I recently received a copy of the RSGB book "Elimination of Electrical Noise." The author recommends that a/c mains filters, consisting of three a/c-rated capacitors across the hot to ground, neutral to ground, and hot to neutral (translating from UK terminology) should be installed both at your and your neighbors' QTHs. Interesting. This will help to bypass any RFI to ground (earth) that might be present on the line.
I've never seen this recommendation in any other publication, i.e., installation of bypass circuitry on the power lines to your and your neighbors' houses.
Think about the system.and how it really works. There are two paths for noise:
1.) There are power lines and other wiring that act like an antenna, and couple noise through space to our antennas.
2.) There are conducted emissions that reach the shack from noise sources. Based on my measurements, the typical modern radio has about 75 dB minimum common or differential mode rejection.
The worse radio I have is an ICOM 751A, and it was -75 dB rejection on 30 MHz, based on injection into the power leads compared to a matched injection into the antenna port for the same S-meter scale reading on any radio. That same ICOM has about 100 dB common or differential mode rejection from power leads on 1.8 MHz. My other radios are all better, except for old tube type radios. This is based on injected RF current levels. In other words I injected enough power supply RF current to read something like S-5 on the meter, and then moved the generator to the antenna port and found the current level required to obtain the exact same S meter reading.
Several samples of 50 and 75 ohm cable show in excess of 80 dB CM rejection on 160 meters, and they get better going up in frequency.
It looks like, if we have no cable defects or connection issues, noise current on the power mains ***if it 100% coupled into the radio's power supply connection***, would be at least 80 dB down and probably a whole lot more. Most of my radios were well over 100 dB down worse case!
So now we have to ask ourselves two questions:
1.) Would noise following the lines couple into our radios more through coupling to our antennas, or through power wiring?
2.) If coupling is through wires, and not by radiation, would a bypass capacitor or mains filter actually reduce the power mains signal level significantly enough to make a change?
The test for number 1 is very simple.
a.) We have to go out to our antenna and replace the antenna with a load resistor, but still have the antenna and any ground connected to the coax shield without altering that setup. This shows us any common mode response into the radio, including via feedlines.
b.) We have to disconnect the antenna from the radio and terminate the antenna port. This shows us radio shielding issues from noise ingress.
In almost all cases, except those where we have bad antenna designs or poor feeder connections, noise ingress will be through radiation and not conducted currents. Since the noise transmitting antenna is all the wire from the house to the noise source, and all down the street, how much are we altering that system by bypassing only the house entrance?
I think it is a pretty safe bet the reason mains filters are not popular at all, except right at a known noise source or at a screen room entrance, is that in nearly all cases noise ingress is almost always by radiation coupling.
The reason we do not see many bypass or line filters used, almost never outside of at noise sources or screen room entrances, is because noise or EMI almost never leaks into equipment connected to outdoor antennas through the mains.
In my 40-50 years of working 160 meters and working weak signals, I have ONE case of power line conducted noise where bypassing at my house changed things. In that case my neighbor had a battery charger that generated hash. The hash followed the mains, and the noise radiated from my house wiring to my antennas that were near my house. In that case I could move capacitors on a plug around my house. By trying different outlets, I could find "sweet spots" in my house wiring where charger noise would almost go away on any given band. This is because I was either changing the radiation pattern of my house wiring, or using the power line wiring as a tuned stub.
I've personally never had a case where noise came into the mains and into a radio.
My opinion is installing a filter as prevention is probably pretty silly, because my only experience where bypassing at my house made a difference in external noise required I move the filter around! It would not even work at the mains box unless I used a series L/C circuit and tuned it for a certain band, and then not nearly as well as a "sweet outlet bypass"!
Making things worse, if someone plugged in something new in my house, the sweet outlet for any band would move, and sometimes not even work.
The solution for me was to buy them a new battery charger.
My opinion is, if there ever is a need for a line filter, it really belongs at the noise source.