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Station RFI Ground
Second Floor Grounding
Long wire
antenna random wire
Common mode
current, which is
what typically
bothers consumer
devices or causes
consumer devices to
bother us, comes
from a variety of
causes. Sometimes
several different
things combine to
make a problem. Very
rarely are harmonics
or other transmitter
maladies at fault.
The exception is VHF
or HF transmitter
harmonics to a
television or FM
receiver, and this
is very rare.
Common Mode RFI in
Consumer Devices
To have common
mode RFI there has
to be a path THROUGH
the gear, not just
to the gear. The
path is always over
the outside of a
shielded cable,
although it can be
on either or all
conductors of an
unshielded cable. With unshielded wires and cables, the problem is common mode
on all conductors including any ground wires. Once in a while differential mode
currents can cause a problem, but not nearly as often as common mode currents.
The problems is almost always never direct pickup, but most often from the
power line acting like an antenna or ground and some other lead, such as speaker
wires or antenna leads, providing the other “half” of the undesired antenna
system. The problem
with anything like
this is very often
multiple paths are
involved, but power
mains are usually
one of them because
of the sheer size of
the “antenna” formed
by the power mains. Whatever the path, almost always multiple wires or cables
are involved in
the path. The path can be
from the power mains
through the
electronics to the
speaker leads, it
can be from speaker
leads to the chassis
of some other device
connected to the
amplifier and back
through power lines,
it can almost any
path.
RF can flow along
any path between
external
connections A, B,
and C. Most
often B, the power
cord, is involved.
The last thing we
want to do is have individual pieces of
equipment in an equipment group or
cluster plugged into
different electrical
outlets.
The RFI path is
normally not a differential mode signal path. This means long conductors, like speaker
leads for external speakers, contribute to problems even when rectification is
not in the audio
output. Think of the
problem as the
speaker leads acting
like an antenna or a
ground, with the other leads (like power line or CATV lines) acting as the other
half or “pole” of a big dipole.
Switching a front panel switch to a different input port
might not change the interference, even when a cable to a particular port is
involved,
because cable shields
and other conductors remain
connected. Only
center conductors
switch, and with any properly connected
shielded cable the
center conductor always has
zero common-mode RF
current. Common mode RF current on shielded cables is always on the outside of
the shield. Switching off the center conductor often has very little effect.
Curing RFI in a
Typical Installation
The installation
below is in my
family room. It has
been there for about
7 years with no RFI
or lightning damage.
I modified this
box by adding two
.01uF 250 VAC rated
UL/CSA/VDE approved
capacitors from each
power lead to the
safety ground. The
75-ohm cable
fittings were
already grounded to
the safety ground,
and the telephone
jack bypasses were
grounded there also.
Every power
connector from the
entertainment
center, without
exception, routes to
this common outlet
and cable box.
Hidden behind my
stereo amplifier (and
not shown in the
picture) the
external speaker
leads are bypassed
through .01 uF disc
capacitors to a lug
under a screw in the
stereo amplifier’s
metal case. That
unit’s case is grounded to
the electrical
safety ground
through the three
wire cord.
This connects or
bypasses every wire
entering or leaving
the immediate area
of the entertainment
center to the common
“ground” point
provided by the
outlet strip.
Computer Center RFI
At every computer
center, I also use a
single outlet strip.
The strips I use
have internal MOV’s,
but I add .01uF 250
VAC UL/CSA/VDE
approved capacitors
as bypasses from
each power line lead
to the safety
ground.
If a Telecom line
has to come into the
same area, I use a
protection strip as
used in my
television
installation above.
Every connection
must always run
through and be
bypassed to one
single point.
This not only
helps for RFI coming
in, it also protects
against lightning
damage and RFI going
out.
Using the above
methods I have
virtually no TVI or
RFI either to or
from my consumer
devices.
Background to the
Above Method of RFI
Protection
In the 70’s, I
engineered systems
for a small cable
operation with
plants over
northwestern Ohio
and Southeastern
Michigan. We had a
few hundred systems
of various sizes. We
inherited a very
large apartment
complex from a cable
operation because
areas of the complex
were built over the
radial field of a 5
kW AM and 50 kW FM
broadcast station.
The original cable
operator installed
double shield cables
and TVI filters
throughout the
system before giving
up.
I had our crews
replace all the
cables with normal
single-shield 75-ohm
hardline, grounding
our cables to the
service feeds for
each building. At
those points we
installed our
distribution
splitters, and
followed the power
feeds with our
distribution cables
to each apartment.
At the apartments,
we mounted our
outlets next to the
power feeds used by
TV sets, and bonded
out box grounds to
the outlet box.
Where this was not
possible I made my
own boxes, where the
TV set power feed
and CATV feeds were
bonded.
The result of
this was a nearly
perfect RFI cure.
Out of dozens of TV
sets that were
formerly
unwatchable, only
one or two
televisions required
additional work.
If you have a
serious problem and
need help with
correcting RFI,
need help figuring
out how to modify a
box, or are chicken
and would like to
purchase modified
outlet boxes,
contact me.
c2010 W8JI
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