DX Engineering/s RSC-2 splitter appears to have all three antenna
terminals connected to a metal case;
i.e., the grounds are all connected together.
In ON4UNs 5th edition, he appears to recommend that the common connector
(the one to the
braid-breaker 7:5 matching transformer) not be tied to the grounds of the
two output connectors,
and shows a picture of his splitter/combiner which appears to be in a
plastic box.
Should I construct my RX splitter with all three shields connected
together,
or should I isolate the common (middle) feedline shield from the other
two?
This is to be used in the shack to feed tor receivers.
Terry,
The entire issue can be pretty complex, and could be one of those ten
thousand page posts where people argue a needle in a haystack of other more
critical things.
The only significant point of ingress for common mode is at a shield
termination, such as the antenna feedpoint or a poor or improper shield
connection. An exception to could be a very strong common mode RF current
from a SMPS with an external ground loop for harmonics through cable
shields, where a shield's -80 dB or more isolation might make am in-band
birdie audible. As a general rule an unwanted signal that strong would
radiate to the antenna anyway, and really needs addressed at the source.
Even worse, a plastic case (or lack of a groundplane upon which connectors
are mounted) just sets people up for ingress issues significantly worse than
a normal shield would allow.
Personally, I would not force an intentional shield discontinuity in the
shack, near noise sources, or at any potentially critical location. I would
use a shielded box with connectors through the wall, or a suitable
well-thought groundplane with connector shells connected right at the
groundplane (no wire shield leads), always.
If I worried about common mode near noise sources or near potential noise
sources, I'd use beads over cables and ground connectors the way the are
designed and intended to be used....with a good groundplane right at the
connector common between ALL cables.
The idea of switching shields, or floating a connector shell at a box
entrance, when this is done near a potential noise source can in no way be
considered good engineering or planning.
Places where the idea of floating a shield from the enclosure or groundplane
works and has logical justification is right at an unavoidable ingress
point, like an antenna feedpoint. We always have ingress and egress at a
point like that, and the only way to reduce it is to isolate the common mode
path. This is because the cable becomes UNshielded at that point!
At all other points in the system, the shield should be a shield. The
connector shell should mount directly to the groundplane, and the
groundplane should be as close to zero impedance between shields as
practical. This even means no wired leads as a ground path leading from
outside into the inside of the box.
If you have common mode, fix any source first through isolation or
containment at the source. Then, if you need to do something further, don't
break the shield. Use beads over an unbroken shield.
This is a complex issue and it varies with how a shield behaves. In practice
a shield works differently at audio, for example, because of skin depth and
the huge levels of common mode current at low frequencies. But for 160 meter
and higher frequency signals, I'd used a closed box or a connector through a
groundplane near noise sources on every lead except an intentionally
balanced line.
73 Tom
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