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I wonder whether anyone has experienced this. My coax cable drop from the cable company emerged through my basement wall close to my power circuit breaker panel.

Initially I hooked up the coax cable directly to my cable modem (Motorola MB8600) as straight down as possible (to avoid bends) to the left of the circuit panel (see picture below) but the cable was close to several thick power cables just to the left of the circuit panel.

But in just 10 hours, my modem shows LOTS of corrected & uncorrected errors (average 40% uncorrected) as well as particularly high numbers (25% uncorrected) for the 732MHz OFDM PLC band (the other channels are QAM256) with SNR around 33 dB and power around -8 dBmV. See screenshot below: the stats at the first location

So I tried to bend the coax cable and putting the modem as far away from the circuit breaker and got a VERY DIFFERENT result. After 24 hours, my modem shows 0 errors except the 732MHz OFDM PLC band which still has a lot of corrected errors, but only 0.00006% of them uncorrected. See screenshot below:

the stats at the second location

Here's a picture of the circuit panel and the current location of the modem. The red line was the cable path before the move (modem was hanging in the air). picture of the circuit panel and the modem

I noticed that in the new location, the power level and the SNR is a lot better, which may explain the improvement in errors. And in between the two locations, I have disconnected the cable from the modem to perform some other experiments, but I think I screwed on the cable to the modem with equal tightness.

Anyone has this experience and can explain what happened?

Addendum. Before I moved the modem to the basement, the original location of the cable is marked by the red line, and the cable from the outside terminates in a splitter that is also grounded. See picture below (red line traces where the outside cable used to go): original cable path

A short cable from the splitter extends it upstairs, through a coupler (second red box above), terminating in a wall jack in a room directly above the current location of the modem (so it is a short cable). Another cable connects the wall jack to the modem inside that room. With this set up, the errors in most channels were better, but still significant. See screenshot below: the stats when modem was upstairs the splitter

I thought the splitter and/or the multiple cable segments was the culprit, so I bypassed the splitter by moving the modem to the basement and connecting the outside black cable directly into the modem. But several users commented that it's the modem's proximity to the power panel that is the culprit, so what I'm going to do next is to move the modem back upstairs and connect the outside cable directly to the coupler as far away from the power panel as possible.

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    – DavidPostill ♦
    Commented Aug 3, 2023 at 15:42
  • Coax cables are not created equally - is it a quad-shielded RG6 (it should say on the cable itself since it's always a printed spec)? It doesn't seem likely a cable modem being close to the electrical panel is the issue, and seems far more likely the EMF from RG6 running parallel to AC electrical cables is (AC electricity we use is actually the magnetic field surrounding the exterior an AC wire, not the AC wire itself). Low voltage cables should always be run at right angles to AC electrical cables - small bends in RG6 doesn't affect it (this is how it's shipped/stored)
    – JW0914
    Commented Jun 27 at 11:28

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From one of your pictures, you say that your modem was very close to the power panel. The power panel emits the largest amount of EMF than most items in your home. Too much EMF too close to your modem definitely causes interference resulting in a large number of errors.

Moving the modem away from the power panel was certainly what fixed the main issue.

In my case, I had two routers next to each other and that was enough to cause many errors. Moving the routers apart by about 1 meter was sufficient to fix the issue (EMF levels drop quickly with distance).

The coaxial cable emits a really very small amount of EMF. First it does not use 110V at 15A, but it is also built in a way that reduces outward emissions and it also protects the signals by blocking incoming EMF, to some level. That is, if the EMF level are really high, it will still disturb the signal. However, many coaxial cables running next to each other, should have no issue since their signals are self contained.

One problem, for coaxial cables, is that everything has to be as symmetrical as possible. The build and the current used within the cables. I would think that your cable company would have a lot of problems with they did not probably pretty good cables and signals. But it could be somewhat lower standard and if the cables are not that great, placing them close to a large EMF source (i.e. your power panel) will also affect the cable. For the power levels within the coaxial cable to not be equal would sound really strange. It should be really easy to keep it the same for the small distances we run those cables to a house. So I would not worry about that part.

If you'd like to know more about those cables, I suggest further reading:

https://electronics.stackexchange.com/questions/64659/coaxial-cable-theory

https://electronics.stackexchange.com/questions/417483/coax-cable-shielding-understanding-at-ac

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