Topband: RX splitter - ground common or not?

[Tom W8JI]

> 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