -
> >
> > I liked your explanation except for the part below.
> > Considering the choke impedance to be part of a voltage
> > divider along with the ground resistance, you surely need a
> > choke with at least several thousand Ohms of impedance. With
> > a poor ground connection (sandy soil), you can need even
> > more. I won't bore everyone with the explanation.
> >
> > Bottom line: I don't think you can ever get even close to
> > that with the over-the-cable chokes. I can recommend only
> > the chokes made with mini-coax and high permeability (at
> > least 2000) material.
> >
> >
> > Chuck
> >
> > > From: k9ao@xxxxxxxxxxx
> > > To: irca@xxxxxxxxxxxxxxxx
> > > Date: Mon, 6 Sep 2010 13:20:37 -0400
> > > Subject: Re: [IRCA] Cable connectors
> > >
> > >
> > > Mark,
> > >  (CLIP)
> >  Too little added impedance and the effects
> > > aren't all they should be. Usually, paying attention
> > to the core material for
> > > the band in question, and getting large enough
> > over-the-cable tubular chokes,
> > > or winding an appropriate number of turns of cable
> > through a toroid core, will
> > > do the trick.
> > > (CLIP)
> > >
> > > Rick Kunath
> > > _______________________________________________
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