Topband: Grounds

[Tom W8JI]

> I'm not saying that there's not a case for suppressing common mode as you
> and others suggest.
>

It all comes down to impedance ratios and signal levels. Any choke or filter 
is like a pi network or attenuator pad. It is ***always*** more than the 
series element alone.

It the line has a very low common mode shunting path impedance, a very small 
series impedance adds a very large amount of additional isolation. This is 
the way most lines in houses and Ham shacks are, because they connect to 
things have low shunt impedances.

Cables that are buried, or very long, establish fairly low impedances. 
Ground rods might help, or might not.

Just think of the shield as an antenna you are feeding. Imagine an aerial 
span over a road or driveway. If you had a little 20 foot long half square 
feed through a buried single wire feeder, that "antenna" would not be a good 
antenna at all. It could not greatly affect the rest of the system, because 
its small contribution to the shield would be lost in attenuation of the 
lossy single wire feeder near earth.

Also, if you have a GOOD system that meets codes, you have every single 
shield grounded at the house entrance point. That alone establishes a low 
impedance for things conducted in or out on the line shield. It takes a very 
minimal series shield impedance to make an effective barrier at that point, 
if anything at all. (My system require nothing at all.)

I don't use ground rods on shields, and generally don't use beads, unless 
the connector is problematic. I have Beads slipped over cables (just ONE 
pass) to eliminate problems from my phono connectors that swap RX antenna to 
the contest barn. If a connector comes up to a fraction of an ohm, even a 
30-40 ohm bead isolates the line. If it is high resistance or open, nothing 
would fix it. So in this case a just a single bead is more than enough. The 
"system" determines what is required, not some arbitrary number.

> I do have a problem here lately with RFI from an electronic fluorescent
> ballast on 160 (10 feet from where I operate the radio), and I'm not sure
> how it's getting into my NE Beverage. Big #43 beads on the ballast leads
> didn't fix it. But that's another story.

There are two forms of noise egress, common mode and differential mode. As a 
matter of fact if you get into the system deep enough, there has to be a 
differential mode somewhere at the source or load or the common mode is 
meaningless.

Everyone ignores that and the shunting impedances, and that is what gets 
them in trouble and require the abnormal impedances some systems might 
require.

We can string beads over the wires for a hundred feet and not change a thing 
if the excitation is differential mode.

For common mode, we add beads with hardly any change for common mode if the 
shunt impedances are high. At the same time, one single 20-100 ohm impedance 
bead can clean a system right up with 50 dB attenuation or more, if the 
shunt impedances are low.

It seems very few people look at the system, which isn't that hard to do.

73 Tom