Mixing power and signal cables
Power line noise can couple into low voltage signal cable easily when the cables travel close to each other in a parallel direction for any amount of distance. As you may guess the longer the parallel run and the closer the two cables the more chance of coupling and disturbance.

There are additional considerations to consider as well...


Coupling potential falls off dramatically with increased separation between the cables. Doubling the distance can make a huge difference.
Quality of cable can make a huge difference. There are many grades of TV signal cables and ones with high quality construction and internal shielding can make a huge difference.
Noise on the power line can often be attributed to other equipment plugged into the circuit. This can even include equipment that is plugged into other circuits in the building but fed from the same power panel. Sometimes noise on the power line can be significantly reduced by using a plug-in power line filter between the noisy piece of equipment and the wall receptacle.