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Re: Topband: Mother of all ferrite common-mode coaxial chokes

To: <topband@contesting.com>, "Victor Goncharsky US5WE" <us5we@bk.ru>
Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Fri, 13 Jul 2012 06:03:41 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
Hi Victor,

> Using them a lot between VHF and UHF boxes to break ground RF paths.

Beads and common mode chokes only modify common mode impedance on the 
outside of cable shield. They do this by allowing a voltage difference to 
appear across the bead or choke. This means the cable shield going in floats 
at a different RF potential than the cable shield exiting.

It is almost never a good thing to have potential differences across cable 
shields on a desk full of lower-level lines.

If any of us have bothersome or significant RF currents in our shacks on the 
outside of cables, we really should learn why the RF is there and fix the 
real problem. The last thing we should be doing is floating ground 
connections (which is what the outside of the shield provides) between 
different equipment on a  desk. The most we can expect from isolating the 
outer shield path is moving equalizing currents to other cables and wires 
between gear.

Beads over transmitting cables between major equipment just isn't a good 
idea at all as a general practice.

> Really helps to prevents all kinds of unpredictable oscillation effects.

If the equipment system changes by moving wires around, or changing the 
impedance of the shield outside, there must be a problem with connector 
mounting and shield impedance, or poor cabinet design.

As for equipment, there are a few cases of terrible cabinets. At least one 
commercial antenna tuner has an intentionally insulated cover, and another 
tuner uses a single core 4:1 current balun that forces output lines into 
significant voltage unbalance.  No one can dispute there are occasional 
serious errors in equipment design.

The correction for those and other errors is not in grounding connections to 
a ground buss, or isolators, or strings of beads. The fix is correcting the 
defect in the gear. Otherwise, when we throw an isolator or beads on cables, 
we simply move the problem someplace else in the wiring where it waits to 
cause problems in the future.

Isolators have a place in the world of antennas and feedlines outside the 
ham shack. They can be important when a system is neither perfectly balanced 
nor perfectly unbalanced. Marconi verticals with less than perfect grounds 
are examples of systems operating in the netherworld between perfectly 
unbalanced and perfectly balanced.  But in cases like that, we want the 
isolation that allows voltages to be different along a section of coaxial 
cable OUTSIDE the house, not inside the house where noise and devices 
sensitive to RF live.

Inside the house, we really want all cabinets and device chassis to have the 
same RF potential. We do not want to isolate cabinets from each other, 
allowing them to float to different potentials to ground.

If throwing beads or isolators on an RF line inside the shack changes 
something, that line has a problem that needs fixed or at least needs 
understood.  The only beads in my shacks on any RF cables are on receiver 
leads that have phono plugs, because the layout of the plugs and the shield 
connections are less than ideal. I live with those connections because that 
is the type of connector used. I understand what the problem is and choose 
to work around it, and I keep an eye on it.

I can't imagine having connections like that on a 100-watt transmitting 
line, let alone 1500 watts. Transmitters deserve real connectors mounted 
properly, and cabinets with RF integrity at joints.

This is all measureable and rationally explainable. It isn't voodoo science, 
like stringing beads everywhere to "make signals stay inside cables". Beads 
don't make anything stay inside something, they allow a larger longitudinal 
voltage difference to occur along a small length of conductor.  We want that 
between radials and a coax feedline shield outside the house, but why would 
we intentionally want two shield ends of a shielded RF cable between two 
cabinets to have greater potential difference on our desks? We really should 
never want shields to be that way, if we have any choice at all.

73 Tom 

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