Topband: Filters and grounding

[Tom Rauch]

Raoul,

Gary alluded to BC work. In well-designed BC station studios 
the audio lines are all balanced shielded lines. The normal 
procedure, and the proper procedure, is to ground the shield 
at one end only. This ground normally is at the input end of 
the equipment since that is the most sensitive spot for RF 
ingress.

Large RF and AC differentials between equipment or rooms is 
prevented by using a wide copper flashing that bonds 
equipment cases together. In a BC environment with proper 
balanced line installations the bonding of case does, as 
Gary suggested, help the situation. But this is because the 
cable shield is intentionally open at one end.

Actually any balanced line filter in a line with no, poor, 
or open ended shield integrity, say for example a power line 
filter with no shield or an audio cable shield that is open 
at one end,  needs a bonding ground or a ground path.

Unbalanced lines operating above the frequency where the 
shield is several skin depths thick, like normal shielded 
cables in amateur gear, are another case entirely.

> I am just wondering here: The outer of the coax and the 
> connectors all have
> a finite resistance. In the presence of a very strong 
> field, the  impedance
> reduction brought about by another path to ground should 
> reduce the voltage
> on the cable?

Any voltage drop stays outside the cable. That's how a 
shield several skin depths thick works. That's why the 
shielded cables that run for miles in CATV systems are tight 
as a drum unless the shield breaks or opens at one point. 
That's why a "shielded" loop, when you short the gap, goes 
dead. The gap is the point of ingress. Effectively nothing 
goes through the shield wall anywhere else.

> Is this not the reason why we sometimes bury our  coax 
> cables
> in addition to ferrite chokes and other common mode noise 
> reducing
> techniques?

No. I bury my cables to keep them out of harm's way. I can 
suspend them 10 feet in the air or one foot under ground and 
the performance of the shield stays the same. They will be 
virtually stone dead unless the shield is actually broken or 
has a high resistance at some narrow point.

I might bury them near an antenna or decople them to prevent 
unwanted currents from following the OUTSIDE of the shield 
to the antenna, which then can pick up the signal radiated 
from the outside of the shield, but you better not have the 
antenna in the house!!


>If the coax shield and connectors were perfect then it 
>would not
> have been needed?

If they are normal cables and not broken, a ground makes no 
difference. The exception is lightning or out near the 
antenna, which is always a point of RF ingress or it would 
not be called an antenna.

 >I am not arguing any point here, just seems to me as if
> you both are right, if you have a poor contact/connector 
> somewhere: fix it.
> If it is all ok within limits, maybe additional 
> precautions like additional
> grounding is needed?

Not in any system I have ever seen. The only thing grounding 
the outside of the cable does at 1.8 MHz is change the 
common mode impedance of the cable. This includes grounding 
the case of an unbalanced filter. If you find the filter 
case needs to be grounded to improve reception you really 
need to find out what is bad in the system. Something is 
probably wrong because that certainly isn't normal, and the 
last place you want the coaxial lines to be acting like 
antennas is in the house where all the noise is.

Certain balanced lines are an exception, as are lines 
operated below the frequency where skin effect isolates the 
inner and outer walls of the shield. We are, of course, 
talking 160 meters and higher frequencies so the shield has 
very good isolation between the inside and outside. Nothing 
can pass through those walls that are many skin depths 
thick. It's basic field behavior.

73 Tom