SAT Coax cable the screen was corroded away - Page 1
Forum section: electronics / beginners

author: bingo600
author_rank: Super Contributor
timestamp: April 27, 2013, 04:25:04 pm
content: I just spend the whole day , replacing my sat-cable , going from my dish (diseq) to the receiver.I have my dish hanging on the garage (W...'s order) , as it wasn't allowed to hang it on the houseThis means i have around 30m (120dB) quad-screened cable , 15m "dug down outside" from the garage to the house , and 15m in the 90cm "crawl cellar" to the receiver.When i installed it it was working excellent , lot's of signal (120 cm Alu Dish - 4 heads) , 120dB cable and SPAUN diseq.Last year (May) we lost the signal , and was suspecting a large tree nearby. But the signal didn't come in the autumn , where the leaves fall off.I had the cable running in some "cheap flex tube" on the outside path , and was then expecting that this flex-tube was somehow breached , and that the cable had been under water for a long time. The cable crosses a place that is under water during heavy rain.Today i replaced the cable , and was quite astonished when i discovered that the error was inside the "crawl cellar".When i withdrew the cable fully ,i discovered there was a spot that was "rust colored" , and felt wrong. The strange part was that the cable "outer protection" plastic was intact , just discoloured on the "rusty" spot.I did cut the cable around the rusty spot , and opened it up.There was absolutely no screen left , just "white powder" ....I don't know what happened , but have a suspicion that the cable ... Lying in the sandy ground in the "crawl cellar" might have been on some metal at that spot and that some kind of "electrolysis" or whatever has happened there.It takes a full day to replace , due to digging it down in the lawn 4 meters , taking up the concrete tiles from the tereasse for 8 meters , digging it down there. Refit the concrete tiles and curse when they doesn't allign nicely. And then there is the 15 meters in the "crawl cellar" ...So this time i decided to lay the "outside" 15m cable inside a garden water hose (for protection) , and then run the last 15m in the crawl cellar in flextube. So the cable doesn't come in contact with the "ground" at any place.I have just finished , and we can now see sat-tv againBut what did happen with the 15cm cable where the screen became "white powder"Even through the outer plastic-protection/Bingo

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: April 27, 2013, 04:46:53 pm
content: I've seen it many times when repairing faults in buried cables, even a tiny pinhole will cause electrolysis and the aluminum turns into aluminum oxide powder.

author: JackOfVA
author_rank: Unknown
timestamp: April 27, 2013, 09:08:12 pm
content: If your antenna has an amplifier at the dish and the amplifier is powered over the coax cable - which I believe it will have - then the coax carries both the RF signal and significant DC voltage and current. The DC power accelerates the galvanic action, as it makes the coax cable into an electrolysis cell.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 28, 2013, 03:05:21 am
content: Some cable TV systems use 60V AC power distribution to avoid that very problem.

author: bingo600
author_rank: Super Contributor
timestamp: April 28, 2013, 05:33:57 am
content: Quote from: JackOfVA on April 27, 2013, 09:08:12 pmThere is power applied , as the diseq protocol requires it. So you could be quite right.I have just never thought that the outer plastic, was penetratable for outside thing.I have had sat dishes with cables running in free air (on the house wall) for 10 years , and have never experienced anything like this. So there must be a "new" component in the mix , and that might be the "rusty spot". The housebuilders have used the crawl cellar as a dumpster , and there is lot's of small sheet metal pieces.Well i think you guyzz have solve dthe mystery.But be warned, about laying the cable directy on sandy ground , if there is any metal there.And maybe the same applies to other paths where the cable could come in contact with some "rust"./Bingo

author: G7PSK
author_rank: Super Contributor
timestamp: April 28, 2013, 08:04:41 am
content: A lot of sand contains iron and other minerals as well it is also sharp and abrasive so could wear through the outer sleve over time with movement caused through heat and clod plus vibration. Also if pulling long lengths through you can get both scuffing and tearing taking place.

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2013, 10:43:05 am
content: Under the house the cable will be damp from ground water evaporation, and as the modern PVC id often slightly permeable to moisture it will penetrate and cause corrosion, especially if there is a slight nick or abrasion caused by bending or dragging during installation. Best is to have a joint box outside the house, and a barrel connector sealed with self amalgamating tape over the join inside the box. That way you only have to replace a smaller section.The LNB and DisEQ are powered and controlled over the coax, with a 13/17V voltage for power and to select polarisation, and a 22kHz tone at around 2Vpp sent down the cable as well for selection. Often it is better ( and here you do this as a matter of course) to install 2 cables, as a lot of the newer decoders have 2 tuners inside ( needed for the HD channels as they need to use 2 8MHz slots for the data rate) that each need a feed to the LNB.

author: Strada916
author_rank: Frequent Contributor
timestamp: April 28, 2013, 10:57:07 am
content: Even better use gel quadshield cable.

author: bingo600
author_rank: Super Contributor
timestamp: April 28, 2013, 11:00:18 am
content: Quote from: SeanB on April 28, 2013, 10:43:05 amI have 4 heads on the Gibertini 100cm dish , and chose to make it one long unbroken run , in order to optimize the cable loss @30 meter.http://www.hm-sat-shop.de/antennen-gibertini-xp-premium/spiegel-gibertini-xp-premium-100-cm.htmlhttp://www.hm-sat-shop.de/antennen-gibertini-xp-premium/schielhalter-gibertini-mfp-4-23-xp-premium-4-fach.htmlBut i didn't know the part about needing 2 cables to the lnb (must be a dual lnb or ?)Ohh i got "bonus points from the W... for matching the dish color with the house"/Bingo

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2013, 11:25:27 am
content: 4 LNB's to see 4 different satellites, very nice. Here you have a choice of 2, and they are so far apart you need 2 separate dishes. Would be good to go and get some covers for the LNB's though, the front cover tends to crack with time. You can also paint some dishes with acrylic paint, certain colours are not terribly lossy at satellite frequencies, though you might want a slightly bigger dish.Here all satellite TV is subscription, and digital. You can go FTA though, though that is an option they will try to steer you away from, as it does not net them any money. Then you get 3 channels from the state broadcaster, 1 independent, Parliament TV and for some reason China TV.