> Figure 5 of W1HIS's writeup. 15 pounds of ferrite beads.
>
> http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf
Unfortunately that can lead to a waste of time and material, and even has
some bad advice.
Common mode chokes NEVER belong between transmitting device like a radio and
an amplifier, or between things on the desk in a system running chassis
mounted connectors. The reason we do not need beads or common mode chokes in
this application, and should never use beads or common mode chokes, is
simple and obvious.
If we have a potential difference between chassis of gear causing common
mode, we need to fix the source of that problem. The very last thing we ever
want to do, unless our goal is to make money selling choke, is increase
impedance between things on the desk.
The above rule applies to connectors that are properly mounted to enclosures
or shield walls.
The reason we throw beads at audio lines and other lines inside the shack
and see improvements is actually rooted in improper connector mounting or
connector grounding to the enclosure. This virtually never applies to
transmitting systems, where enclosures are metal and connectors are bolted
to the enclosure. Beads or CM chokes on lines only applies to systems with
inherent flaws in cable entrance or exit shield integrity. For example, a
power cord is not shielded for any of the length, let alone the entire
length exiting the area of the station. There can be no shield integrity, so
beads and CM filters are useful. They are also useful near where the system
is nether perfectly balanced nor perfectly unbalanced, like at many
antennas, or where a plastic box is used.
It is absolutely certain though, common mode isolation does not belong on RF
cables between enclosures with higher power RF on someone's desk. With a
defect in the antenna system, they can be useful just outside the room at
the cable entrance or exit to the station area, but not inside the station.
Used inside, on unbalanced RF cables that properly enter or exit boxes, they
will increase problems more easily than decreasing problems.
73 Tom
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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