by Niall » Sun Jun 05, 2016 2:50 pm
There are a few factors here. Presumably the commercial broadcasting was done with the cooperation of the power company and steps would be taken to limit and control the amount of power line actually used by filtering at the desired end point.
From an ad hoc point of view, a reference point would be those "internet over mains" things which hams and swls hate, which have to have notch filtering in appropriate bands to stop them radiating there. As they are legal, there must be a defined level of RF which is allowed to be coupled into the mains. It would be interesting to know what the maximum is, and how well it would work injecting a modulated RF signal at that level. It would only work if the site was fed from an overhead line, and would be affected by the frequency used depending on what happened at the pole transformer.