All of this centers around one thing, at lower frequencies devices that
generate noise need "antennas" to cause problems. The cures will all prevent
the devices from effective "antennas".
The original post contained information about an unusual thing we should
keep in mind, that some devices have battery power. When the mains were
turned off remotely, the battery took over. Since the device was not
disconnected from the "antenna", the radiation was not altered. Had it been
unplugged, even with the battery, the noise would have either greatly
dropped or disappeared.
This should serve as a warning to NOT just depend on flipping breakers off
to find noise. There is no assurance removing power will alter noise,
because some devices run without line power. Radiating devices all change
significantly when the radiating "antenna" or "loop path" is opened (or
shorted), but not always when mains power is removed.
Outside of new equipment I design or special situation, I virtually never
use chokes and beads. From an engineering standpoint a series choke system
is actually an unpredictable solution. I tend to avoid unplanned "throw
something at it" cures.
My first step is **always** to close the loop. I buy a cheap TV/power outlet
surge protector outlet, and I run every cable and wire in a local equipment
cluster through that point. I make sure the line strip has suitable bypass
capacitors that are across the line, and from neutral to ground. I make sure
the coaxial line shield is bypassed to the common ground point.
This does several very important things, including things beads cannot do:
1.) It closes the loop for lightning and surge ingress into the system. Most
lightning damage is common mode stuff that loops through gear, like into the
cable port via the shield, through equipment, and out the power line. The
external common point, even without a ground, keeps most of the unwanted
current out of the equipment.
2.) RF from our transmitters follows the same path. Instead of going through
the gear, the RF is "shorted" and bypassed around the gear. The sensitive
equipment is not in the loop.
3.) RF inside any device cannot drive the power lines and other cables in
"push pull". This is exactly what a bead does, but better. Instead of adding
a series impedance via a choke and depending on the ratio of the newly added
series impedance being very high compared to the outside world impedance,
this method "shorts out" the outside world path for RF.
Human focus tends to mindlessly follow the herd of sheep. Since soft iron
cores began, and the first TV deflection yokes were split apart to wind
power cords around, we have focused on throwing beads at systems. We have
not considered the path is two modes, and we have not considered how a bead
actually works.
The primary problem paths are either differential between conductors in one
cord or cable, differential between two cables, or a mixture of the two.
A bead or choke does nothing for differential on a given cable or cord. It
does very little for lightning. Its effectiveness is also highly dependent
on the differential impedance between inlet and outlet at the insertion
point.
A bypassing arrangement greatly reduces differential excitation of the power
feed, something the choke does not do at all. A bypassing arrangement also
greatly reduces differential excitation of the mains against other cables.
It is generally far more effective than a choke for suppression, because
external leads almost always have modest to high differential impedance.
I can often do, with just a simple piece of hookup wire, a better job than a
choke. I cured an apartment complex from severe RFI that Buckeye Cable had
given up on mostly with wire. Buckeye used chokes, filters, double and
triple shielded cables, and were generally just marginally successful. I
fixed almost all of it with simple jumpers. Just a few apartments out of
hundreds needed anything more. I'd guess they invested many thousands of
dollars, and the nearly perfect fix really just cost a few dollars per
building. :)
73 Tom
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