Since most of the RFI most of us experience is self inflicted, it
might help to understand how the RF noise
we generate in our homes reaches our receivers. The FCC EMI/EMC/RFI
regulations are based on the fairly
accurate idea that for frequencies above 30MHz mainly are a problem
because of direct radiation becasue
at higher frequencies require smaller conductors to be effective
radiative antennas. And for frequencies below
30MHz conduction of the RF noise on to the AC mains, or other
conductor, and once the noise is one the
AC mains it is either radiated and reaches our antennas, or, somewhat
less likely, is conducted via the
AC mains directly to the receiver where is enters the receiver via the
power cable.

Balanced transmission lines radiate very little, but the typical home
AC mains wiring is far from a balanced
RF transmission. The presence of the ground wire does some very odd
things to the balance, and every
light circuit. outlet, and device connected to the AC mains, upsets
the "balance". the net result is the typical
house AC mains wiring is very often a very effective antenna. I have
found some references that attempt to
detail 3 conductor balanced lines, but they disagree and based on my
personal experience, don't have
any relationship to reality.



Most self generated noise happens when an electric circuit ether
closes or opens, (makes or breaks).
And from personal experience, the opening of an electric circuit
produces much more noise then
closure. As you may have noticed, every time you toggle a light
switch, your radio will likely produce
a "pop" . Since very few people flip a light switch constantly,
switches are seldom worth the effort
to mitigate.

However any device that uses DC has at least one diode, that acts like
a switch. A diode only allows
current to flow in on direction and turns on for one half of the AC
power cycle. Many devices use
2 diodes and a transformer so both halves of the AC sine wave can be
converted to DC, and a lot
of equipment uses four diodes to convert both halves of the AC to
DC.

Modern Silicon diodes turn on and turn off at very high speeds. This
high switching speed can produce
a spike at every turn off. this spike is very narrow and acts a lot
like a miniature arc and creates a
very strong and broadband RF noise that covers from ~10KHz to at
least 200MHz.

Mitigating diode commutation or switching noise is best accomplished
by the use of a "snubber" cirucit
that absorbs the electrical transient. People have obtained PHDs on
optimal snubber design. If you
are lucky you can get by with a single low value capacitor placed
directly across each rectifier diode.
Values in the 0.1 to 0.001uF are suitable and I have found that 0.01uF
is often a good compromise.
A better circuit adds a resistor in series with the capacitor and when
my wife modified a couple of new
AC-ONE heavy duty commercial power supplies we ended up with a 100 Ohm
resistor in series with
a 0.01uF cap. Do not use wire wound resistors, they will act like
inductors and reduce the effectiveness
of the circuit. In applications where transformers are used, a 1.0uF
and a 0.01uF in parallel across
the transformer secondary can help reduce the switching noise.

In general it is better to tackle the noise at the individual
component then to apply filtering at the AC
mains connection.


For devices with thermostatic switches, it is important to understand
that with age the contacts may
wear and fail to open or close smoothly. This leads to an arc when the
switch cycles and may produce
a noise burst that can last as long as 5 seconds. A snubber is the
answer here. Since most thermostats
are connected directly to the AC mains care must be taken to insure
you don't endanger yourself while
adding the snubber, or that you don't create a hazard for others after
the mods are made.

Because of the high voltage in most thermostats, X Class capacitors
must be used. X Class capacitors
are designed for direct connection across the AC mains and designed to
fail in a safe manor.
I have found that a 0.01uF is a good value to start with. You may have
to try several different values
to find the correct value for your situation.

Some devices, like water bed heaters and aquarium heaters may be
sealed an require replacement
or a cheat to succeed. Will's water bed heater thermostat had become
extremely noisy. Since he
didn't want to drain and refill the mattress, I cheated and used a
zero crossing triac switch with a
9V low noise DC power supply and a resistor to limit the current to
~5mA with plenty of 0.01 caps
to filter the minor RFI left to eliminate the noise. I have a similar
circuit on our water heater. I was
going through thermostats at about 1 per 6 months and got tired of
changing them out. When I
create the web page I will include typical diagrams.

HVAC often use 24AC control voltages and standard lower voltage
capacitors may be used.

For devices like vacuum cleaners, mixers, it may be useful to open the
device and install a 0.01 X class
capacitor across the brushes. Sadly even with a snubber and a high
quality AC mains RFI filter, such
devices are very difficult to effectively reduce the RFI to an
acceptable level.

For many modern devices one of the most useful steps will be to add an
external, in line, AC RFI
filter. I am partial to the Curtis two stage RFI filter that is
available on the surplus market for $0.35.
These units are good for 3A. RFI filters are widely available on the
surplus market and heavy duty
RFI filters are very reasonable when purchases on the surplus.


W1HIS has written a very detailed and useful article, "Common Mode
Chokes", that details many steps
that can be taken to reduce self generated noise.
www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf


Chuck Counselman, in a private email, mentioned the use of 0.01uF X
class capacitors installed in
cheap rubber AC male power connectors and installing them through out
his home and the significant
noise reduction that he experienced. I was given a bunch of GE MOV
over voltage clamps that I opened
and removed the MOV and added the X class 0.01uF and reassembled. This
allowed me to install a
filter at every AC outlet in our home and didn't block on outlet.


MOVs can produce serious RFI during the early failure mode. This may
take a few seconds to a few weeks.
I have 5 "plug in" MOVs that produce serious RFI. The current through
these MOVs starts out at a few
uA and gradually creeps up to 50mA after an hour. I am afraid to run a
prolonged test because quite
frankly I don't need a house fire. But these suckers all sound
different, and all produce broadband RFI
that is at least S5 when tested in a friend's home.

A rant against MOVs. This correctly doesn't belong in a RFI thread,
but MOVs are very dangerous.
Unless they are equipped with thermal cutouts, or placed in a fire
proof container, they are incendiary
devices that can set your house on fire. I know of three house fires
that were started by damaged
MOVs. Sadly when MOVs fail they often undergo thermal runaway and I am
sadly amazed at their
omni presence.


John Bryant found a way to use common mode chokes to reduce unwanted
RF ingress into his
antenna system.
http://www3.telus.net/7dxr/ircatech/snake.pdf#search='bryant%20lead
%20in%20antenna

His toroid design is useful for laptop SM power supplies as well as in
taming unwanted and perhaps
unexpected RF ingress.


Next: NEC Safety Grounds and "RF" grounds.

Terry