Antenna/Cable TV Ground loops.
OK so you’ve narrowed your ground loop issue down to your cable connection or your rooftop antenna. You’re not alone; this is one of the more common ground loop issues, but I don’t necessarily want to want to get into the gory details, so here are a few links.
Look in this link above and scroll down to the “Grounding Outdoor Antennas” section and you can see why it’s so easy to have a ground loop here.
Ground isolation is really one of the only answers here due to the grounding requirements of aerial antennas and cable systems. Isolation transformers or capacitors are commonly used. Both types will break the ground loop and provide isolation.
Breaking the ground loop can either be done on the coax side of the TV tuner/cable box or on the audio side between TV tuner/cable box and the audio preamp/receiver. When breaking the loop between the TV tuner/Cable box and the audio preamplifier, a wideband audio transformer for the line level signal is probably my best recommendation.
BEWARE… many of the inexpensive audio isolation transformers are based on telephone transformers and don’t have the bandwidth required for hi-fi audio. Perhaps this made little difference in the past, but many shows are now broadcast with much better audio quality than before. Read the specs… if the audio bandwidth is not specified keep looking.
Also keep in mind that the ground loop can be connected to your system on the video cables if your receiver is doing the video switching. In addition, DC level is important in video signals so whatever you do may have an impact on video quality as well.
Ground loops on the video side of things can also cause “hum bars” to appear on your video screen (horizontal stripes that move slowly across the screen).
New cable standards, including HDTV and digital cable, along with widespread use of cable modems for high speed internet access have placed serious constraints on the use of ground isolation equipment if you choose to isolate your ground on the coax side of the cable box. So again if this affects you, read the specs and make sure the unit you are purchasing will not prevent you from using your cable or internet access the way you plan to.
My best recommendation, if your equipment has it, is to use the optical audio output on your cable box or HDTV receiver to connect to your audio equipment. Maybe our readers will have some more recommendations.
Other than this I can’t make a specific recommendation since I don’t have cable… you know, that whole tightwad thing… J