AM radio interference from telephone line. - Page 1
Forum section: electronics / rf-microwave

author: oliver602
author_rank: Regular Contributor
timestamp: January 15, 2017, 06:43:46 pm
content: Have issues with ADSL for quite a while now(very high error rate and fluctuating SNR). I'm not here to trouble shoot my internet but I have a question.Using an AM radio close to the telephone drop line(several meters) I get a lot of noise that seams to cover the whole AM band. The affect changes as I turn the tuning knob. I get similar results on the car radio as I follow the over head cables along the road for a mile or so. I have attached a recording from the radio.Is this in any way typical behaviour? Whats happening here?

author: German_EE
author_rank: Super Contributor
timestamp: January 15, 2017, 07:45:02 pm
content: ADSL uses 26.075 kHz to 137.825 kHz for upstream communication and 138 kHz – 1104 kHz for downstream, this covers a large chunk of the Long Wave and Medium Wave bands and is a major pain in the ass for those of us who used to do Medium Wave DX before ADSL came along

author: Red Squirrel
author_rank: Super Contributor
timestamp: January 16, 2017, 04:55:34 pm
content: Sometimes phone lines go to a cross box, to the customer, but also continue on to other cross boxes, unterminated at the other end. Basically creating an antenna. I could definitely see it causing issues. It can also cause issues for the DSL connection itself as it will pickup lot of noise.

author: retrolefty
author_rank: Super Contributor
timestamp: January 17, 2017, 12:56:23 am
content: Such induced noise is usually rooted in bad power insulators on power poles which are often shared by telephone cables. Ham radio operators have a lot of experience hunting these down often as you did with a simple AM radio. Once isolated to a specific power pole a call to the utilities co. will often get repairs made. A power pole troubleshooter from our local company gave a talk on his job at our local ham radio club back in the 70s. The problem can be quite intermittent, changing with weather, etc. He said they often start by hitting the pole with a sledgehammer and listening for changes in the noise on the AM receiver. Dirty or damaged insulators is the frequent problem, requiring special cleaning or replacement.

author: jh15
author_rank: Frequent Contributor
timestamp: January 18, 2017, 05:28:10 am
content: Don't go hitting a suspected noisy pole with a hammer. It may be noisy due to broken hardware arcing, and shaking the pole can bring it down on you.orenlist a politician looking for your vote to do it, lend him a hammer.

author: Lord of nothing
author_rank: Super Contributor
timestamp: January 20, 2017, 09:31:37 pm
content: http://www.rtl-sdr.com/showing-the-hf-interference-problem-from-ethernet-over-powerline-devices/I have excakte the same Problem...

author: oliver602
author_rank: Regular Contributor
timestamp: January 21, 2017, 05:41:21 pm
content: Turns out my problem was mostly caused by a wall wart with bad caps. The offending device was plugged into a cordless phone base station in our house.Why did it wipe out the AM band on the car radio. Shouldn't the twist in the phone line prevent that? Red Squirrel mentions the line may continue past our house unterminated. Would that cause only the section past the cross box to act as an antenna? Thanks

author: SeanB
author_rank: Super Contributor
timestamp: January 21, 2017, 07:34:26 pm
content: ADSL is designed ( because there might be 999 other ADSL signals in that lovely big 1000 pair line leading to the DSLAM) to be relatively non radiating on proper wiring. The signal is carried on a twisted pair, and is overall designed to have almost no common mode component, as that will radiate. The phone wart though, will put a common mode signal down the line, and the whole line, both upstream to the DSLAM and carrying on past you, will radiate it as it is coupled to the other pairs.With your phone line it is best to put an ADSL line filter right at the line entry, and split the phone line into 2 branches. One set, with the ADSL signal greatly attenuated, goes to the phone wiring in the house, while the other signal, with the ADSL and the POTS as well, goes via a very short twisted pair cable ( I used some old CAT5cable, as it was both free and recovered cable and also I only needed 1 pair) to the modem.That way you get both better ADSL data rates, as there are reduced reflections on the stub lines, and there is lower line loss up and down. Plus you also have less phone line interference, and as a bonus less common mode hash from phone line power supplies carried up the line. I will guess the guilty party was a Siemens DECT phone base station, with the tiny power supply they use on them. Amazing that the whole board in those phones is double sided SRBP, with electroplated traces on the 2 sides, and conductive ink via's to join the 2 sides. Then the RF stuff that needs controlled impedance is done with a small module, with mouse bites, soldered to the board.

author: Co6aka
author_rank: Unknown
timestamp: January 23, 2017, 12:22:40 am
content: FWIW, when I've encountered raucous radiated noise from balanced data transmission lines it has typically been because of an imbalance somewhere along the line.Re RF noise: It has a special ghostly and ghastly properties that allow it to couple into any and all conductors that might enable it to cause the greatest amount of grief to anyone operating a receiver. Place no moving electron beyond suspicion!

author: oliver602
author_rank: Regular Contributor
timestamp: January 28, 2017, 03:41:35 pm
content: You're right SeanB! it was a Siemens DECT phone. We also have a filter at the entry point and our ADSL modem is connected right there by a short length length of twisted pair cable.I found another problem. After replacing the bad wall wart the FEC error graph from our broadband router looks surprisingly similar to our solar production graph...We have an immersion heater controller that diverts electricity that would otherwise be exported to the grid. When this is disabled our internet works better.

author: SeanB
author_rank: Super Contributor
timestamp: January 28, 2017, 04:22:56 pm
content: Time to find a 20A common mode mains choke then, or find one from a scrapped inverter aircon to fit there in the line. You probably have a lot of mains harmonics being imposed on the phone wiring, probably best to replace the twisted pair with STP ethernet cable, and put a LPU on the incoming phone line, with the shield of this cabling connected to this, and then the ground wire led through a short minimal bend 4mm cable to a ground rod outside. Should reduce the pick up of EMI on the phone cable, though you might also try using a small mains common mode filter assembly on the DSL modem supply, to reduce conducted noise into the PSU. Make sure to use one with VDR protection as well, helps prolong the life of the SMPS primary side from transients.

author: cdev
author_rank: Super Contributor
timestamp: January 28, 2017, 05:09:06 pm
content: A broadband SDR that can receive HF is really good for finding RFI sources. Definitely worth the effort of buying or building one.

author: oliver602
author_rank: Regular Contributor
timestamp: January 28, 2017, 08:06:27 pm
content: QuoteWhat is LPU?

author: helius
author_rank: Super Contributor
timestamp: January 28, 2017, 08:22:30 pm
content: Lightning Protection Unit, a surge arrestor.

author: SeanB
author_rank: Super Contributor
timestamp: January 28, 2017, 08:36:44 pm
content: LPU Line Protection Unit, basically a dual spark gap unit attached to the line, a pair of 22R 1W fusible resistors and a special 3 terminal TO220 dual diac clamp that is designed to shunt lightning to the common ground connection. This then is connected to a ground rod, not mains earth, to reduce the noise. This is a recommended device with overhead lines feeding the property, and while it does have some loss with the DSL connection, especially at the high end, it does reduce damage with local EMP from lightning.There is a 10 pair unit that is clipped into the Krone blocks by the DSLAM, and which is grounded to the termination frame simply by plugging it in, and the frame is grounded with a 6mm copper wire to the common DSLAM ground bus. There are single units as well for Krone available, but the more common versions are inline LPU units for use with Fax machines. They typically have RJ11 connections or a cable out each end, and a ground wire at the side.Saved my bacon a few times, clamping close strikes to a survivable level. Thjat, and a lot of VDR units scattered in equipment was a cheap investment, though a few did fail short with time and surges.

author: jackzzj
author_rank: Newbie
timestamp: February 13, 2017, 08:33:30 pm
content: ADSL sucks...

author: Monkeh
author_rank: Super Contributor
timestamp: February 13, 2017, 09:07:23 pm
content: Quote from: jackzzj on February 13, 2017, 08:33:30 pmThank you for that highly valuable and informed contribution.

author: wkb
author_rank: Super Contributor
timestamp: February 13, 2017, 10:43:15 pm
content: Quote from: jackzzj on February 13, 2017, 08:33:30 pmNope. Stringing DSL lines, telephone lines and power lines overhead is bad news. Bury the whole lot underground and things work just fine.It helps to live in a country founded on mud (NL) and not on granite

author: jonovid
author_rank: Super Contributor
timestamp: February 13, 2017, 11:50:00 pm
content: I have had poor shortwave radio reception from metropolitan telephone line noise. on a 22 meter long wire reception antenna. I find unplugging the landline phone, fixes the problem. as most of my AM & SSB shortwave listening is at night, when the phone is not in use. no solar inverters running esc.