Re: voltage in cable coaxial
Ground loop --- go back and think about what causes a ground loop --- a potential difference between two interconnected components.
In this case the cable company is grounded on their end ---- your home is connected to a different ground reference ---- which equals two different potentials. The cable resistance from the distribution to your home increases creating more voltage potential drop.
This is why it's important for the cable to be properly bonded to your home ground in order to try and bring both points to the same potential.
There also can be ground loops within your own home when different components are interconnected while being plugged into different receptacles.
Differences in power supplies and electronic designs also affect ground loops with interconnected equipment that use ground reference in their designs.
One thing you might try is bring the modem to the point of entry --- eliminating all splitters , interconnects , etc..
This way the modem directly connects to the incoming cable and would eliminate any local ground loops.
If all else fails you might try an isolation transformer on the cable connection.
"" an ounce of perception -- a pound of obscure "