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Author Topic: Ingress below 40MHz in CATV indoor cable run  (Read 1183 times)
N2NJO
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Posts: 186




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« on: December 17, 2017, 10:23:37 AM »

I have been having problems with dropped Internet connectivity. I swapped my Modem (with a backup, same model) and swapped my Router with another model, but the problems continued.

 No pattern (naturally). I contacted the MSO (Spectrum/TWC) more than once, as usual they only checked for signal level and S/N ratios & return power, all showed good, which I already knew. This went on fro probably a month or more and there were other issues, separate from this that affected my VoIP phone service which I also pointed out where I believed the problem was (using multiple Trace Route programs).

 After blasting them with multiple e-mails (I have their lead techs & their field supervisors e-mail address thru a PSC complaint I placed regarding the VoIP issues from twp months ago), they finally sent someoen out with a 'real ' piece of test equipment (impressive in it's own right).

 After checking the 'run' from the basement to the upper 'flat' (about 25'), was found ingress below 40MHz, especially below 30MHz. I fired up my Siglent SA in the same range with a 'whip' attached to the input and found nothing. I did see the display on their meter and levels were under -20dBmV, most were less than -30dBmV, but in their 'return/upstream band which could affect Internet access (Modems return channel).
 Cable replaced, ingress dropped 15dB or more, but it's too soon to see if that solves the problem.

So, my question is: what the hell is this garbage below 40MHz inside a house with NO MOCA in use that I can't find?? There is a nearby Romex line run parallel to the coxial cable, but 60Hz is a long way away from 30 MHz.

 Oh, the replaced cable after I stripped a section and removed the jacket was only 60% braid (and foil), not 99% braid which would explain the difference in ingress levels.
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KC9NVP
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Posts: 217




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« Reply #1 on: December 18, 2017, 05:27:14 AM »

Start with standard interference search - isolate and identify the source.  Determine if the noise generator is inside or outside of your home (shut off main power breaker and see if the issue goes away).  If the noise is gone, you have reduce your search area to just you home, if not then you have larger area to search outside of the house.  Based on the noise going away when you shut off the main power breaker, you need turn back on the main power breaker and verify that everything is back up and running (computers, TV, radios, heaters, etc) and that the noise has returned.  the next step is to turn off each individual sub breakers to individual rooms or circuits till the noise goes away.  Once you have identified which room or circuit the noise is on, you need to check everything that is plugged into that circuit.

I would not be surprise if the noise is being generated by a switching power supply (under cabinet lighting/computer/etc), pet heating pad, touch lamp, washer/dryer, HVAC fan motors, etc.  You are most likely seeing the harmonics of the original noise which could be around 20-50 kHz.  It is possible for an arcing contact in a switch or equipment to generate noise well into the MHz range.  Regarding the use of the SA, you may need to open up the band width in order to see what the Tech was seeing on his equipment, because you can have narrow band and wide band noise.  It sounds like you have wide band noise issue.

I have read some stories on the internet (yea, I know not to trust anything on the internet - but it can be your friend as well) where people found laptop power supplies, under cabinet lighting, battery chargers have generated noise that cause issue with Wi-Fi internet connection and what they did to eliminate the problem.
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N2NJO
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Posts: 186




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« Reply #2 on: December 18, 2017, 07:53:38 AM »

The problem is, even after doing an additional sweep using a single 24 gauge 'wire antenna' dropped down the vertical portion of the route, I don't see what their SLM saw. Only spikes at 33 & 35MHz and some broadband noise below 15MHz.

I didn't figure in switching PS's.  I did power down my PC at the time since that was the 1st 'source' that came to mind and a 'scanner' receiver, but that didn't make any difference. This isn't a typical suburban home, no kids, some electronics, but no where near what most seem to have that are 'wireless' crazy.
There are 9 power strip mounted switching PS's but they all go into a TrippLite Isobar filtered power strip (not a simple surge protector).

One more thing (I didn't originally think of). I do have a "RF" ground (only 8 gauge) running with that Romex down to the basement tied into the electrical ground then to two 8" ground rods.

I just didn't figure harmonics going that high from power line 'noise', even switching PS's.
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K8BYP
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« Reply #3 on: Yesterday at 06:33:11 AM »

Theyre checking "within" their system ( I assume) 'inside' their cables. I assume they connect a sniffer TO the cable and read whats bouncing up and down the line.

Youre (for the most part) checking outside their cables (RF sniffing) with an aerial.

Apples, meet oranges.

There are two kinds of RFI, conducted and radiated. Must figure out which it is before proceeding.
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N2NJO
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Posts: 186




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« Reply #4 on: Yesterday at 06:57:05 AM »

No, the check was a specific length of inside wiring NOT connected to their outside plant. When they 1st did the check the far end was not terminated. I had them re-do it after I placed a terminator on it. The 'ingress' was from inside the house, not from their system.

If nothing else, that proved the need for 99% braided shield, not 60% when it comes to Internet usage due to it being prone to low band ingress (in this case) with the 'return' channels. Now, I understand with the elimination of all analog channels, many systems are moving those 'return' channels up into the VHF high band to get away from low band noise which should help.

Spectrum (former TWC) has been gradually doing dropping analog for the past 2-3 years. Our system is next. A small system east of us was just done with a number of unhappy campers (which I'm sure has been the case every place else). AFAIK, they now can (thanks for more anti-regulation that reverses the existing order) encript even the OTA stations.  Angry : Smiley
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