Noise on Cable TV/Internet Coax - Page 1
Forum section: electronics / general

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 29, 2025, 12:20:41 pm
content: I have underground coax going from the (Comcast) pedestal in my front yard to the side of my house. I have had no problems recently and am getting the full rated speed. Yesterday, I looked outside and saw a Comcast truck and a technician poking around the pedestal. I went outside and the tech told me that there is an intermittent noise problem that is affecting other customers in the neighborhood. He showed me the spectrum and said the noise was below 50 MHz.He disconnected my line at the pedestal and the noise vanished, so he concluded that the noise was coming from my underground coax. He then disconnected my coax at the house and concluded that my house is not the source of the noise, so somehow some "foreign" noise was getting induced onto the coax between the pedestal and my house.He hooked up is time domain reflectometer and saw a massive impedance bump 7 feet from my house and a smaller impedance bump about 30 feet from that. Hmm, 7 feet from my house is about where the electric utility dug a massive hole two years ago to replace my electric feed. I remember seeing the coax stretching across the hole.The Comcast tech then put a 30 MHz high pass filter at the pedestal so the noise wouldn't leak back into the system, causing a slight decrease in my upload speed, and put in an order to have a new underground line installed.This all sounds kind of strange to me. How does "noise" leak into a coax cable and why am I not seeing any performance degradation on my end given the reflectometer readings.Note: As far as the intermittent noise goes, the only thing I can think of is that my heat pump ECM fan motor does emit a lot of RF noise when it is on. I can see it on my ham radio transceiver around 13-16 MHz and it consists of spikes every 18 kHz. But I don't remember when the Comcast tech was measuring things whether or not the heat pump was on or not. And I wouldn't expect that to leak much into a coax cable. My basement lab is only a few feet from the heat pump and I've never seen any heat pump noise pickup in any of my test equipment BNC cabling.Anybody have any insights?

author: u666sa
author_rank: Frequent Contributor
timestamp: March 29, 2025, 12:53:39 pm
content: Get aUV-K5 radioIt's good.Get it from ali.Then when it arrives install s0vahttps://t.me/uvk5_spectrum_talkHere is a more specific link for youhttps://t.me/uvk5_spectrum_talk/54231/106440You will get a spectrum analyzer in your hands, with which you will be able to locate the source of your noise in your house.Option 2 is to get an actual spectrum analyzer. For exampleTinySA Ultra.You should have both by the way. First one besides spectrum analyzer has freq catch feature. Spectrum analyzer will show you just a spectrum, and you'd have to change frequencies to find the source of your noise. Guy once discovered a multiplex in a cable he thought was long cut, as a result added some free channels to his TV.

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 29, 2025, 01:28:23 pm
content: I've got the suitable equipment and I already know that the heat pump does generate HF noise. I am only guessing that the heat pump could be a source of noise that the cable guy is seeing. He did break the connection at my house and measured no noise coming from my house. But whether or not the various measurements he was making at that the time coincided with my heat pump being on or off, I wasn't looking at the pump while he was doing it. I think I remember him measuring the house, seeing no noise, and then measuring the underground cable a few seconds later, seeing noise, so if it was the heat pump, it would have had to turn on exactly between those two measurements. Probably not.But all of this heat pump nonsense isn't my basic question. My question is what is the mechanism that allows noise onto a coax line given that he measured those large impedance bumps on his TDR which point to a flaw in the underground cable.When Comcast eventually gets around to replacing the underground cable, hopefully they will do some more measurements and at that point, I will manually cycle the heat pump on and off and we'll see.

author: tszaboo
author_rank: Super Contributor
timestamp: March 29, 2025, 09:16:13 pm
content: Quote from: wn1fju on March 29, 2025, 12:20:41 pmIt's the usual people with skills enough to operate a shovel and not much else.They came to do whatever at work. We go out, point out exactly where the internet cables go, 20 minutes later, no internet, because they -of course- cut it.

author: EPAIII
author_rank: Super Contributor
timestamp: April 01, 2025, 02:03:31 pm
content: The mechanism?That electric crew obviously damaged the coax. It could have been only a small hole or slice into the insulation: practically invisible but enough to let some moisture in over time. So the shield began to slowly corrode. That corrosion lead to an ever increasing opening in the shield.Now, shields are never 100%. They always leak some small amount of radiation. The manufacturers of coaxial cables will list the amount of shielding that the shield on a given type of coax will protect against vs. the frequency of the interfering signal. Of course electrical noise usually contains a broad spectrum of frequencies so it is hard to predict what the overall effect will be.Now, the earth's ground is partially conductive. That is how ground rods can function. I would be sure that the conductivity of the earth varies with frequency so the coax will be bathed in the radiation from the heat pump and other sources. And some fraction of that radiation will find it's way inside the coaxial cable where it can be picked up by the center conductor and get into your signal.Another way would be if the shield were completely separated at that point, disconnecting the two halves of it. Now each length of the shield, disconnected from the other at the point of damage, can act as an antenna which induces the RF field in the ground and passes (capacitively coupled) that RF field (AKA noise) into the center conductor and from there to one end or both of the coaxial cable.You could spend a lot of money investigating the exact mechanism of this. But, no matter what it turns out to be, there is only one solution: REPLACE THE COAXIAL CABLE. The present one has been damaged and, while not impossible, it would be difficult to repair the existing cable. Any repair would need to first replace at least part of the cable where moisture has penetrated. And no matter how much you choose to replace, you will never know if it was enough. Some moisture may lurk past the point where you cut out a segment. On top of that you would need to make not one, but two splices. The easiest and probably best way to do that would be with connectors and barrels. That's four connectors and two barrels, each of which is another point of failure. And finally it would need to be completely sealed against moisture. That's a ton of work and a lot of future failure points. It just isn't worth it.Replace it! But that's what you say they are going to do anyway, hopefully at no extra charge. Replace it and don't worry about it. If you want reassurance that is is OK, have them use the TDR again. If it doesn't show any problems for the full length of the cable, you are OK.PS: any splices that were done WOULD show up on a TDR display. That's how we would look for any problems on a run of solid coax going up a tall tower to the antenna at the top. Every joint in the coax, no matter how well it was made, would show up as a small discontinuity. Usually all evenly spaced on the display depending on the length of the coaxial segments used. If you see one bigger than the others you send a tower climber up to that point, he puts his hand on it, and if it is hot with the transmitter on he takes it apart and cleans everything before assembling it again.And the next time the electric crew wants to dig up your lawn, break out your shotgun.Quote from: wn1fju on March 29, 2025, 01:28:23 pm

author: wn1fju
author_rank: Frequent Contributor
timestamp: April 02, 2025, 07:12:20 pm
content: Thanks, EPAIII, for the explanations.I might have also contributed to things because a few years ago, before the electric utility dug up my yard, I noticed a couple of slices into the cable sheath thanks to the landscapers and their careless use of weed-whackers. At that time, I taped over them with electrical tape. But the slices were only about two feet from the cable termination and the Comcast tech told me that his TDR had the fault seven feet from the end (consistent with the utility digging). But who knows how accurate his TDR was.I took my spectrum analyzer and hooked it up to the cable at my TV's desktop Comcast box (disconnected from the box). Everybody was still sleeping in my house so there was no internet activity going on. The spectrum shown goes from 0 to 30 MHz because that's where the Comcast tech said the problem was.There is plenty of activity down low plus a spike around 25 MHz that magically disappeared a few minutes later. I'm not sure if the dBm levels are high enough to cause a problem, so please enlighten me if you know.I did rule out my heat pump as a source of noise. Taking spectra with and without the heat pump on shows a small 10 dB increase in the noise floor from about 13 to 16 MHz. Doesn't appear to be the major noise source in my opinion.Hopefully Comcast will replace the cable as promised in the next few weeks. On the other hand, they could just be happy with the 30 MHz low pass filter they stuck on the line and ignore the whole thing. We'll see.

author: audiotubes
author_rank: Regular Contributor
timestamp: April 02, 2025, 08:40:09 pm
content: Quote from: wn1fju on April 02, 2025, 07:12:20 pmCould it be the dreaded rogue CB operator with unlawful kilowatt 10M liner amplifier?

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2025, 08:47:07 pm
content: The low end of your SA scan looks like AM broadcast and maybe ham activity: what are the spikes at 6 and 9 MHz?

author: wn1fju
author_rank: Frequent Contributor
timestamp: April 02, 2025, 11:25:33 pm
content: The low end of the spectrum is indeed full of AM broadcast signals. At one point, I set the spectrum analyzer from 0 to 2 MHz so I could more accurately get the frequencies of the peaks. They coincided with the frequencies of all my favorite AM radio stations in the area.I don't know what the other HF signals are and stupidly didn't zoom in and get the exact frequencies like I did for the AM broadcast stuff. Except for the ~25 MHz signal that was only there for a short time, those other HF signals persisted over the 10 minutes that I was doing the measurements. My only thought on the ~25 MHz signal was perhaps a local ham radio station on 12 meters.

author: tszaboo
author_rank: Super Contributor
timestamp: April 06, 2025, 03:44:06 pm
content: Quote from: audiotubes on April 02, 2025, 08:40:09 pmIt could be one of those mysterious "numbers stations" that they have somehow very little idea what it's purpose is.