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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