On Tue, 15 Jan 2008 04:32:04 -0500, Tom Rauch wrote:
>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.
That is the "old" way. Bill Whitlock published an AES paper (JAES
Jun 1995) that convinced everyone who read it that the proper
shield connection for a "one end only" bond is at the SENDING end,
not the receiving end. Further, virtually every audio authority is
convinced that the shields of balanced audio cables ought to be
one end only bonded at AUDIO frequencies, and BOTH ends bonded at
RF. What Ott calls "hybrid" grounding.
However, BALANCED shielded cables are VERY different from coax
with respect to the behavior of the shield. With balanced cables,
the shield is effective only against the electric field, NOT a
magnetic field. With coax, the shield is not really a shield, it
is one conductor of a transmission line, and the rejection of
external fields comes from the mutual coupling between the
"shield" and the center conductor. With twisted pair balanced
cables, the primary rejection mechanisms are mutual coupling and
twisting, with twisting being the most powerful.
>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.
I disagree. With wide flashing, we lower the resistance a lot and
the inductance a bit.
The primary reason that balanced audio cable shields must be
opened at one end is THE PIN 1 PROBLEM. I STRONGLY suspect a pin 1
problem in the equipment being discussed here (that is, where an
EARTH connection for the filter improves things). In this case,
common mode current on the shield is likely flowing into the
equipment via a pin 1 problem at the connector where that antenna
is connected. I have found pin 1 problems in virtually every piece
of ham gear I have ever opened. For more on this, see the
publications section of my website. Pin 1 problems are a PRIMARY
cause of baseband (hum, buzz) and RFI. MOST of the problems we
call "RF in the shack" have pin 1 problems as their fundamental
means of ingress (and poor antenna design often puts more RF in
the shack).
http://audiosystemsgroup.com/RFI-Ham.pdf
http://audiosystemsgroup.com/publish
>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.
More correctly, the "ground" connection needs to be to the
shielding enclosure of the equipment being filtered, or in the
absence of a shielding enclosure, the reference plane.
Pin 1 problems can be "band-aided" by killing the common mode
current with a good common mode choke at the point where the cable
enters the equipment, or by bonding the shield to the shielding
enclosure.
73,
Jim Brown K9YC
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