Earth loop impedance for mais support ply into your is established in multiple ways.
Earth loop through ground (literally), impedance being a chain of cable impedance, ground rod (usually copper or brass) and soil, rods may be placed at your home, at the transformer feeding the neighborhood or both
Impedance of the feeder back to the central earthing point usually that pion is at the transformer feeding your neighborhood
The lower that resistance the better EMI pollution can be filtered i.e. dissipated to ground.
High impedance earth loops are consequently more polite with EMI, topically even the most expensive EMI filters have very little effect if earth loop resistance is high.
It’s consequently more important to try and invest in lower earth loop resistance than in expensive filters. In Europe, due to the EMI directive every audio device has some sort of mains filter, which will function better with low impedance earth loop.
Our friend @Dirk lives in low impedance earth loop Valhalla, due to the amount of rain up in Scotland the soil stays wetter and contact resistance between the earth rods and ground is usually very low.
Apart from the fact that Dirk knows how to build those filters. He might be able to shed more light on this topic.